**Introduction**

**Chapter 1**

**Provisional chapter**

**Introductory Chapter: Relevance of Soil pH to**

**Introductory Chapter: Relevance of Soil pH to** 

DOI: 10.5772/intechopen.82551

Soil pH is a master variable in soils because it controls many chemical and biochemical processes operating within the soil. It is a measure of the acidity or alkalinity of a soil. The study of soil pH is very important in agriculture due to the fact that soil pH regulates plant nutrient availability by controlling the chemical forms of the different nutrients and also influences their chemical reactions. As a result, soil and crop productivities are linked to soil pH value. Though soil pH generally ranges from 1 to 14, the optimum range for most agricultural crops is between 5.5 and 7.5. However, some crops have adapted to thrive at soil pH values outside this optimum range. The United States Department of Agricultural National Resources Conservation Service groups soil pH values as follows: ultra acidic (<3.5), extremely acidic (3.5–4.4), very strongly acid (4.5–5.0), strongly acidic (5.1–5.5), moderately acidic (5.6–6.0), slightly acidic (6.1–6.5), neutral (6.6–7.3), slightly alkaline (7.4–7.8), moderately alkaline (7.9–8.4),

Soil pH is affected by the mineral composition of the soil parent material and the weathering reactions undergone by that parent material. For instance, in humid environments, soil acidification occurs for a long time as the products of weathering leached by water moving laterally or downwards through the soil, while in the dry environments, soil weathering and

> © 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

© 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use,

distribution, and reproduction in any medium, provided the original work is properly cited.

**Agriculture**

**1. Why soil pH?**

**Agriculture**

Suarau Odutola Oshunsanya

Suarau Odutola Oshunsanya

http://dx.doi.org/10.5772/intechopen.82551

Additional information is available at the end of the chapter

strongly alkaline (8.5–9.0) and very strongly alkaline (>9.0) [1].

leaching are less intense, and soil pH is often neutral or alkaline [2].

Additional information is available at the end of the chapter

#### **Introductory Chapter: Relevance of Soil pH to Agriculture Introductory Chapter: Relevance of Soil pH to Agriculture**

DOI: 10.5772/intechopen.82551

Suarau Odutola Oshunsanya Suarau Odutola Oshunsanya

Additional information is available at the end of the chapter Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/intechopen.82551

#### **1. Why soil pH?**

Soil pH is a master variable in soils because it controls many chemical and biochemical processes operating within the soil. It is a measure of the acidity or alkalinity of a soil. The study of soil pH is very important in agriculture due to the fact that soil pH regulates plant nutrient availability by controlling the chemical forms of the different nutrients and also influences their chemical reactions. As a result, soil and crop productivities are linked to soil pH value. Though soil pH generally ranges from 1 to 14, the optimum range for most agricultural crops is between 5.5 and 7.5. However, some crops have adapted to thrive at soil pH values outside this optimum range. The United States Department of Agricultural National Resources Conservation Service groups soil pH values as follows: ultra acidic (<3.5), extremely acidic (3.5–4.4), very strongly acid (4.5–5.0), strongly acidic (5.1–5.5), moderately acidic (5.6–6.0), slightly acidic (6.1–6.5), neutral (6.6–7.3), slightly alkaline (7.4–7.8), moderately alkaline (7.9–8.4), strongly alkaline (8.5–9.0) and very strongly alkaline (>9.0) [1].

Soil pH is affected by the mineral composition of the soil parent material and the weathering reactions undergone by that parent material. For instance, in humid environments, soil acidification occurs for a long time as the products of weathering leached by water moving laterally or downwards through the soil, while in the dry environments, soil weathering and leaching are less intense, and soil pH is often neutral or alkaline [2].

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

### **2. Soil acidification**

Soil acidification is brought about by a number of processes such as high rainfall, crop growth, the use of fertilizers, acid rain and oxidative weathering.

Both acid and alkaline soils have influence on crop growth and development. For instance, agricultural crops grown in acid soils may experience some stresses such as AI, H and Mn toxicity as well as Ca and Mg nutrient deficiencies. Aluminum toxicity, which is the most widely spread problem of acid soils, occurs when aluminum is present in ionic Al3+ form. Aluminum ion Al3+ is the most soluble of all forms of aluminum at soil pH less than 5.0 (acidic condition). Aluminum is not a plant nutrient but an ionic form of Al3+ that enters crop roots passively through the process of osmosis. Aluminum inhibits root growth and development by interfering uptake and transport of essential nutrients, cell division, cell wall formation and enzyme activity. However, strong alkaline soils (sodic soils) are characterized with slow infiltration, reduced hydraulic conductivity and poor soil water retention capacity that make crops to experience water stress. Generally, agricultural crops are varied in terms of suitability for soil pH range. Some crops can be intolerant of a particular soil pH due to a particular mechanism. For instance, soil pH 5.5 is not suitable for soybean plants when molybdenum is low in the soil, but the same pH 5.5 becomes optimum for soybean when molybdenum is sufficient in the soil. Most agricultural crops perform optimally around soil pH 7.0 (neutral). This shows that it is very important to bring both acidic and alkaline soils to neutral soil pH value for optimum per-

Introductory Chapter: Relevance of Soil pH to Agriculture

http://dx.doi.org/10.5772/intechopen.82551

5

The pH of acidic soil can be increased by using finely ground agricultural lime (limestone or chalk). The buffering capacity of the soil determines the amount of lime needed to increase pH of acidic soil. The buffering capacity of the soil largely depends on the amount of clay and organic matter present. Soils with high clay and organic matter will have high buffering capacity. Apart from limestone, other amendments such as wood ash, industrial calcium oxide (burnt lime), magnesium oxide, basic slag (calcium silicate) and oyster shells can be used to increase pH of acidic soils. On the other hand, the pH of alkaline soils can be decreased by using acidifying fertilizers or organic materials. Acidifying fertilizers include ammonium sulphate, ammonium nitrate and urea, while acidifying organic materials are peat or sphagnum peat moss. Elemental sulfur (90–99% S) has been successfully used at application rates of

to reduce the pH of an alkaline soil. Therefore, farmers must be encouraged

formance of crops.

300–500 kg ha−<sup>1</sup>

**Author details**

Nanning, China

Suarau Odutola Oshunsanya1,2

**4. Amendment of soil acidity and alkalinity**

to regulate the soil pH value for optimal crop performance.

Address all correspondence to: soshunsanya@yahoo.com

1 Department of Agronomy, University of Ibadan, Nigeria

2 Key Laboratory of Agro-Environment and Agro-Product Safety, Guangxi University,

#### **2.1. High rainfall**

Soils usually become acidic under heavy rainfall. This is because rainwater is slightly acidic (about 5.7) due to a reaction with CO2 in the atmosphere that forms carbonic acid. As this rainwater passes through soil pores, it leaches basic cations from the soil as bicarbonates, which increases the percentage of AI3+ and H+ relative to other cations in the soil. Root respiration and decomposition of organic matter by microorganisms also release CO<sup>2</sup> that increases the carbonic acid (H2 CO3 ) concentration resulting to leaching.

#### **2.2. Crop growth**

Soil nutrients are taken up by crop roots in the form of ions (NO3 <sup>−</sup>, NH4 + , Ca2+ and H2 PO4 <sup>−</sup>). Crop roots often take up more cations than anions. But crops must maintain a neutral charge in their roots for normal physiological processes to take place. H+ ions are released by root crops to compensate for the extra positive charges resulting to acid soils.

#### **2.3. Use of fertilizers**

Some fertilizers such as ammonium (NH4 + ) fertilizers undergo nitrification process to form nitrate (NO3 <sup>−</sup>), and during this process, H+ ions are released leading to acid soils.

#### **2.4. Acid rain**

Oxides of sulfur and nitrogen are released into the atmosphere when burning fossil fuels. Released oxides react with rainwater in the atmosphere to form tetraoxosulphate (vi) acid and trioxonitrate (v) acid.

#### **2.5. Oxidative weathering**

Sulphides and other compounds containing Fe2+ produced acidity during oxidation process.

#### **3. Soil alkalinity**

Soil alkalinity increases with weathering of silicate, aluminosilicate and carbonate mineral compounds that contain Na+ , Ca+ , Mg2+ and K+ . The fore-listed minerals are usually added to the soil by the deposition of eroded sediments by water or wind. Soil alkalinity can also be increased by addition of water containing dissolved bicarbonates especially when irrigating with high-bicarbonate water. Insufficient water flowing to leach soluble salts can lead to accumulation of alkalinity in a soil. This is common in arid areas or poor internal soil water drainage situations, where the water that comes in is either transpired by crops or evaporates rather than flowing through the soil.

Both acid and alkaline soils have influence on crop growth and development. For instance, agricultural crops grown in acid soils may experience some stresses such as AI, H and Mn toxicity as well as Ca and Mg nutrient deficiencies. Aluminum toxicity, which is the most widely spread problem of acid soils, occurs when aluminum is present in ionic Al3+ form. Aluminum ion Al3+ is the most soluble of all forms of aluminum at soil pH less than 5.0 (acidic condition). Aluminum is not a plant nutrient but an ionic form of Al3+ that enters crop roots passively through the process of osmosis. Aluminum inhibits root growth and development by interfering uptake and transport of essential nutrients, cell division, cell wall formation and enzyme activity. However, strong alkaline soils (sodic soils) are characterized with slow infiltration, reduced hydraulic conductivity and poor soil water retention capacity that make crops to experience water stress.

Generally, agricultural crops are varied in terms of suitability for soil pH range. Some crops can be intolerant of a particular soil pH due to a particular mechanism. For instance, soil pH 5.5 is not suitable for soybean plants when molybdenum is low in the soil, but the same pH 5.5 becomes optimum for soybean when molybdenum is sufficient in the soil. Most agricultural crops perform optimally around soil pH 7.0 (neutral). This shows that it is very important to bring both acidic and alkaline soils to neutral soil pH value for optimum performance of crops.

#### **4. Amendment of soil acidity and alkalinity**

The pH of acidic soil can be increased by using finely ground agricultural lime (limestone or chalk). The buffering capacity of the soil determines the amount of lime needed to increase pH of acidic soil. The buffering capacity of the soil largely depends on the amount of clay and organic matter present. Soils with high clay and organic matter will have high buffering capacity. Apart from limestone, other amendments such as wood ash, industrial calcium oxide (burnt lime), magnesium oxide, basic slag (calcium silicate) and oyster shells can be used to increase pH of acidic soils. On the other hand, the pH of alkaline soils can be decreased by using acidifying fertilizers or organic materials. Acidifying fertilizers include ammonium sulphate, ammonium nitrate and urea, while acidifying organic materials are peat or sphagnum peat moss. Elemental sulfur (90–99% S) has been successfully used at application rates of 300–500 kg ha−<sup>1</sup> to reduce the pH of an alkaline soil. Therefore, farmers must be encouraged to regulate the soil pH value for optimal crop performance.

#### **Author details**

**2. Soil acidification**

(about 5.7) due to a reaction with CO2

4 Soil pH for Nutrient Availability and Crop Performance

increases the percentage of AI3+ and H+

CO3

Some fertilizers such as ammonium (NH4

<sup>−</sup>), and during this process, H+

, Ca+

, Mg2+ and K+

**2.1. High rainfall**

carbonic acid (H2

**2.2. Crop growth**

**2.3. Use of fertilizers**

trioxonitrate (v) acid.

**3. Soil alkalinity**

pounds that contain Na+

**2.5. Oxidative weathering**

nitrate (NO3

**2.4. Acid rain**

the use of fertilizers, acid rain and oxidative weathering.

Soil acidification is brought about by a number of processes such as high rainfall, crop growth,

Soils usually become acidic under heavy rainfall. This is because rainwater is slightly acidic

water passes through soil pores, it leaches basic cations from the soil as bicarbonates, which

Crop roots often take up more cations than anions. But crops must maintain a neutral charge

Oxides of sulfur and nitrogen are released into the atmosphere when burning fossil fuels. Released oxides react with rainwater in the atmosphere to form tetraoxosulphate (vi) acid and

Sulphides and other compounds containing Fe2+ produced acidity during oxidation process.

Soil alkalinity increases with weathering of silicate, aluminosilicate and carbonate mineral com-

the deposition of eroded sediments by water or wind. Soil alkalinity can also be increased by addition of water containing dissolved bicarbonates especially when irrigating with high-bicarbonate water. Insufficient water flowing to leach soluble salts can lead to accumulation of alkalinity in a soil. This is common in arid areas or poor internal soil water drainage situations, where the water that comes in is either transpired by crops or evaporates rather than flowing through the soil.

+

and decomposition of organic matter by microorganisms also release CO<sup>2</sup>

Soil nutrients are taken up by crop roots in the form of ions (NO3

in their roots for normal physiological processes to take place. H+

crops to compensate for the extra positive charges resulting to acid soils.

) concentration resulting to leaching.

in the atmosphere that forms carbonic acid. As this rain-

relative to other cations in the soil. Root respiration

<sup>−</sup>, NH4 +

) fertilizers undergo nitrification process to form

. The fore-listed minerals are usually added to the soil by

ions are released leading to acid soils.

that increases the

PO4 <sup>−</sup>).

, Ca2+ and H2

ions are released by root

Suarau Odutola Oshunsanya1,2

Address all correspondence to: soshunsanya@yahoo.com

1 Department of Agronomy, University of Ibadan, Nigeria

2 Key Laboratory of Agro-Environment and Agro-Product Safety, Guangxi University, Nanning, China

#### **References**

[1] Soil Survey Staff. Soil survey laboratory methods manual. In: Burt R, editor. Soil Survey Investigations Report No. 42, Version 5.0. 5th ed. U.S. Department of Agriculture, Natural Resources Conservation Service. 2014. pp. 276-279

**Section 2**

**Soil Fertility and Plant Nutrition**

[2] Bloom PR, Skyllberg U. Soil pH and pH buffering. In: Huang PM, Li Y, Sumner ME, editors. Handbook of Soil Sciences: Properties and Processes. 2nd ed. Boca Raton, FL: CRC Press; 2012. pp. 19-14. ISBN: 9781439803059

**Soil Fertility and Plant Nutrition**

**References**

6 Soil pH for Nutrient Availability and Crop Performance

[1] Soil Survey Staff. Soil survey laboratory methods manual. In: Burt R, editor. Soil Survey Investigations Report No. 42, Version 5.0. 5th ed. U.S. Department of Agriculture,

[2] Bloom PR, Skyllberg U. Soil pH and pH buffering. In: Huang PM, Li Y, Sumner ME, editors. Handbook of Soil Sciences: Properties and Processes. 2nd ed. Boca Raton, FL:

Natural Resources Conservation Service. 2014. pp. 276-279

CRC Press; 2012. pp. 19-14. ISBN: 9781439803059

**Chapter 2**

**Provisional chapter**

**Effects of Acid Soils on Plant Growth and Successful**

Acid soils are caused by mining, potentially causing the death of plants. Although soil pH is one of the useful indicators to evaluate acid soil conditions for successful revegetation, the dissolution of harmful elements under acidic conditions should be considered in addition to the tolerance mechanism of plants in mines. Thus, this study aims to report the current situation of acid soils and plant growth in mine site and to elucidate the effects of acid soils on plant growth over time through field investigation and a vegetation test. The results showed that the dissolution of Al from acid soils which were attributed to the dissolution of sulfides influenced plant growth. Not only soil pH but also the assessment of the dissolution of sulfides over time is crucial for successful revegetation, suggesting that net acid producing potential (NAPP) and net acid generation (NAG) pH, which are used for evaluating the formation of acidic water, are useful to evaluate soil conditions for the revegetation. Furthermore, acid-tolerant plant survived under acidic conditions by increasing the resistance against acidic conditions with the plant growth. Such factors and the proper selection of plant species play an important role in achieving successful revegetation in mines.

**Keywords:** acid soils, plant growth, revegetation, mine site, acidic water, Al tolerance,

**Effects of Acid Soils on Plant Growth and Successful** 

DOI: 10.5772/intechopen.70928

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution,

© 2017 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use,

distribution, and reproduction in any medium, provided the original work is properly cited.

and reproduction in any medium, provided the original work is properly cited.

Acid soils are formed with human activities, such as construction and mining, potentially causing the death of plants [1]. Plants wither due to not only low pH conditions in acid soils

**Revegetation in the Case of Mine Site**

**Revegetation in the Case of Mine Site**

Shinji Matsumoto, Hideki Shimada,

Shinji Matsumoto, Hideki Shimada, Takashi Sasaoka, Ikuo Miyajima,

Ginting J. Kusuma and Rudy S. Gautama

Additional information is available at the end of the chapter

Additional information is available at the end of the chapter

Takashi Sasaoka, Ikuo Miyajima,

Ginting J. Kusuma and

Rudy S. Gautama

**Abstract**

sulfides

**1. Introduction**

http://dx.doi.org/10.5772/intechopen.70928

**Provisional chapter**

### **Effects of Acid Soils on Plant Growth and Successful Revegetation in the Case of Mine Site Revegetation in the Case of Mine Site**

**Effects of Acid Soils on Plant Growth and Successful** 

DOI: 10.5772/intechopen.70928

Shinji Matsumoto, Hideki Shimada, Takashi Sasaoka, Ikuo Miyajima, Ginting J. Kusuma and Rudy S. Gautama Takashi Sasaoka, Ikuo Miyajima, Ginting J. Kusuma and Rudy S. Gautama Additional information is available at the end of the chapter

Shinji Matsumoto, Hideki Shimada,

Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/intechopen.70928

#### **Abstract**

Acid soils are caused by mining, potentially causing the death of plants. Although soil pH is one of the useful indicators to evaluate acid soil conditions for successful revegetation, the dissolution of harmful elements under acidic conditions should be considered in addition to the tolerance mechanism of plants in mines. Thus, this study aims to report the current situation of acid soils and plant growth in mine site and to elucidate the effects of acid soils on plant growth over time through field investigation and a vegetation test. The results showed that the dissolution of Al from acid soils which were attributed to the dissolution of sulfides influenced plant growth. Not only soil pH but also the assessment of the dissolution of sulfides over time is crucial for successful revegetation, suggesting that net acid producing potential (NAPP) and net acid generation (NAG) pH, which are used for evaluating the formation of acidic water, are useful to evaluate soil conditions for the revegetation. Furthermore, acid-tolerant plant survived under acidic conditions by increasing the resistance against acidic conditions with the plant growth. Such factors and the proper selection of plant species play an important role in achieving successful revegetation in mines.

**Keywords:** acid soils, plant growth, revegetation, mine site, acidic water, Al tolerance, sulfides

#### **1. Introduction**

Acid soils are formed with human activities, such as construction and mining, potentially causing the death of plants [1]. Plants wither due to not only low pH conditions in acid soils

Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2017 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons

but also dissolution of harmful elements, such as Al, Fe, and Mn, dissolving under acidic conditions. Although soil pH is one of the useful indicators to evaluate acid soil conditions for successful revegetation and/or farming, the dissolution of harmful elements under acidic conditions should be taken into account [2]. For example, Al, which constitutes approximately 7% of the Earth's mass, is easily released in water with the change of pH, thereby inhibiting plant growth, including root growth and its function [3]. Al generally existing as Al(OH)<sup>3</sup> , which is insoluble in soils, dissolves in water as Al3+ under acidic conditions (pH < 4.5) and is released as Al(OH)<sup>4</sup> − under alkaline conditions. The Al3+ easily reacts with phosphoric acid and then it causes phosphorus deficiency on plants with the formation of insoluble aluminum phosphate in soils [4]. Other harmful elements, such as Fe and Mn, also inhibit plant growth. Therefore, acid soils affect plant growth through indirect factors like dissolution of harmful elements, indicating that the understanding of the effect of acid soils on plant growth in terms of not only soil pH but also harmful elements is important for successful revegetation.

The research on the effects of acid soils on plant growth has been performed in the world; however, it is still a serious problem, especially in mine site where revegetation is necessary for environmental reclamation as shown in **Figure 1**. The formation of acid soils resulted from construction and/or resource exploitation has been often highlighted as the cause of plant death. The information on the effects of acid soils on plant growth in mine site is crucial for successful revegetation. Therefore, this study aims to report the current situation of acid soils and plant growth in mine site and to elucidate the effects of acid soils on plant growth over time through field investigation and a vegetation test. On the basis of the results, the key to successful revegetation was discussed from the point of view of soil acidification and tolerance mechanism of plants.

Effects of Acid Soils on Plant Growth and Successful Revegetation in the Case of Mine Site

http://dx.doi.org/10.5772/intechopen.70928

11

Plant species were investigated at two points (namely, Point A and Point B) in the waste dump in a coal mine in Indonesia in conjunction with literature research on plant species focusing on fast-growing characteristics and acid tolerance in order to understand the effects of acid soils on plant growth. Point A is about 400 m away from Point B in the same waste dump. This waste dump was constructed more than a year ago by piling up waste rocks, followed by revegetation which is mandatory for environmental conservation in mines. The revegetation in the research area had been conducted in the two stages (primary and secondary revegetation) in terms of growth rate of plants based on the experiences of the revegetation. Fast-growing trees were planted in the first stage of the revegetation for 3 years to improve soil conditions by increasing organic matter, followed by planting local plants in the second stage. In this mine, the death of plants has been reported along with the formation of acidic water at Point A as shown in **Figure 1**; on the other hand, it has not been

In addition, three samples of *Eleusine indica (E. indica)* and *Melaleuca leucadendra (M. leucadendra)* (*Melaleuca cajuputi*) which were observed at the both points were taken, and they were separated into leaves, stems, and roots. The samples were washed with deionized water using a sonication (UT-106H, SHARP) at room temperature to remove soil particles. Finally, they were dried at 60°C for 72 hours and pulverized using mortar and pestle by each part of the plants. 0.25 g of each part of the samples were digested by 5 mL of a mixture of 61% nitric acid (HNO<sup>3</sup>

and 35% hydrochloric acid (HCl) at a ratio of 3:1 at 110°C in a DigiPREP Jr. (SCP Science, Quebec, Canada) until they were completely digested with reference to the method by Quadir et al. [11]. In the case that they were not dissolved in the mixture, 1 mL of the mixture was added and the dissolution process was repeated. After the dissolution process, the volume of the solution was adjusted to 20 mL by adding deionized water. The solutions were subjected to Inductively Coupled Plasma-Atomic Emission Spectrometry (ICP-AES, VISTA-MPX ICP-OES [Seiko Inst., Japan]) after the filtration with 0.45 μm of membrane filter in order to quantify the content of Al, B, Fe, Mn, S, and Zn, which are thought to affect plant growth and be linked with the formation of acidic water, in each part of plant body. The results were calculated with mg

)

**2. Methods**

**2.1. Vegetation survey**

reported at Point B.

With respect to plant species, there are some plants that can survive in acid soils owing to tolerance characteristics, such as acid tolerance and Al tolerance. Acid-tolerant plants can survive under low soil pH conditions by setting up several tolerance mechanisms, such as the increase of soil pH around the root apices [5, 6]. The plants with Al tolerance are resistant to the effects of Al as described above. They are separated into Al-tolerant plant and Al-stimulated plant and additionally categorized as Al-excluders, Al root-accumulators, and Al-accumulators [7]. While *Camellia sinensis* localizes Al in the cell walls of epidermal cells of the leaves against Al toxicity [8], *Acacia mangium (A. mangium)* excludes Al from the root apices [9]. Furthermore, Saifuddin et al. found that the photosynthetic rates rose by increasing soil pH in *Leucaena leucocephala* after the pre-aluminum treatment [9, 10]. Thus, Al-tolerant mechanism of plants depends on the plant species, and the effects of acid soils on plant growth change according to the species. This indicates that tolerance characteristics of plants should be considered in regard to the effect of acid soils on plant growth in addition to soil pH.

**Figure 1.** Death of plants under acidic condition in the waste dump in mine site.

The research on the effects of acid soils on plant growth has been performed in the world; however, it is still a serious problem, especially in mine site where revegetation is necessary for environmental reclamation as shown in **Figure 1**. The formation of acid soils resulted from construction and/or resource exploitation has been often highlighted as the cause of plant death. The information on the effects of acid soils on plant growth in mine site is crucial for successful revegetation. Therefore, this study aims to report the current situation of acid soils and plant growth in mine site and to elucidate the effects of acid soils on plant growth over time through field investigation and a vegetation test. On the basis of the results, the key to successful revegetation was discussed from the point of view of soil acidification and tolerance mechanism of plants.

#### **2. Methods**

,

but also dissolution of harmful elements, such as Al, Fe, and Mn, dissolving under acidic conditions. Although soil pH is one of the useful indicators to evaluate acid soil conditions for successful revegetation and/or farming, the dissolution of harmful elements under acidic conditions should be taken into account [2]. For example, Al, which constitutes approximately 7% of the Earth's mass, is easily released in water with the change of pH, thereby inhibiting plant growth, including root growth and its function [3]. Al generally existing as Al(OH)<sup>3</sup>

which is insoluble in soils, dissolves in water as Al3+ under acidic conditions (pH < 4.5) and

and then it causes phosphorus deficiency on plants with the formation of insoluble aluminum phosphate in soils [4]. Other harmful elements, such as Fe and Mn, also inhibit plant growth. Therefore, acid soils affect plant growth through indirect factors like dissolution of harmful elements, indicating that the understanding of the effect of acid soils on plant growth in terms of not only soil pH but also harmful elements is important for successful revegetation.

With respect to plant species, there are some plants that can survive in acid soils owing to tolerance characteristics, such as acid tolerance and Al tolerance. Acid-tolerant plants can survive under low soil pH conditions by setting up several tolerance mechanisms, such as the increase of soil pH around the root apices [5, 6]. The plants with Al tolerance are resistant to the effects of Al as described above. They are separated into Al-tolerant plant and Al-stimulated plant and additionally categorized as Al-excluders, Al root-accumulators, and Al-accumulators [7]. While *Camellia sinensis* localizes Al in the cell walls of epidermal cells of the leaves against Al toxicity [8], *Acacia mangium (A. mangium)* excludes Al from the root apices [9]. Furthermore, Saifuddin et al. found that the photosynthetic rates rose by increasing soil pH in *Leucaena leucocephala* after the pre-aluminum treatment [9, 10]. Thus, Al-tolerant mechanism of plants depends on the plant species, and the effects of acid soils on plant growth change according to the species. This indicates that tolerance characteristics of plants should be considered in

regard to the effect of acid soils on plant growth in addition to soil pH.

**Figure 1.** Death of plants under acidic condition in the waste dump in mine site.

under alkaline conditions. The Al3+ easily reacts with phosphoric acid

is released as Al(OH)<sup>4</sup>

−

10 Soil pH for Nutrient Availability and Crop Performance

#### **2.1. Vegetation survey**

Plant species were investigated at two points (namely, Point A and Point B) in the waste dump in a coal mine in Indonesia in conjunction with literature research on plant species focusing on fast-growing characteristics and acid tolerance in order to understand the effects of acid soils on plant growth. Point A is about 400 m away from Point B in the same waste dump. This waste dump was constructed more than a year ago by piling up waste rocks, followed by revegetation which is mandatory for environmental conservation in mines. The revegetation in the research area had been conducted in the two stages (primary and secondary revegetation) in terms of growth rate of plants based on the experiences of the revegetation. Fast-growing trees were planted in the first stage of the revegetation for 3 years to improve soil conditions by increasing organic matter, followed by planting local plants in the second stage. In this mine, the death of plants has been reported along with the formation of acidic water at Point A as shown in **Figure 1**; on the other hand, it has not been reported at Point B.

In addition, three samples of *Eleusine indica (E. indica)* and *Melaleuca leucadendra (M. leucadendra)* (*Melaleuca cajuputi*) which were observed at the both points were taken, and they were separated into leaves, stems, and roots. The samples were washed with deionized water using a sonication (UT-106H, SHARP) at room temperature to remove soil particles. Finally, they were dried at 60°C for 72 hours and pulverized using mortar and pestle by each part of the plants. 0.25 g of each part of the samples were digested by 5 mL of a mixture of 61% nitric acid (HNO<sup>3</sup> ) and 35% hydrochloric acid (HCl) at a ratio of 3:1 at 110°C in a DigiPREP Jr. (SCP Science, Quebec, Canada) until they were completely digested with reference to the method by Quadir et al. [11]. In the case that they were not dissolved in the mixture, 1 mL of the mixture was added and the dissolution process was repeated. After the dissolution process, the volume of the solution was adjusted to 20 mL by adding deionized water. The solutions were subjected to Inductively Coupled Plasma-Atomic Emission Spectrometry (ICP-AES, VISTA-MPX ICP-OES [Seiko Inst., Japan]) after the filtration with 0.45 μm of membrane filter in order to quantify the content of Al, B, Fe, Mn, S, and Zn, which are thought to affect plant growth and be linked with the formation of acidic water, in each part of plant body. The results were calculated with mg per dry unit weight (mg/g) and compared with the waste water quality, which was recorded at Point A and Point B, so as to understand the linkage between the formation of acidic water and the accumulation of the elements in the plants.

site in Indonesia. The 9 flower pots with different content of pyrite were used for the vegetation test. In this test, five plants were planted in pots by the content of pyrite, and the height and the diameter were measured every week. About 500 mL of water was supplied to the pots every 3–4 days. The liquid fertilizer HYPONeX-R (N-P-K = 6-10-5) diluted to 1000 mg/L with water was, additionally, added to them once per week. The test was continued for 133 days

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To understand the effects of chemical and physical conditions of the prepared acid soils on plant growth, paste pH, NAG test, ABA test, Atterberg Limits test, and particle size distribution test were conducted according to the standard of AMIRA [13], ASTM D4318-05 [15], and ASTM D422-63 [16]. Moreover, the leachate from the bottom of the pots was taken every week to monitor the change of pH. At the end of the vegetation test, each part of the plants was digested by the acids in the same way as in Section 2.1 [11]. On the basis of the concentration of Al, B, Fe, Mn, S, and Zn in each part of the plants, the effect of the elements on plant growth

**Tables 1** and **2** summarize the main species of plants, which were planted in each stage of the revegetation in the mine and were characterized according to the literatures. The plants, which are acid tolerant, were planted in the first stage of the revegetation, and most of them were classified into fast-growing species. The plants generally planted for soil improvement, such as *Calliandra calothyrsus, Gliricidia sepium, Pterocarpus indicus,* and *Paraserianthes falcataria*, were planted in the first stage, aiming at improving soil conditions by increasing organic matter in the waste dump before planting local plants that are not acid tolerant in the second stage. Such fast-growing species shorten the time for revegetation and enable us to perform an earlier improvement of soil conditions in the waste dump. On the other hand, local plants which were planted in the second stage are utilized for industrial use as timber and ecosystem protection, such as *Intsia bijuga* and *Inocarpus fagifer*. The results indicated that plants were transplanted in the waste dump in the two stages for different purposes for successful

The plants, which were observed in the waste dump at Point A and Point B, are summarized in **Table 3**. *Swietenia macrophylla*, *Mimosa pudica*, and cover crop (*Convolvulaceae*) plants, which are not acid tolerant, were not observed at Point A. Although *Intsia bijuga*, which is not acid tolerant, was observed at Point A, some of them withered. By contrast, *Paraserianthes falcataria*, and *M. leucadendra*, which are acid tolerant, were found at Point A and Point B, indicating that the plant growth in the waste dump may have been affected by acidic conditions. Additionally, the revegetation failed at Point A since *Swietenia macrophylla* which was planted in the second stage of the revegetation and the cover crop (*Convolvulaceae*) plants which are

until a clear distinction is observed.

over time was elucidated.

revegetation in this mine.

**3. Results and discussion**

**3.1. Effects of acid soils on plant growth in mine site**

important for improving soil conditions withered.

#### **2.2. Soil analysis**

Soils within the waste dump were sampled until 100 cm depth with a 8-cm diameter hand auger (DIK-100A-55) at Point A and Point B in order to understand the current soil conditions. Soil pH was measured at each depth using Soil Acid meter (SK-910A-D, Sato, and DM-13, Takemura Electric Works Ltd.). The samples were separated by 20-30 cm and supplied to X-ray diffraction (XRD) and X-ray fluorescence (XRF) analysis, paste pH test, acid base accounting (ABA) test, and net acid generation (NAG) test so as to investigate the cause of acidic water in this area [12, 13] . The XRD analysis was conducted using wide angle goniometer RINT 2100 XRD after the drying process at 50°C for 48 hours in a nitrogen atmosphere under the following conditions: radiation CuKα, operating voltage 40 kV, current 26 mA, divergence slid 1°, anti-scatter 1°, receiving slit 0.3 mm, step scanning 0.050°, scan speed 2.000°/min, and scan range 2.000–65.000°. In paste pH test, the change of pH was reported as paste pH after 12 hours of the dissolution process at the 1:2 of mixing ratio of sample and deionized water (pH1:2w). The ABA and NAG test were performed to evaluate the acid producing potential of the soils. Net acid producing potential (NAPP) was calculated with acid potential (AP) and acid neutralizing capacity (ANC) of the samples as an acid producing potential in addition to NAG pH [12, 13]. Soils with NAG pH < 4.5 and NAPP > 0 were considered the source of acidic water, which can produce acids [14]. Acid producing potential generally rises with the increase of NAPP and the decrease of NAG pH.

#### **2.3. Vegetation test**

The vegetation test was conducted with a focus on the effect of acidic conditions on plant growth over time based on the results of the vegetation survey. Simulated acid soil was prepared by mixing sand, clay, and pyrite. Pyrite was mixed in the simulated soils, which were prepared using sand and clay in conformity to the soil texture and the physical properties of the topsoil in the mine site, aiming at setting the sulfur content as from 0 to 30% by weight on the basis of the result of XRF analysis. Each sample was labeled as S0.0, S0.5, S1.5, S3.0, S5.0, S7.5, S10.0, S15.0, and S20.0. The prepared acid soils were evenly mixed in each flower pot at a constant rate to uniform physical conditions in each pot, and used for the vegetation test. In this study, *A. mangium* which inhabits tropical forests in Southeast Asian countries, including Indonesia, and has acid tolerance was used: the seeds were obtained in Japan. *A. mangium* occurs naturally in the humid tropical lowland and has been successfully applied in reclamation in post-mine lands for bauxite, copper, coal, and iron in the world. It grows in compacted soils, dry area, on the slopes of hills, and humid area owing to high adaptability. *A. mangium* is designated an Al-excluder plant as well as *M. leucadendra* (*M. cajuputi*) [7].

In order to elucidate the effects of acidic conditions, including low pH, Al, B, Fe, Mn, S, and Zn on plant growth, *A. mangium* was planted on the prepared acid soils in the phytotron glass room G-9 in Biotron Application Center, Kyushu University under the following conditions: at 30°C room temperature and 70% relative humidity assuming the local climate in the mine site in Indonesia. The 9 flower pots with different content of pyrite were used for the vegetation test. In this test, five plants were planted in pots by the content of pyrite, and the height and the diameter were measured every week. About 500 mL of water was supplied to the pots every 3–4 days. The liquid fertilizer HYPONeX-R (N-P-K = 6-10-5) diluted to 1000 mg/L with water was, additionally, added to them once per week. The test was continued for 133 days until a clear distinction is observed.

To understand the effects of chemical and physical conditions of the prepared acid soils on plant growth, paste pH, NAG test, ABA test, Atterberg Limits test, and particle size distribution test were conducted according to the standard of AMIRA [13], ASTM D4318-05 [15], and ASTM D422-63 [16]. Moreover, the leachate from the bottom of the pots was taken every week to monitor the change of pH. At the end of the vegetation test, each part of the plants was digested by the acids in the same way as in Section 2.1 [11]. On the basis of the concentration of Al, B, Fe, Mn, S, and Zn in each part of the plants, the effect of the elements on plant growth over time was elucidated.

#### **3. Results and discussion**

.

per dry unit weight (mg/g) and compared with the waste water quality, which was recorded at Point A and Point B, so as to understand the linkage between the formation of acidic water and

Soils within the waste dump were sampled until 100 cm depth with a 8-cm diameter hand auger (DIK-100A-55) at Point A and Point B in order to understand the current soil conditions. Soil pH was measured at each depth using Soil Acid meter (SK-910A-D, Sato, and DM-13, Takemura Electric Works Ltd.). The samples were separated by 20-30 cm and supplied to X-ray diffraction (XRD) and X-ray fluorescence (XRF) analysis, paste pH test, acid base accounting (ABA) test, and net acid generation (NAG) test so as to investigate the cause of acidic water in this area [12, 13]

The XRD analysis was conducted using wide angle goniometer RINT 2100 XRD after the drying process at 50°C for 48 hours in a nitrogen atmosphere under the following conditions: radiation CuKα, operating voltage 40 kV, current 26 mA, divergence slid 1°, anti-scatter 1°, receiving slit 0.3 mm, step scanning 0.050°, scan speed 2.000°/min, and scan range 2.000–65.000°. In paste pH test, the change of pH was reported as paste pH after 12 hours of the dissolution process at the 1:2 of mixing ratio of sample and deionized water (pH1:2w). The ABA and NAG test were performed to evaluate the acid producing potential of the soils. Net acid producing potential (NAPP) was calculated with acid potential (AP) and acid neutralizing capacity (ANC) of the samples as an acid producing potential in addition to NAG pH [12, 13]. Soils with NAG pH < 4.5 and NAPP > 0 were considered the source of acidic water, which can produce acids [14]. Acid producing poten-

The vegetation test was conducted with a focus on the effect of acidic conditions on plant growth over time based on the results of the vegetation survey. Simulated acid soil was prepared by mixing sand, clay, and pyrite. Pyrite was mixed in the simulated soils, which were prepared using sand and clay in conformity to the soil texture and the physical properties of the topsoil in the mine site, aiming at setting the sulfur content as from 0 to 30% by weight on the basis of the result of XRF analysis. Each sample was labeled as S0.0, S0.5, S1.5, S3.0, S5.0, S7.5, S10.0, S15.0, and S20.0. The prepared acid soils were evenly mixed in each flower pot at a constant rate to uniform physical conditions in each pot, and used for the vegetation test. In this study, *A. mangium* which inhabits tropical forests in Southeast Asian countries, including Indonesia, and has acid tolerance was used: the seeds were obtained in Japan. *A. mangium* occurs naturally in the humid tropical lowland and has been successfully applied in reclamation in post-mine lands for bauxite, copper, coal, and iron in the world. It grows in compacted soils, dry area, on the slopes of hills, and humid area owing to high adaptability. *A. mangium* is designated an

In order to elucidate the effects of acidic conditions, including low pH, Al, B, Fe, Mn, S, and Zn on plant growth, *A. mangium* was planted on the prepared acid soils in the phytotron glass room G-9 in Biotron Application Center, Kyushu University under the following conditions: at 30°C room temperature and 70% relative humidity assuming the local climate in the mine

tial generally rises with the increase of NAPP and the decrease of NAG pH.

Al-excluder plant as well as *M. leucadendra* (*M. cajuputi*) [7].

the accumulation of the elements in the plants.

12 Soil pH for Nutrient Availability and Crop Performance

**2.2. Soil analysis**

**2.3. Vegetation test**

#### **3.1. Effects of acid soils on plant growth in mine site**

**Tables 1** and **2** summarize the main species of plants, which were planted in each stage of the revegetation in the mine and were characterized according to the literatures. The plants, which are acid tolerant, were planted in the first stage of the revegetation, and most of them were classified into fast-growing species. The plants generally planted for soil improvement, such as *Calliandra calothyrsus, Gliricidia sepium, Pterocarpus indicus,* and *Paraserianthes falcataria*, were planted in the first stage, aiming at improving soil conditions by increasing organic matter in the waste dump before planting local plants that are not acid tolerant in the second stage. Such fast-growing species shorten the time for revegetation and enable us to perform an earlier improvement of soil conditions in the waste dump. On the other hand, local plants which were planted in the second stage are utilized for industrial use as timber and ecosystem protection, such as *Intsia bijuga* and *Inocarpus fagifer*. The results indicated that plants were transplanted in the waste dump in the two stages for different purposes for successful revegetation in this mine.

The plants, which were observed in the waste dump at Point A and Point B, are summarized in **Table 3**. *Swietenia macrophylla*, *Mimosa pudica*, and cover crop (*Convolvulaceae*) plants, which are not acid tolerant, were not observed at Point A. Although *Intsia bijuga*, which is not acid tolerant, was observed at Point A, some of them withered. By contrast, *Paraserianthes falcataria*, and *M. leucadendra*, which are acid tolerant, were found at Point A and Point B, indicating that the plant growth in the waste dump may have been affected by acidic conditions. Additionally, the revegetation failed at Point A since *Swietenia macrophylla* which was planted in the second stage of the revegetation and the cover crop (*Convolvulaceae*) plants which are important for improving soil conditions withered.


**Table 1.** Plant species in the first stage of the revegetation and the growth characteristics and the acid tolerance: + indicates that it has the characteristics; − indicates it does not.

In regard to waste water quality in the waste dump, electric conductivity (EC) and oxidation reduction potential (ORP) exhibited higher values at Point A (EC = 3.00 mS/cm, ORP = 588 mV) than at Point B (EC = 0.62 mS/cm, ORP = 568 mV). Moreover, a higher concentration of total Fe,

**Table 3.** Plant species, which were observed in the waste dump at Point A and Point B, and its acid tolerance: ○ indicates that it was observed; × indicates that it was not observed; + indicates acid tolerant; and − indicates that it is not acid

**Name of plants Point A Point B Acid tolerance** *Paraserianthes falcataria* ○ ○ + *Anthocephalus cadamba* ○ ○ + *Melaleuca leucadendra* ○ ○ + *Intsia bijuga* ○ ○ − *Eleusine indica* ○ ○ + *Swietenia macrophylla* × ○ − *Mimosa pudica* × ○ − *Cover crop (Convolvulaceae)* × ○ −

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2−, and Al related to the formation of acidic water was recorded at Point A than at Point B, suggesting the formation of acidic water with the dissolution of sulfides at Point A based on the XRD results. This implies that acid soils can be formed with the intrusion of acidic water into the ground at Point A. While acid soils in Southeast Asian countries are often attributed to sulfuric sediments formed in mangrove, acidic water caused by the exposure of sulfides to oxygen and water with the excavation in mine site may have resulted in the formation of acid soils in this case [33]. The geochemical properties of the soils at Point A and Point B are summarized in **Table 4**. Paste pH showed similar values at each depth at the points. Furthermore, there was not a significant difference in soil pH showing 5.0–6.4 at each depth at the points. This was resulted from acid soils, which are common in the Southeast Asian countries. Meanwhile, there was a large difference in sulfur content, NAPP, and NAG pH between the points. Sulfur content showed larger values especially at 30–70 cm depth at Point A than at Point B. The results of XRD analysis suggested the presence of sulfides at the points, thus showing that the difference in the content of sulfides led to the difference in sulfur content at 30–70 cm depth between Point A and Point B. Since NAPP is calculated by subtracting ANC from AP, NAPP showed negative values at Point B where ANC showed positive values because of neutralization by carbonate and/or clay minerals. Besides, considering the similar values of paste pH and soil pH between

ences in NAG pH between the points were triggered by the abundance distribution of sulfides. The change of paste pH is not greatly affected by the dissolution of sulfides as soluble minerals mostly affect paste pH in a relative short time as well as soil pH. On the other hand, NAG pH significantly depends on the dissolution of sulfides owing to the complete dissolution of

water, the presence of sulfides leads to the continuous formation of acidic water for a long time,

in NAG test. Therefore, NAG pH was lower at Point A than Point B since

O2

O2

. With respect to the formation of acidic

in NAG test, the differ-

Point A and Point B and complete dissolution of the samples with H<sup>2</sup>

SO4

tolerant.

samples with H2

O2

sulfides at Point A were sufficient to react with H<sup>2</sup>


**Table 2.** Plant species in the second stage of the revegetation and the growth characteristics and the acid tolerance: ++ indicates that it has the characteristics; + indicates moderate; and − indicates that it does not.


**Table 3.** Plant species, which were observed in the waste dump at Point A and Point B, and its acid tolerance: ○ indicates that it was observed; × indicates that it was not observed; + indicates acid tolerant; and − indicates that it is not acid tolerant.

In regard to waste water quality in the waste dump, electric conductivity (EC) and oxidation reduction potential (ORP) exhibited higher values at Point A (EC = 3.00 mS/cm, ORP = 588 mV) than at Point B (EC = 0.62 mS/cm, ORP = 568 mV). Moreover, a higher concentration of total Fe, SO4 2−, and Al related to the formation of acidic water was recorded at Point A than at Point B, suggesting the formation of acidic water with the dissolution of sulfides at Point A based on the XRD results. This implies that acid soils can be formed with the intrusion of acidic water into the ground at Point A. While acid soils in Southeast Asian countries are often attributed to sulfuric sediments formed in mangrove, acidic water caused by the exposure of sulfides to oxygen and water with the excavation in mine site may have resulted in the formation of acid soils in this case [33]. The geochemical properties of the soils at Point A and Point B are summarized in **Table 4**. Paste pH showed similar values at each depth at the points. Furthermore, there was not a significant difference in soil pH showing 5.0–6.4 at each depth at the points. This was resulted from acid soils, which are common in the Southeast Asian countries. Meanwhile, there was a large difference in sulfur content, NAPP, and NAG pH between the points. Sulfur content showed larger values especially at 30–70 cm depth at Point A than at Point B. The results of XRD analysis suggested the presence of sulfides at the points, thus showing that the difference in the content of sulfides led to the difference in sulfur content at 30–70 cm depth between Point A and Point B. Since NAPP is calculated by subtracting ANC from AP, NAPP showed negative values at Point B where ANC showed positive values because of neutralization by carbonate and/or clay minerals. Besides, considering the similar values of paste pH and soil pH between Point A and Point B and complete dissolution of the samples with H<sup>2</sup> O2 in NAG test, the differences in NAG pH between the points were triggered by the abundance distribution of sulfides. The change of paste pH is not greatly affected by the dissolution of sulfides as soluble minerals mostly affect paste pH in a relative short time as well as soil pH. On the other hand, NAG pH significantly depends on the dissolution of sulfides owing to the complete dissolution of samples with H2 O2 in NAG test. Therefore, NAG pH was lower at Point A than Point B since sulfides at Point A were sufficient to react with H<sup>2</sup> O2 . With respect to the formation of acidic water, the presence of sulfides leads to the continuous formation of acidic water for a long time,

**Name of plants Fast growing Acid tolerance References**

**Name of plants Fast growing Acid tolerance References**

14 Soil pH for Nutrient Availability and Crop Performance

*Paraserianthes falcataria* + + Evans and Szott [17]

*Pterocarpus indicus* − + Evans and Szott [17]

*Michelia champaca* − + Orwa et al. [20]

*Gliricidia sepium* + + Orwa et al. [20]

*Senna siamea* − + Orwa et al. [20] *Calliandra calothyrsus* + + Orwa et al. [20] *Melaleuca leucadendra* + + Masitah et al. [23]

*Nauclea orientalis* − − Orwa et al. [20]

*Anthocephalus cadamba* + + Irawan and Purwanto [22]

*Anthocephalus macrophyllus* + + Irawan and Purwanto [22]

Krisnawati et al. [18]

Evans and Szott [17]

Krisnawati et al. [18]

Krisnawati et al. [18]

Evans and Szott [17]

Turnbull [24] Nakabayashi et al. [25]

Thomson [19]

Fern [21]

*Ficus benjamina* ++ − Fern [21]

*Inocarpus fagifer* + + Pauku [28] *Tectona grandis* ++ − Orwa et al. [20] *Hevea brasiliensis* − − Orwa et al. [20] *Pericopsis mooniana* − − Ishiguri et al. [29] *Swietenia macrophylla* − − Krisnawati et al. [18]

*Intsia bijuga* − − Thaman et al. [30] *Schima wallichii* − − Orwa et al. [20] *Mimusops elengi* − − Kadam [31] *Fagraea fragrans* − − Steinmetz [32]

indicates that it has the characteristics; + indicates moderate; and − indicates that it does not.

*Samanea saman* ++ − National Research Council [26]

**Table 2.** Plant species in the second stage of the revegetation and the growth characteristics and the acid tolerance: ++

*Aquilaria* spp*.* − − Soehartono and Newton [27]

*Enterolobium cyclocarpum* + + National Research Council [26]

**Table 1.** Plant species in the first stage of the revegetation and the growth characteristics and the acid tolerance: +

*Pterospermum javanicum* − −

indicates that it has the characteristics; − indicates it does not.

*Shorea balangeran* − −


**Table 4.** Geochemical properties of the samples in the waste dump at Point A and Point B.

which is considered as a lag time due to the difference in the solubility of sulfur in various minerals, such as sulfates and sulfides [34]. The continuous dissolution of sulfides for a long time resulted in a high concentration of total Fe, Al, and SO4 2− in the waste water at Point A. Additionally, NAG pH < 4.5 and NAPP > 0 at Point A indicated the source of acidic water. This suggested that the evaluation of soil pH combined with NAPP and NAG pH, which are used to predict the formation of acidic water, enables us to understand the formation of acidic water and acid soils over time. Hence, in this case, the formation of acidic conditions in soils triggered by acidic water for a long time with the continuous dissolution of sulfides caused plant death at Point A. It is necessary to consider a lag time of the dissolution of sulfur in addition to soil pH for successful revegetation.

**Figure 2.** Concentration of Al, As, B, Fe, Mn, S, and Zn in each part of the plant body of *E. indica* at Point A and

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**Figure 3.** Concentration of Al, As, B, Fe, Mn, S, and Zn in each part of the plant body of *M. leucadendra* at Point A and

Point B.

Point B.

**Figures 2** and **3** show the concentration of Al, As, B, Fe, Mn, S, and Zn in each part of *E. indica* and *M. leucadendra*, which were sampled in the waste dump at Point A and Point B. In addition, the standard deviation of the results was summarized by the species as shown in **Figure 4**. In particular, Fe and Al were accumulated in the roots of the plants. S was accumulated in the roots and leaves of the plants, and the concentrations of the elements were higher at Point A than that at Point B. This was attributed to the biological action to accumulate the excess of the harmful elements for plant growth on the roots. As the accumulation of Al in the roots causes the death of plants by preventing the absorption of nutrients from the roots, the high concentration of Al caused the inhibition of the growth of *Intsia bijuga*, *Swietenia macrophylla*, *Mimosa pudica*, and cover crop (*Convolvulaceae*) at Point A [35]. Moreover, a high concentration of Fe and S, which were derived from the dissolution of sulfides such as FeS<sup>2</sup> , suggested that the dissolution of Al under acidic conditions was caused by the formation of acidic water with the dissolution of sulfides. The higher concentration of Fe, S, and Al was, additionally, obtained in *M. leucadendra*, which is acid tolerant, than that in *E. indica*, indicating that the accumulation capacity of the elements in the plant body depends on the species. Compared to the standard deviation of B, Mn, and Zn with that of Al, Fe, and S, the standard deviation of B, Mn, and Zn was near zero as shown in **Figure 4**, suggesting that B, Mn, and Zn were ubiquitous in the plants. B is an essential element for the maintenance of cell wall and carbohydrate metabolism [36], and the atomic number of B is similar with that of C, which is utilized for organic substances through the formation of carbon Effects of Acid Soils on Plant Growth and Successful Revegetation in the Case of Mine Site http://dx.doi.org/10.5772/intechopen.70928 17

**Figure 2.** Concentration of Al, As, B, Fe, Mn, S, and Zn in each part of the plant body of *E. indica* at Point A and Point B.

which is considered as a lag time due to the difference in the solubility of sulfur in various minerals, such as sulfates and sulfides [34]. The continuous dissolution of sulfides for a long

Additionally, NAG pH < 4.5 and NAPP > 0 at Point A indicated the source of acidic water. This suggested that the evaluation of soil pH combined with NAPP and NAG pH, which are used to predict the formation of acidic water, enables us to understand the formation of acidic water and acid soils over time. Hence, in this case, the formation of acidic conditions in soils triggered by acidic water for a long time with the continuous dissolution of sulfides caused plant death at Point A. It is necessary to consider a lag time of the dissolution of sulfur in addition to soil

**Figures 2** and **3** show the concentration of Al, As, B, Fe, Mn, S, and Zn in each part of *E. indica* and *M. leucadendra*, which were sampled in the waste dump at Point A and Point B. In addition, the standard deviation of the results was summarized by the species as shown in **Figure 4**. In particular, Fe and Al were accumulated in the roots of the plants. S was accumulated in the roots and leaves of the plants, and the concentrations of the elements were higher at Point A than that at Point B. This was attributed to the biological action to accumulate the excess of the harmful elements for plant growth on the roots. As the accumulation of Al in the roots causes the death of plants by preventing the absorption of nutrients from the roots, the high concentration of Al caused the inhibition of the growth of *Intsia bijuga*, *Swietenia macrophylla*, *Mimosa pudica*, and cover crop (*Convolvulaceae*) at Point A [35]. Moreover, a high concentration of Fe and S, which were

acidic conditions was caused by the formation of acidic water with the dissolution of sulfides. The higher concentration of Fe, S, and Al was, additionally, obtained in *M. leucadendra*, which is acid tolerant, than that in *E. indica*, indicating that the accumulation capacity of the elements in the plant body depends on the species. Compared to the standard deviation of B, Mn, and Zn with that of Al, Fe, and S, the standard deviation of B, Mn, and Zn was near zero as shown in **Figure 4**, suggesting that B, Mn, and Zn were ubiquitous in the plants. B is an essential element for the maintenance of cell wall and carbohydrate metabolism [36], and the atomic number of B is similar with that of C, which is utilized for organic substances through the formation of carbon

2− in the waste water at Point A.

**SO4**

**/ton) NAG pH**

, suggested that the dissolution of Al under

time resulted in a high concentration of total Fe, Al, and SO4

**Table 4.** Geochemical properties of the samples in the waste dump at Point A and Point B.

**Point Depth (cm) Paste pH S (%) AP ANC NAPP (kg H<sup>2</sup>**

16 Soil pH for Nutrient Availability and Crop Performance

Point A 0–20 4.47 0.13 4.1 0.0 4.1 4.24

Point B 0–20 4.74 0.09 2.9 43.9 −41.0 4.39

30–50 4.72 0.75 23.1 0.0 23.1 3.76 50–70 5.05 1.06 32.5 3.2 29.3 2.74 70–100 4.41 0.52 16.0 0.0 16.0 3.22

30–50 4.76 0.08 2.5 43.6 −41.1 4.57 50–70 4.57 0.11 3.4 43.4 −39.9 4.30 70–100 4.63 0.11 3.3 43.4 −40.1 4.06

derived from the dissolution of sulfides such as FeS<sup>2</sup>

pH for successful revegetation.

**Figure 3.** Concentration of Al, As, B, Fe, Mn, S, and Zn in each part of the plant body of *M. leucadendra* at Point A and Point B.

**Figure 4.** Standard deviation of the concentration of Al, As, B, Fe, Mn, S, and Zn in each part of the plant body: the distribution of the elements is homogeneous when standard deviations are nearly zero.

the plants under the condition of S1.5–S20.0 withered after 56 days (8 weeks). By contrast, the plants under the condition of S0.0–S0.5 had grown after 56 days. For these results, the excess of sulfides in soils inhibited the growth of *A. mangium*, and the sulfur content for the limitation of the growth of *A. mangium* lied between 1.13 and 1.94% in this case. Furthermore, the plant growth declined with the increase in the mixing ratio of pyrite as indicated by the decrease of the height and the diameter as shown in **Figure 7**. The sulfur content at Point A showed 0.13–1.06%, especially 1.06% at 50–70 cm depth, as shown in **Table 4**, suggesting that the growth of plants which are subject to effects of acid soils compared to *A. mangium* can be inhib-

**Figure 5.** (a) Relationship between IP and WL of the prepared acid soils (C: clay; M: silt; H: high liquid limit; and L: low

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In **Figure 8**, the changes of pH of the leachate from the bottom of the pots during the vegetation test are plotted. The pH of the leachate ranged from pH 2.0 to pH 4.0 in S1.5–S20.0 from the beginning of the experiment, resulting in the death of *A. mangium* contrary to the growth with constant pH 7.0 in S0.0. On the other hand, *A. mangium* in S0.5 had grown even if pH dropped

**Table 5.** Geochemical properties of the prepared acid soils: the mixing ratio of pyrite in the prepared soils is labeled as

**SO4**

**/ton) NAG pH**

ited under the soil conditions in the mine site.

**Sample Sulfur content (%) NAPP (kg H<sup>2</sup>**

sample names, e.g. the mixing ratio of pyrite is 0.5% in S0.5.

S0.0 0.04 −8.9 5.67 S0.5 1.13 24.1 2.34 S1.5 1.94 48.8 2.24 S3.0 3.83 106.1 2.14 S5.0 6.86 198.2 2.05 S7.5 9.08 265.3 1.92 S10.0 11.60 340.1 1.98 S15.0 18.20 541.7 1.98 S20.0 28.20 846.0 1.91

liquid limit). (b) Particle size distribution of the prepared acid soils.

hydride in plant body. Thereby, B was widely distributed in the body of both plants in the similar way with C as a consequence of the transport mechanism of Mn to the leaves for photosynthesis [37, 38]. Zn is also one of the essential elements for plant growth as coenzyme to accelerate photosynthesis and DNA synthesis [39, 40]. Consequently, the dissolution of Al in acid soils triggered by the continuous formation of acidic water along with the dissolution of sulfides influenced the plant growth at Point A. For the presence of Al, the neutralization of soil conditions with limestones and/or chemicals is not always useful to improve soil conditions in acid soils because Al is released as Al(OH)<sup>4</sup> − under alkali conditions. The evaluation of soil conditions before revegetation and/or farming is more important than the treatment of such acidic conditions. Likewise, even if the plants which are planted in the first stage of revegetation are acid tolerant, it is still necessary to select a proper plant for successful revegetation from the point of view of Al tolerance of plants and the dissolution of Al with the formation of acidic water over time in this case.

#### **3.2. Tolerance characteristics of plants to acid soils**

**Figure 5(a)** and **(b)** shows the relationship between plasticity index (IP) and liquid limit (WL), which shows soil conditions under different water content and particle size distribution of the prepared acid soils, respectively. Soil classification is closely associated with physical characteristics of soils such as particle size distribution, which affects plant growth [41]. All of the prepared acid soils were categorized as high WL silt, and there were not significant differences in the particle size distribution among the samples as shown in **Figure 5**. This indicated that the growth of plants was not affected by the physical characteristics of the soil samples during the vegetation test.

The geochemical properties of the soil samples are summarized in **Table 5**, and **Figure 6** describes the content of Al, Fe, and S in the prepared acid soils. Besides, **Figure 7** shows the changes of the height of seedlings and the diameter of stem of *A. mangium* during the vegetation test. In **Table 5**, NAG pH dropped between S0.0 and S0.5, showing that NAG pH was significantly affected by the content of sulfides such as pyrite. The content of Fe and S rose with the increase in the mixing ratio of pyrite in **Figure 6**, whereas that of Al decreased, attributing to the decrease in the mixing ratio of simulated soils containing Al. In **Figure 7**,

Effects of Acid Soils on Plant Growth and Successful Revegetation in the Case of Mine Site http://dx.doi.org/10.5772/intechopen.70928 19

**Figure 5.** (a) Relationship between IP and WL of the prepared acid soils (C: clay; M: silt; H: high liquid limit; and L: low liquid limit). (b) Particle size distribution of the prepared acid soils.

**Figure 4.** Standard deviation of the concentration of Al, As, B, Fe, Mn, S, and Zn in each part of the plant body: the

hydride in plant body. Thereby, B was widely distributed in the body of both plants in the similar way with C as a consequence of the transport mechanism of Mn to the leaves for photosynthesis [37, 38]. Zn is also one of the essential elements for plant growth as coenzyme to accelerate photosynthesis and DNA synthesis [39, 40]. Consequently, the dissolution of Al in acid soils triggered by the continuous formation of acidic water along with the dissolution of sulfides influenced the plant growth at Point A. For the presence of Al, the neutralization of soil conditions with limestones and/or chemicals is not always useful to improve soil conditions in acid soils because Al

tion and/or farming is more important than the treatment of such acidic conditions. Likewise, even if the plants which are planted in the first stage of revegetation are acid tolerant, it is still necessary to select a proper plant for successful revegetation from the point of view of Al tolerance of plants and the dissolution of Al with the formation of acidic water over time in this case.

**Figure 5(a)** and **(b)** shows the relationship between plasticity index (IP) and liquid limit (WL), which shows soil conditions under different water content and particle size distribution of the prepared acid soils, respectively. Soil classification is closely associated with physical characteristics of soils such as particle size distribution, which affects plant growth [41]. All of the prepared acid soils were categorized as high WL silt, and there were not significant differences in the particle size distribution among the samples as shown in **Figure 5**. This indicated that the growth of plants was not affected by the physical characteristics of the soil samples during

The geochemical properties of the soil samples are summarized in **Table 5**, and **Figure 6** describes the content of Al, Fe, and S in the prepared acid soils. Besides, **Figure 7** shows the changes of the height of seedlings and the diameter of stem of *A. mangium* during the vegetation test. In **Table 5**, NAG pH dropped between S0.0 and S0.5, showing that NAG pH was significantly affected by the content of sulfides such as pyrite. The content of Fe and S rose with the increase in the mixing ratio of pyrite in **Figure 6**, whereas that of Al decreased, attributing to the decrease in the mixing ratio of simulated soils containing Al. In **Figure 7**,

under alkali conditions. The evaluation of soil conditions before revegeta-

distribution of the elements is homogeneous when standard deviations are nearly zero.

is released as Al(OH)<sup>4</sup>

the vegetation test.

−

18 Soil pH for Nutrient Availability and Crop Performance

**3.2. Tolerance characteristics of plants to acid soils**

the plants under the condition of S1.5–S20.0 withered after 56 days (8 weeks). By contrast, the plants under the condition of S0.0–S0.5 had grown after 56 days. For these results, the excess of sulfides in soils inhibited the growth of *A. mangium*, and the sulfur content for the limitation of the growth of *A. mangium* lied between 1.13 and 1.94% in this case. Furthermore, the plant growth declined with the increase in the mixing ratio of pyrite as indicated by the decrease of the height and the diameter as shown in **Figure 7**. The sulfur content at Point A showed 0.13–1.06%, especially 1.06% at 50–70 cm depth, as shown in **Table 4**, suggesting that the growth of plants which are subject to effects of acid soils compared to *A. mangium* can be inhibited under the soil conditions in the mine site.

In **Figure 8**, the changes of pH of the leachate from the bottom of the pots during the vegetation test are plotted. The pH of the leachate ranged from pH 2.0 to pH 4.0 in S1.5–S20.0 from the beginning of the experiment, resulting in the death of *A. mangium* contrary to the growth with constant pH 7.0 in S0.0. On the other hand, *A. mangium* in S0.5 had grown even if pH dropped


**Table 5.** Geochemical properties of the prepared acid soils: the mixing ratio of pyrite in the prepared soils is labeled as sample names, e.g. the mixing ratio of pyrite is 0.5% in S0.5.

**Figure 6.** Content of Al, Fe, and S in the prepared acid soils with different contents of pyrite.

from pH 8.0 to ca. pH 2.0 after 70 days. It would appear that *A. mangium* can survive under acidic conditions by increasing the resistance against acidic conditions with the plant growth during 70 days [7]. The 8 cm height of seedlings were transplanted at the beginning of the experiment, and they died when they were exposed to pH 2.0 in S1.5–S20.0. However, *A. mangium* in S0.5 was exposed to pH 2.0 after 70 days when the height became 15 cm, leading to the existence of the seedlings due to the development of acid tolerance with the plant growth. Additionally, the sudden drop of the pH of the leachate may have resulted from the lag time of the dissolution of sulfides. Sulfides gradually dissolve in water over time, causing the formation of acidic water for a long time [34]. Thus, both formation of acidic water over time and the development of acid tolerance with the plant growth have to be taken into account for successful revegetation in the waste dump in mine site.

and the leaves. Moreover, the accumulation of Al in the roots significantly rose with the increase in the mixing ratio of pyrite as shown in **Figure 9**. The high concentration of Al in the roots resulted in the death of *A. mangium* by preventing the absorption of nutrients from the roots [10]. In **Figure 10**, the standard deviation of Al, Fe, and S showed more than 0.5, whereas that of the other elements ranged from 0.01 to 0.1. The standard deviation of Al, Fe, and S, besides, rose with the increase in the mixing ratio of pyrite, revealing the accumulation of Al, Fe, and S in the plant body: the standard deviation of Al, Fe, and S was 2.98, 13.17, and 1.43 in S1.5, respectively. This was caused as a consequence of the biological action to accumulate the excess of the harmful elements for plant growth on the roots in the same case as in Section 3.1. The results in Section 3.1 also support that B, Mn, and Zn were distributed through the body of *A. mangium*. Furthermore, a larger amount of Fe, S, and Al was obtained in *M. leucadendra* compared to

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**Figure 8.** Change of pH of the leachate from the pots with different contents of pyrite in soils in the vegetation test.

**Figure 9.** Concentration of Al, As, B, Fe, Mn, S, and Zn in each part of *A. mangium*.

In **Figure 9**, the concentration of Al, As, B, Fe, Mn, S, and Zn in each part of *A. mangium* is summarized, and in **Figure 10**, the standard deviation of the results is described by the soil samples. Compared to the results in **Figures 2** and **3** and the concentration of the elements in *A. mangium* in **Figure 9**, Fe and Al were equally accumulated in the roots and S was accumulated in the roots

**Figure 7.** Change of the height and the stem diameter of *A. mangium* with different contents of pyrite in soils: the values were calculated based on the average of five samples by each content.

Effects of Acid Soils on Plant Growth and Successful Revegetation in the Case of Mine Site http://dx.doi.org/10.5772/intechopen.70928 21

**Figure 8.** Change of pH of the leachate from the pots with different contents of pyrite in soils in the vegetation test.

from pH 8.0 to ca. pH 2.0 after 70 days. It would appear that *A. mangium* can survive under acidic conditions by increasing the resistance against acidic conditions with the plant growth during 70 days [7]. The 8 cm height of seedlings were transplanted at the beginning of the experiment, and they died when they were exposed to pH 2.0 in S1.5–S20.0. However, *A. mangium* in S0.5 was exposed to pH 2.0 after 70 days when the height became 15 cm, leading to the existence of the seedlings due to the development of acid tolerance with the plant growth. Additionally, the sudden drop of the pH of the leachate may have resulted from the lag time of the dissolution of sulfides. Sulfides gradually dissolve in water over time, causing the formation of acidic water for a long time [34]. Thus, both formation of acidic water over time and the development of acid tolerance with the plant growth have to be taken into account for successful revegetation in the

**Figure 6.** Content of Al, Fe, and S in the prepared acid soils with different contents of pyrite.

In **Figure 9**, the concentration of Al, As, B, Fe, Mn, S, and Zn in each part of *A. mangium* is summarized, and in **Figure 10**, the standard deviation of the results is described by the soil samples. Compared to the results in **Figures 2** and **3** and the concentration of the elements in *A. mangium* in **Figure 9**, Fe and Al were equally accumulated in the roots and S was accumulated in the roots

**Figure 7.** Change of the height and the stem diameter of *A. mangium* with different contents of pyrite in soils: the values

were calculated based on the average of five samples by each content.

waste dump in mine site.

20 Soil pH for Nutrient Availability and Crop Performance

and the leaves. Moreover, the accumulation of Al in the roots significantly rose with the increase in the mixing ratio of pyrite as shown in **Figure 9**. The high concentration of Al in the roots resulted in the death of *A. mangium* by preventing the absorption of nutrients from the roots [10]. In **Figure 10**, the standard deviation of Al, Fe, and S showed more than 0.5, whereas that of the other elements ranged from 0.01 to 0.1. The standard deviation of Al, Fe, and S, besides, rose with the increase in the mixing ratio of pyrite, revealing the accumulation of Al, Fe, and S in the plant body: the standard deviation of Al, Fe, and S was 2.98, 13.17, and 1.43 in S1.5, respectively. This was caused as a consequence of the biological action to accumulate the excess of the harmful elements for plant growth on the roots in the same case as in Section 3.1. The results in Section 3.1 also support that B, Mn, and Zn were distributed through the body of *A. mangium*. Furthermore, a larger amount of Fe, S, and Al was obtained in *M. leucadendra* compared to

**Figure 9.** Concentration of Al, As, B, Fe, Mn, S, and Zn in each part of *A. mangium*.

**Figure 10.** Standard deviation of the concentration of Al, As, B, Fe, Mn, S, and Zn in each part of *A. mangium.*

*A. mangium* although both *M. leucadendra* and *A. mangium* are acid tolerant. This indicated that the accumulation capacity of the elements in the plant body depends on the species.

> pH 2.0 after 70 days. In contrast, the accumulation of Al in the roots resulted in the inhibition of the root elongation and the death of *A. mangium* because of the large amount of dissolved Al and its accumulation in plants at pH 3.0 in S1.5. In S3.0–S20.0, *A. mangium* died resulting from the dissolution of Al and its accumulation in the roots with the continual production of

> **Figure 12.** Change of the height of the seedlings and the length of the roots of *A. mangium* at the end of the vegetation

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 at pH 2.0 from the beginning of the vegetation test. Therefore, the timing of the transplant of plants and acidification of soils over time should be taken into account for the revegetation.

Acid-tolerant plants can survive in acid soils by setting up several tolerance mechanisms, such as the increase of soil pH around the root apices [5, 6]. However, the results in this study suggest that Al tolerance of plants has to be considered in addition to acid tolerance in the case that the accumulation of Al in plants inhibits plant growth. As shown in Ref. [7], there are differences in the Al-tolerant mechanism, such as Al-excluder and Al-accumulator. In regard to the effects of Al accumulation in plant body on plant growth, the plant classified as Al-excluder seems to be a suitable plant for the first stage of the revegetation in which acidic conditions are formed. Likewise, the content of Al was 6.45 mg/g in the roots of *A. mangium* in the vegetation test and it was 9.05 mg/g in *M. leucadendra,* which survived in acid soils in this study, although both plants are similarly categorized as Al-excluder in Ref. [7]. This indicates that *M. leucadendra* is more suitable for the first stage of the revegetation than *A. mangium* from the point of view of the effects of Al. Thereby, the plant for revegetation should be carefully selected from various perspectives, such as acid tolerance and Al tolerance of plants, and soil conditions. The evaluation in terms of not only soil conditions but also plant species has to be highlighted for

In this study, vegetation survey and a vegetation test were conducted to investigate the current situation of acid soils and plant growth in mine site and to understand the effects of acid soils on plant growth over time. The results are summarized with the key to successful revegetation in terms of soil acidification and tolerance characteristics of plants as follows:

H+

test.

successful revegetation.

**4. Conclusions**

**Figure 11** demonstrates the growth of *A. mangium* in S0.0–S1.5, and **Figure 12** shows the change of height and length of the seedlings and roots of *A. mangium*. The number of the leaves and the height of *A. mangium* obviously decreased with the increase in the mixing ratio of pyrite as shown in **Figure 11**. The length of the roots also decreased with the increase of the content of pyrite as shown in **Figure 12**, attributing to the inhibition of the root elongation in response to Al-stress [10, 42]. This result and the increase of the content of Al in the roots of *A. mangium* with the increase of mixing ratio of pyrite revealed that the accumulation of Al in *A. mangium* resulted in the death in S1.5 regardless of the similar content of Al in S0.0–S1.5 as shown in **Figure 6**. In short, the immobilization of Al in soils without absorption in the plant body due to neutral pH led to the growth of *A. mangium* in S0.0, and *A. mangium* survived in S0.5 by increasing the resistance against acidic conditions with the growth and without absorption of Al in the plant body at around pH 7.0 during 70 days even if pH dropped in

**Figure 11.** Growth of *A. mangium* at different mixing ratios of pyrite in the acid soil.

Effects of Acid Soils on Plant Growth and Successful Revegetation in the Case of Mine Site http://dx.doi.org/10.5772/intechopen.70928 23

**Figure 12.** Change of the height of the seedlings and the length of the roots of *A. mangium* at the end of the vegetation test.

pH 2.0 after 70 days. In contrast, the accumulation of Al in the roots resulted in the inhibition of the root elongation and the death of *A. mangium* because of the large amount of dissolved Al and its accumulation in plants at pH 3.0 in S1.5. In S3.0–S20.0, *A. mangium* died resulting from the dissolution of Al and its accumulation in the roots with the continual production of H+ at pH 2.0 from the beginning of the vegetation test. Therefore, the timing of the transplant of plants and acidification of soils over time should be taken into account for the revegetation.

Acid-tolerant plants can survive in acid soils by setting up several tolerance mechanisms, such as the increase of soil pH around the root apices [5, 6]. However, the results in this study suggest that Al tolerance of plants has to be considered in addition to acid tolerance in the case that the accumulation of Al in plants inhibits plant growth. As shown in Ref. [7], there are differences in the Al-tolerant mechanism, such as Al-excluder and Al-accumulator. In regard to the effects of Al accumulation in plant body on plant growth, the plant classified as Al-excluder seems to be a suitable plant for the first stage of the revegetation in which acidic conditions are formed. Likewise, the content of Al was 6.45 mg/g in the roots of *A. mangium* in the vegetation test and it was 9.05 mg/g in *M. leucadendra,* which survived in acid soils in this study, although both plants are similarly categorized as Al-excluder in Ref. [7]. This indicates that *M. leucadendra* is more suitable for the first stage of the revegetation than *A. mangium* from the point of view of the effects of Al. Thereby, the plant for revegetation should be carefully selected from various perspectives, such as acid tolerance and Al tolerance of plants, and soil conditions. The evaluation in terms of not only soil conditions but also plant species has to be highlighted for successful revegetation.

#### **4. Conclusions**

*A. mangium* although both *M. leucadendra* and *A. mangium* are acid tolerant. This indicated that

**Figure 11** demonstrates the growth of *A. mangium* in S0.0–S1.5, and **Figure 12** shows the change of height and length of the seedlings and roots of *A. mangium*. The number of the leaves and the height of *A. mangium* obviously decreased with the increase in the mixing ratio of pyrite as shown in **Figure 11**. The length of the roots also decreased with the increase of the content of pyrite as shown in **Figure 12**, attributing to the inhibition of the root elongation in response to Al-stress [10, 42]. This result and the increase of the content of Al in the roots of *A. mangium* with the increase of mixing ratio of pyrite revealed that the accumulation of Al in *A. mangium* resulted in the death in S1.5 regardless of the similar content of Al in S0.0–S1.5 as shown in **Figure 6**. In short, the immobilization of Al in soils without absorption in the plant body due to neutral pH led to the growth of *A. mangium* in S0.0, and *A. mangium* survived in S0.5 by increasing the resistance against acidic conditions with the growth and without absorption of Al in the plant body at around pH 7.0 during 70 days even if pH dropped in

the accumulation capacity of the elements in the plant body depends on the species.

**Figure 11.** Growth of *A. mangium* at different mixing ratios of pyrite in the acid soil.

**Figure 10.** Standard deviation of the concentration of Al, As, B, Fe, Mn, S, and Zn in each part of *A. mangium.*

22 Soil pH for Nutrient Availability and Crop Performance

In this study, vegetation survey and a vegetation test were conducted to investigate the current situation of acid soils and plant growth in mine site and to understand the effects of acid soils on plant growth over time. The results are summarized with the key to successful revegetation in terms of soil acidification and tolerance characteristics of plants as follows:

1. The dissolution of Al under acidic conditions in acid soils which were attributed to the formation of acidic water triggered by the dissolution of sulfides influenced plant growth in mine site. It is necessary to select a proper plant for successful revegetation from the point of view of Al tolerance and the dissolution of Al with the formation of acidic water over time.

[2] Ueno K.A mechanism of soil acidification in acid sulfate soils. Annual Report of Research

Effects of Acid Soils on Plant Growth and Successful Revegetation in the Case of Mine Site

http://dx.doi.org/10.5772/intechopen.70928

25

[3] Kochian LV, Pineros MA, Hoekenga OA. The physiology, genetics and molecular biology of plant aluminum resistance and toxicity. Plant and Soil. 2005;**274**(1-2):175-195

[4] Matsumoto H. Plant responses to aluminum stress in acid soil molecular mechanism of

[5] Kochian LV, Hoekenga OA, Pineros MA. How do crop plants tolerate acid soils? Mechanism of aluminum tolerance and phosphorus efficiency. Annual Review of Plant

[6] Vitorello VA, Capaldi FR, Stefanuto VA. Recent advances in aluminium toxicity and resistance in higher plants. Brazilian Journal of Plant Physiology. 2005;**17**:129-143

[7] Osaki M, Watanabe T, Tadano T. Beneficial effect of aluminum on growth of plants

[8] Tanikawa N, Inoue H, Nakayama M.Aluminum ions are involved in purple flower coloration in *Camellia japonica*. 'Sennen-fujimurasaki'. The Horticulture Journal. 2016;**85**(4):331-339

[9] Osaki M, Sittibush C, Nuyim T. Nutritional characteristics of wild plants grown in peat and acid sulfate soils distributed in Thailand and Malaysia. In: Vijarsorn P, Suzuki K, Kyuma K, Wada E, Nagano T, Takai Y, editors. A Tropical Swamp Forest Ecosystem and Its Greenhouse Gas Emission. Tokyo: Nodai Research Institute Tokyo University of Agriculture; 1995.

[10] Saifuddin M, Osman N, Idris RM, Halim A. The effects of pre-aluminum treatment on morphology and physiology of potential acidic slope plants. Kuwait Journal of Science

[11] Quadir QF, Watanabe T, Chen Z, Osaki M, Shinano T. Ionomic response of *Lotus japonicus* to different root-zone temperatures. Soil Science and Plant Nutrition. 2011;**57**(2):221-232

[12] Sobek AA, Schuller WA, Freeman JR, Smith RM. Field and laboratory methods applicable to overburdens and minesoils. Report EPA-600/2-78-054. U.S. National Technical Information Service Report PB-280. 1978; 1-204. Available from: http://www.osmre.gov/

[13] AMIRA International. ARD Test Handbook: Prediction & Kinetic Control of Acid Mine Drainage, AMIRA P387A. Reported by Ian Wark Research Institute and Environmental Geochemistry International Ltd. Melbourne, Australia: AMIRA International; 2002. Available from: http://www.amira.com.au/documents/downloads/P387AProtocol

[14] Miller S, Robertson A, Donahue T. Advances in acid drainage prediction using the net acid generation (NAG) test. In: Proceedings of the 4th International Conference on Acid Rock Drainage; May 31, 1997–June 6, 1997; Vancouver, B.C., Canada: Natural Resources

resources/library/ghm/FieldLab.pdf [Accessed: Aug 31, 2017]

adapted to low pH soils. Soil Science & Plant Nutrition. 1997;**43**(3):551-563

aluminum injury and tolerance. Kagaku to Seibutsu. 2000;**38**(7):425-458

Physiology and Plant Molecular Biology. 2004;**55**:459-493

Institute for Biological Function. 2004;**4**:25-33

p. 63-76

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Booklet.pdf [Accessed: Aug 31, 2017]

Canada; 1997. p. 533-549

2. Not only soil pH but also the assessment of the dissolution of sulfides over time is crucial for successful revegetation, suggesting that net acid producing potential (NAPP) and net acid generation (NAG) pH, which are used for evaluating the formation of acidic water, are useful to evaluate soil conditions for the revegetation in addition to soil pH.

3. The effects of acid soils on plant growth change according to plant species because Al-tolerant mechanism of plants depends on the species. Moreover, plants can survive under acidic conditions by increasing the resistance against acidic conditions with the plant growth. Therefore, the timing of the transplant of plants and acidification of soils over time should be taken into account for the revegetation.

#### **Acknowledgements**

The authors would like to express their appreciation to the mine for providing the samples. The experiments were conducted with the kind support of Mr. Shunta Ogata in Department of Earth Resources Engineering of Kyushu University.

#### **Author details**

Shinji Matsumoto1 \*, Hideki Shimada2 , Takashi Sasaoka2 , Ikuo Miyajima3 , Ginting J. Kusuma<sup>4</sup> and Rudy S. Gautama<sup>4</sup>

\*Address all correspondence to: shin.matsumoto@aist.go.jp

1 Geological Survey of Japan, National Institute of Advanced Industrial Science and Technology (AIST), Ibaraki, Japan

2 Department of Earth Resources Engineering, Faculty of Engineering, Kyushu University, Fukuoka, Japan


#### **References**

[1] Shishido M, Ito Y, Tamoto S. A vegetation method using native plants in the acid sulfate soil. Japan Society of Engineering Geology. 2013:123-124

[2] Ueno K.A mechanism of soil acidification in acid sulfate soils. Annual Report of Research Institute for Biological Function. 2004;**4**:25-33

1. The dissolution of Al under acidic conditions in acid soils which were attributed to the formation of acidic water triggered by the dissolution of sulfides influenced plant growth in mine site. It is necessary to select a proper plant for successful revegetation from the point of view of Al tolerance and the dissolution of Al with the formation of acidic water

2. Not only soil pH but also the assessment of the dissolution of sulfides over time is crucial for successful revegetation, suggesting that net acid producing potential (NAPP) and net acid generation (NAG) pH, which are used for evaluating the formation of acidic water, are useful

3. The effects of acid soils on plant growth change according to plant species because Al-tolerant mechanism of plants depends on the species. Moreover, plants can survive under acidic conditions by increasing the resistance against acidic conditions with the plant growth. Therefore, the timing of the transplant of plants and acidification of soils over time should be

The authors would like to express their appreciation to the mine for providing the samples. The experiments were conducted with the kind support of Mr. Shunta Ogata in Department

, Takashi Sasaoka2

1 Geological Survey of Japan, National Institute of Advanced Industrial Science and Techno-

2 Department of Earth Resources Engineering, Faculty of Engineering, Kyushu University,

[1] Shishido M, Ito Y, Tamoto S. A vegetation method using native plants in the acid sulfate

4 Department of Mining Engineering, Institut Teknologi Bandung, Bandung, Indonesia

, Ikuo Miyajima3

, Ginting J. Kusuma<sup>4</sup>

to evaluate soil conditions for the revegetation in addition to soil pH.

taken into account for the revegetation.

24 Soil pH for Nutrient Availability and Crop Performance

of Earth Resources Engineering of Kyushu University.

\*, Hideki Shimada2

\*Address all correspondence to: shin.matsumoto@aist.go.jp

3 Institute of Tropical Agriculture, Kyushu University, Fukuoka, Japan

soil. Japan Society of Engineering Geology. 2013:123-124

**Acknowledgements**

**Author details**

Shinji Matsumoto1

Fukuoka, Japan

**References**

and Rudy S. Gautama<sup>4</sup>

logy (AIST), Ibaraki, Japan

over time.


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[30] Thaman RR, Thomson LAJ, DeMeo R, Areki R, Elevitch CR. *Instia bijuga* (vesi), Ver. 3.1. In: Elevitch CR, editor. Species Profiles for Pacific Island Agroforestry. Holualao: Permanent

[31] Kadam PV, Yadav KN, Deoda RS, Shivatare RS, Patil MJ. *Mimusops elengi*: A review on ethnobotany. Phytochemical and pharmacological profile. Journal of Pharmacognosy

[32] Steinmetz EF. *Fagraea fragrans*. Quarterly Journal of Crude Drug Research. 1961;**1**(2):66-71 [33] Dent DL, Pons LJ. A world perspective on acid sulphate soils. Geoderma. 1995;**67**(3-4):

[34] Matsumoto S, Shimada H, Sasaoka T, Matsui K, Kusuma GJ. Prevention of acid mine drainage (AMD) by using sulfur-bearing rocks for a cover layer in a dry cover system in view of the form of sulfur. Journal of the Polish Mineral Engineering Society. 2015;**2**(36):29-35

[35] Nguyen NT, Nakabayashi K, Thompson J, Fujita K. Role of exudation of organic acids and phosphate in aluminium tolerance of four tropical woody species. Tree Physiology.

[36] Tanaka M, Miwa K, Fujiwara T. Molecular mechanism and regulation of boric acid transport in plants. The Journal of Japanese Biochemical Society. 2010;**82**(5):367-377

[37] Goulet RR, Pick FR. Changes in dissolved and total Fe and Mn in a young constructed wetland: Implications for retention performance. Ecological Engineering. 2001;**17**(4):373-384

[38] Sasaki K, Ogino T, Hori O, Takano K, Endo Y, Sakurai Y, Irie K. Treatment of heavy metals in a constructed wetland, Kaminokuni, Hokkaido: Accumulation of heavy metals in

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[40] Kadlec RH, Knight RL. Treatment Wetlands. Boca Raton: CRC Press/Lewis Publishers;

[41] Huang L, Dong BC, Xue W, Peng YK, Zhang MX, Yu FH. Soil particle heterogeneity affects the growth of a rhizomatous wetland plant. PLoS One. 2013;**8**(7):e69836

[42] Kikui S, Sasaki T, Maekawa M, Miyao A, Hirochika H, Matsumoto H, Yamamoto Y. Physiological and genetic analyses of aluminium tolerance in rice, focusing on root growth during germination. Journal of Inorganic Biochemistry. 2005;**99**(9):1837-1844

emergent vegetations. Journal of MMIJ. 2009;**125**(8):453-460

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[15] ASTM. Standard Test Methods for Liquid Limit, Plastic Limit, and Plasticity Index of

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[18] Krisnawati H, Varis E, Kallio MH, Kanninen M. *Paraserianthes falcataria* (L.) Nielsen: Ecology, Silviculture and Productivity. Bogor: Center for International Forestry Research

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[20] Orwa C, Mutua A, Kindt R, Jamnadass R, Simons A. Agroforestree Database: A Tree Reference and Selection Guide, Ver. 4.0; 2009. Available from: http://www.worldagrofor-

[21] Fern K. *Michelia champaca*, Useful Tropical Plants Database; 2014. Available from: http://tropical.theferns.info/viewtropical.php?id=Magnolia+champaca [Accessed: Aug

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**Section 3**

**Soil Chemistry and Mineralogy**

**Soil Chemistry and Mineralogy**

**Chapter 3**

**Provisional chapter**

**Fluoride Adsorption onto Soil Adsorbents: The Role of**

**Fluoride Adsorption onto Soil Adsorbents: The Role of** 

Soil adsorbents continue to attract increasingly high numbers of researchers in water defluoridation studies. An aspect of solution parameters, that is the aqueous adsorption of fluoride onto soil adsorbents in defluoridation studies, has been reviewed and reported. The pH was found to be the main factor controlling fluoride adsorption on the popular soil adsorbents including: aluminosilicates, iron (hydr)oxides, aluminum (hydr) oxides, apatites, carbonaceous minerals, calcareous soils and zeolites and the other key parameters being temperature, time of contact, and co-existent ions. Fluoride adsorption onto metal-exchanged zeolites and hydroxyapatites (optimum pH = 4–10), iron (hydro) oxide minerals (pH = 2–7), and carbonaceous minerals (pH = 4–12) is relatively pHindependent, and high amounts of fluoride are able to sorb upon the surfaces of these minerals in a wide range of pH values. However, montmorillonites (optimum pH = 5–6), aluminum (hydro)oxide minerals (pH = 5–7), and calcareous minerals (pH = 5–6) only sorb significant amount of fluoride in a narrow range of pH values. The fluoride adsorption onto the latter class of minerals, also generally occurring at slightly above room temperatures, appears to be highly specific and not strongly affected by the presence of

> , and NO3 − .

Adequate dietary levels of fluoride are desired for good oral health and for the proper development of skeletal tissues [1]. Nonetheless, the excessive levels of fluoride in the environment pose major public health challenges in many regions of the world [2]. Dietary fluoride overexposure has been linked to a series of detrimental physiological effects [3] and it is known

**Keywords:** adsorption, defluoridation, drinking water, fluoride, minerals, pH, soil

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

© 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use,

distribution, and reproduction in any medium, provided the original work is properly cited.

DOI: 10.5772/intechopen.74652

**pH and Other Solution Parameters**

**pH and Other Solution Parameters**

Enos Wamalwa Wambu and Audre Jerop Kurui

Enos Wamalwa Wambu and Audre Jerop Kurui

Additional information is available at the end of the chapter

Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/intechopen.74652

coexistent anions including: PO<sup>4</sup>

3− , SO<sup>4</sup> 2− , Cl<sup>−</sup>

**Abstract**

**1. Introduction**

#### **Fluoride Adsorption onto Soil Adsorbents: The Role of pH and Other Solution Parameters Fluoride Adsorption onto Soil Adsorbents: The Role of pH and Other Solution Parameters**

DOI: 10.5772/intechopen.74652

Enos Wamalwa Wambu and Audre Jerop Kurui Enos Wamalwa Wambu and Audre Jerop Kurui

Additional information is available at the end of the chapter Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/intechopen.74652

#### **Abstract**

Soil adsorbents continue to attract increasingly high numbers of researchers in water defluoridation studies. An aspect of solution parameters, that is the aqueous adsorption of fluoride onto soil adsorbents in defluoridation studies, has been reviewed and reported. The pH was found to be the main factor controlling fluoride adsorption on the popular soil adsorbents including: aluminosilicates, iron (hydr)oxides, aluminum (hydr) oxides, apatites, carbonaceous minerals, calcareous soils and zeolites and the other key parameters being temperature, time of contact, and co-existent ions. Fluoride adsorption onto metal-exchanged zeolites and hydroxyapatites (optimum pH = 4–10), iron (hydro) oxide minerals (pH = 2–7), and carbonaceous minerals (pH = 4–12) is relatively pHindependent, and high amounts of fluoride are able to sorb upon the surfaces of these minerals in a wide range of pH values. However, montmorillonites (optimum pH = 5–6), aluminum (hydro)oxide minerals (pH = 5–7), and calcareous minerals (pH = 5–6) only sorb significant amount of fluoride in a narrow range of pH values. The fluoride adsorption onto the latter class of minerals, also generally occurring at slightly above room temperatures, appears to be highly specific and not strongly affected by the presence of coexistent anions including: PO<sup>4</sup> 3− , SO<sup>4</sup> 2− , Cl<sup>−</sup> , and NO3 − .

**Keywords:** adsorption, defluoridation, drinking water, fluoride, minerals, pH, soil

#### **1. Introduction**

Adequate dietary levels of fluoride are desired for good oral health and for the proper development of skeletal tissues [1]. Nonetheless, the excessive levels of fluoride in the environment pose major public health challenges in many regions of the world [2]. Dietary fluoride overexposure has been linked to a series of detrimental physiological effects [3] and it is known

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

to lead to serious mottling of teeth enamel and gross skeletal malformations [4]. Continued dependence on fluoride-enriched water by communities in high-fluoride areas is the principal conduit by which people get exposed to undue levels of fluoride from the environment. Problems linked to prolonged consumption of excessive fluoride through water and food are, for that reason, normally correlated to areas of high-fluoride-bearing rocks and fluorideenriched soil minerals. Even so, the hydro-geological release of soil mineral fluoride and its bioavailability through food chains is dependent on the hydrogeochemical characteristics of the environment.

the mineral for fluoride or for related adsorbates; the ease of availability of the mineral, its procurement, preparation, and applicability under given conditions; as well as by its user and environmental safety considerations. Based on approximate fluoride adsorption capacities of the minerals frequently revealed in the literature, the minerals that have exhibited the most promising potential for water defluoridation in the most recent studies include palygorskite (with a mean fluoride adsorption capacity of 57.97 mg/g), pumice (18.27 mg/g), zeolites (15.65 mg/g), hydroxyapatite (13.27 mg/g), iron-enriched laterites (9.39 mg/g), bauxite (7.53 mg/g), and montmorillonites (4.82 mg/g). The other minerals including kaolinites, ceramics, and quartz normally have mean fluoride adsorption capacities of less than 3.0 mg/g

Fluoride Adsorption onto Soil Adsorbents: The Role of pH and Other Solution Parameters

http://dx.doi.org/10.5772/intechopen.74652

33

The capacity of soil media to sorb large amounts of fluoride is controlled by the predominant surface chemistry of the soil systems. The primary fluoride sorptive sites of clay colloids in the soil minerals comprise mainly the protonated or non-protonated silanol groups and the cationic positive centers provided by prevalent soil cations such as Fe3+, Al3+, and Si4+. Natural soil systems are, however, generally associated with low ion-exchange capacities because the soil surfaces are normally saturated with replaceable counter groups, which mask and neutralize intrinsic surface charge so as to maintain mineral surface stability. The ion-exchange properties of the soil minerals can, however, be enhanced by pre-treatments that are aimed at dislodging the masking ions from the soil surfaces so as to increase the reactivity of the soil surfaces toward the target adsorbate ion and unblock the pores into the crystal lattice structure of the soil systems [16]. This is more so for soil surfaces that possess net charges that repel the adsorbate ions as is usually the case of fluoride adsorption onto clay systems, which are normally characterized by high density of electronegative oxygen groups in their structures that induce a net negative charge in the adsorbent soil surfaces. These surface charges make such soil to naturally repel and keep fluoride in the solution. This necessitates pretreatment to produce soil surface charge reversal in order to enhance their fluoride adsorption affinities

The surface charge reversal for negatively charged soil adsorbents, which is aimed at enhancing electro-activity of their colloid surfaces towards aqueous fluoride particles, may be achieved by impregnating the adsorbent soil structure with multivalent metal ions or by grafting and intercalating the soil adsorbents with charged reactive groups. Hydrothermal activation of soil adsorbents in dilute acids is also a common practice that is not only simpler to apply but also more cost-effective [17, 18]. The latter procedure results in partial dealumination of the clay structure, which increases the proportion of silica and the density of acid silanol groups on soil adsorbent surface leading to increased overall positive charge of

The effects of adsorption solution parameters on the adsorption process spring from their influence on the adsorbents soil surface chemistry and on the flux transport of adsorbate

and do not constitute prospective robust fluoride adsorbents [15].

and capacities.

the clay surfaces [19, 20].

**3. Effect of selected solution parameters**

Because of its detrimental public health effects when consumed in excessive amounts, the World Health Organization (WHO) has set recommended levels of fluoride for drinking water at 0.7 ppm [5]. However, the set maximum permissible levels of 1.5 ppm are the most widely used fluoride standards of drinking water to guard against dental caries and ensure healthy development of teeth and bones [6]. The point-of-use treatment of contaminated drinking water, to remove excessive fluoride while allowing sufficient levels for good oral and skeletal health, is now an indispensable component in many domestic water treatment protocols around the world [7]. Because of the high costs involved, many studies have recently been devoted to investigating the capacity of different materials for fluoride removal from water with a view to device more affordable approaches to water defluoridation [8–10].

Soil adsorbents, in particular, have been among natural media that have been extensively explored by researchers as alternate affordable media in water defluoridation [11–14] as they are normally more readily available and, by and large, possess significant fluoride adsorption capabilities. Furthermore, they are relatively stable and usable in a wider range of water conditions than most other natural media. The soil adsorbents that have attracted highest attention of scientists for water defluoridation include montmorillonites, aluminosilicates, iron and aluminum (hydr)oxides, hydroxyapatites, carbonaceous minerals, calcareous soils, and zeolites [15]. The solution pH, fluoride concentration, temperature, and co-existent ions play a major role in controlling fluoride adsorption onto soil adsorbents. Understanding the influence of these parameters in fluoride removal from water by adsorption using soil adsorbents could present additional insight into the scope of applicability of the geomaterials in water defluoridation.

The present work was initiated to interrogate available literature on water defluoridation by adsorption using soil adsorbents with a view to divulge information that could inform subsequent strategies in water defluoridation-based soil mineral adsorbents.

#### **2. Adsorption surface enhancement for soil adsorbents**

The potential soil adsorbents for fluoride sequestration from water are as diverse as the natural soil systems on earth. However, a glimpse through recent literature reveals that only few minerals have been repeatedly been studied for their potential to sorb fluoride from water over the last few decades. The selection of a soil adsorbent for water defluoridation studies is usually informed by, among other factors, the already known sorption capacities of the mineral for fluoride or for related adsorbates; the ease of availability of the mineral, its procurement, preparation, and applicability under given conditions; as well as by its user and environmental safety considerations. Based on approximate fluoride adsorption capacities of the minerals frequently revealed in the literature, the minerals that have exhibited the most promising potential for water defluoridation in the most recent studies include palygorskite (with a mean fluoride adsorption capacity of 57.97 mg/g), pumice (18.27 mg/g), zeolites (15.65 mg/g), hydroxyapatite (13.27 mg/g), iron-enriched laterites (9.39 mg/g), bauxite (7.53 mg/g), and montmorillonites (4.82 mg/g). The other minerals including kaolinites, ceramics, and quartz normally have mean fluoride adsorption capacities of less than 3.0 mg/g and do not constitute prospective robust fluoride adsorbents [15].

to lead to serious mottling of teeth enamel and gross skeletal malformations [4]. Continued dependence on fluoride-enriched water by communities in high-fluoride areas is the principal conduit by which people get exposed to undue levels of fluoride from the environment. Problems linked to prolonged consumption of excessive fluoride through water and food are, for that reason, normally correlated to areas of high-fluoride-bearing rocks and fluorideenriched soil minerals. Even so, the hydro-geological release of soil mineral fluoride and its bioavailability through food chains is dependent on the hydrogeochemical characteristics of

Because of its detrimental public health effects when consumed in excessive amounts, the World Health Organization (WHO) has set recommended levels of fluoride for drinking water at 0.7 ppm [5]. However, the set maximum permissible levels of 1.5 ppm are the most widely used fluoride standards of drinking water to guard against dental caries and ensure healthy development of teeth and bones [6]. The point-of-use treatment of contaminated drinking water, to remove excessive fluoride while allowing sufficient levels for good oral and skeletal health, is now an indispensable component in many domestic water treatment protocols around the world [7]. Because of the high costs involved, many studies have recently been devoted to investigating the capacity of different materials for fluoride removal from water

Soil adsorbents, in particular, have been among natural media that have been extensively explored by researchers as alternate affordable media in water defluoridation [11–14] as they are normally more readily available and, by and large, possess significant fluoride adsorption capabilities. Furthermore, they are relatively stable and usable in a wider range of water conditions than most other natural media. The soil adsorbents that have attracted highest attention of scientists for water defluoridation include montmorillonites, aluminosilicates, iron and aluminum (hydr)oxides, hydroxyapatites, carbonaceous minerals, calcareous soils, and zeolites [15]. The solution pH, fluoride concentration, temperature, and co-existent ions play a major role in controlling fluoride adsorption onto soil adsorbents. Understanding the influence of these parameters in fluoride removal from water by adsorption using soil adsorbents could present additional insight into the scope of applicability of the geomaterials in

The present work was initiated to interrogate available literature on water defluoridation by adsorption using soil adsorbents with a view to divulge information that could inform subse-

The potential soil adsorbents for fluoride sequestration from water are as diverse as the natural soil systems on earth. However, a glimpse through recent literature reveals that only few minerals have been repeatedly been studied for their potential to sorb fluoride from water over the last few decades. The selection of a soil adsorbent for water defluoridation studies is usually informed by, among other factors, the already known sorption capacities of

quent strategies in water defluoridation-based soil mineral adsorbents.

**2. Adsorption surface enhancement for soil adsorbents**

with a view to device more affordable approaches to water defluoridation [8–10].

the environment.

32 Soil pH for Nutrient Availability and Crop Performance

water defluoridation.

The capacity of soil media to sorb large amounts of fluoride is controlled by the predominant surface chemistry of the soil systems. The primary fluoride sorptive sites of clay colloids in the soil minerals comprise mainly the protonated or non-protonated silanol groups and the cationic positive centers provided by prevalent soil cations such as Fe3+, Al3+, and Si4+. Natural soil systems are, however, generally associated with low ion-exchange capacities because the soil surfaces are normally saturated with replaceable counter groups, which mask and neutralize intrinsic surface charge so as to maintain mineral surface stability. The ion-exchange properties of the soil minerals can, however, be enhanced by pre-treatments that are aimed at dislodging the masking ions from the soil surfaces so as to increase the reactivity of the soil surfaces toward the target adsorbate ion and unblock the pores into the crystal lattice structure of the soil systems [16]. This is more so for soil surfaces that possess net charges that repel the adsorbate ions as is usually the case of fluoride adsorption onto clay systems, which are normally characterized by high density of electronegative oxygen groups in their structures that induce a net negative charge in the adsorbent soil surfaces. These surface charges make such soil to naturally repel and keep fluoride in the solution. This necessitates pretreatment to produce soil surface charge reversal in order to enhance their fluoride adsorption affinities and capacities.

The surface charge reversal for negatively charged soil adsorbents, which is aimed at enhancing electro-activity of their colloid surfaces towards aqueous fluoride particles, may be achieved by impregnating the adsorbent soil structure with multivalent metal ions or by grafting and intercalating the soil adsorbents with charged reactive groups. Hydrothermal activation of soil adsorbents in dilute acids is also a common practice that is not only simpler to apply but also more cost-effective [17, 18]. The latter procedure results in partial dealumination of the clay structure, which increases the proportion of silica and the density of acid silanol groups on soil adsorbent surface leading to increased overall positive charge of the clay surfaces [19, 20].

#### **3. Effect of selected solution parameters**

The effects of adsorption solution parameters on the adsorption process spring from their influence on the adsorbents soil surface chemistry and on the flux transport of adsorbate solutes from the bulk solution through the aqueous matrix to the adsorbent surface. The principal solution parameters that control fluoride sequestration onto soil surfaces include the pH, temperature, contact time, fluoride concentration, and co-existing ions. Other contributing factors comprise: adsorbent dosage, adsorbent particle size, and the rate of agitation. The effect of adsorbent dosage and particle size and those of the adsorbate concentration mirrors each other. This is because both adsorbent dosage and particle size and those of the adsorbate concentration control the availability of reacting "particles" that drive the thermodynamic adsorption equilibrium on either side of the adsorption interface. Increase in the adsorbent dosage and in the adsorbate concentration results in high rates of adsorption as a result of more intensified solute fluxes through aqueous media to the soil surfaces. This influence is, however, extensively discussed elsewhere in the literature [15].

(2)

35

(3)

The pH of the aqueous media is, therefore, the prime factor that controls fluoride uptake by

However, the solution pH of maximum fluoride adsorption varies from one type of soil adsorbent to the other. For iron-enriched laterites [27–29], kaolinites [22, 30–33] and, to a limited extent, for certain hydroxyapatites [34, 35], the maximum fluoride adsorption capacities occur in acidic media at pH values of 5 or less. Fluoride uptake in low pH (3–5) can be attributed to the formation of weak hydrofluoric acid [27]. It, therefore, shows that the adsorbent surfaces

The maximum fluoride adsorption capacities for montmorillonite clays [22, 36, 37], aluminum (hydrox)oxide minerals [38–44] and calcareous minerals [11, 12] are, however, restricted

of expanding smectite clays comprising octahedral sheets of alumina sandwiched between two tetrahedral sheets of silica. The tripartite sheets are then loosely held together by weak oxygen-oxygen and oxygen-cation bonds [22]. The principal fluoride binding sides in montmorillonites are the cationic Fe3+, Al3+ and Si4+ centers. At pH of 4 and less, the capacity of montmorillonites to sorb large amounts of fluoride is greatly compromised due to their dis-

fluoride in a montmorillonite-water system exists in the form of aqueous iron and aluminum

Conversely, certain soil sorbents, which include pumice [45, 46]; palygorskites [47]; and particular ferric oxide minerals such as hematite [48, 49] are able to sorb high amounts of fluoride over an entire range of pH values from 2 to about 8. Furthermore, fluoride adsorption onto natural and metal-exchanged zeolites [50] and onto a class of carbonaceous adsorbents including lignite [51, 52] and coal [52, 53] appear to be quite pH-independent and high amounts of fluoride adsorption based on this class of adsorbents occur over the wide range of pH values of 4–12.

In general, therefore, montmorillonites normally tend to solubilize in low pH media and get

have narrow fluoride sorption edge within the neutral pH values. Like for montmorillonites, the usual pH for effective fluoride removal from water using metal-enhanced palygorskite is usually in the range of 2–8. Fluoride adsorption onto metal-exchanged zeolites and onto certain synthetic hydroxyapatites is, however, relatively independent of pH, and the adsorbents are able to take up high fluoride adsorption over a wide choice of pH values of 4–10. Aluminum oxide minerals usually have a narrower fluoride sorption edge in the pH range of 5.5–6.5 as is maximum fluoride adsorption onto Ca-based minerals, which occurs within the pH values of 5–6. On the other hand, high fluoride uptakes by hematite occur over a wider range of acidic pH values of 2–7. In the same way, optimum fluoride removal using carbona-

ceous adsorbents can be achieved at room temperature in the pH range of 4–12.

ruptive dissolution of the mineral structure with release of Fe3+, Al3+ and SiO<sup>2</sup>

complexes, and only a small fraction is able to sorb onto the clay surface.

(OH)2 (Si4

Fluoride Adsorption onto Soil Adsorbents: The Role of pH and Other Solution Parameters

ions in alkaline media. For this reason, montmorillonites usually

O10)].nH2

http://dx.doi.org/10.5772/intechopen.74652

O, are a group

. A major part of

for these minerals have affinity for HF aqueous species.

to pH values of 5–6. Montmorillonites, Mx[(Mg, Al, Fe)2

soil surfaces.

poisoned by excessive OH−

#### **3.1. Effects of pH**

Speciation and aqueous availability of fluoride in water is the function of pH, concentration, and the presence of cations such as: Al3+, Fe3+, Mn2+, Ca2+, and Mg2+ [21]. At low pH values of 4 and less, for example, the molecular HF fluoride species predominates aqueous fluoride speciation in solution. The formation of HF, which favors solubility and aqueous availability of fluoride, increases with decreasing pH of the media [22]. The fluoro-aluminum complexes that include AlF2+, AlF2 + , and AlF<sup>3</sup> 0 and other metallo-fluoro complex species involving other multivalent cations such as Fe3+, emerge in the pH range of 4–6 and the concentration of free fluoride ions in this pH range is only 21.35% [23]. At higher pH values, the stability of metallofluoro complexes decreases and the free fluoride anions, F− , predominate. All fluorides exist as free anions, F− , at pH values of 8–9, where all forms of aluminum species form the aluminate, [Al(OH)4 ]− , complexes in the presence of excess OH− species [24].

In the same way, the solution pH controls the ionization of reactive surface groups in the colloid soil surfaces and determines the nature and the intensity of the soil surface charge and the adsorption potential at the soil surfaces [25]. Calcareous minerals, for instance, facilitate pH-dependent fluoride solubility according to the mass balance Eq. (1) as follows [26]:

$$\text{CaCO}\_3(\text{s}) + \text{H}^+(\text{aq}) + 2\text{F}^-(\text{aq}) \qquad \overleftrightarrow{\text{g} \, ^\circ \text{CaF}\_2(\text{s}) - \text{HCO}\_3^-(\text{aq})} \tag{1}$$

This equation relates calcite and fluorite in the natural soil environments when both salts are in contact with the water. Accordingly, the increase in pH and in the concentrations of HCO<sup>3</sup> − increases water fluoride concentrations and vice versa.

In addition, anionic adsorption onto soil adsorbents can proceed through specific or nonspecific adsorption. The former is based on ligand-exchange reactions where the anions displace OH− and H2 O from the soil surfaces, whereas the latter involves electrostatic coulombic forces and mainly depends on the pH of zero net charge (pHpzc) of the adsorbent soil surface. Above pHzpc, the soil surface assumes positive charge, whereas below net positive surface charge persists [27]. The specific adsorption of fluoride by metal oxyhydroxide surface sites, for example, occurs by ligand exchange according to Eqs. (2) and (3) for protonated and nonprotonated sites, respectively, as follows:

Fluoride Adsorption onto Soil Adsorbents: The Role of pH and Other Solution Parameters http://dx.doi.org/10.5772/intechopen.74652 35

(3)

The pH of the aqueous media is, therefore, the prime factor that controls fluoride uptake by soil surfaces.

solutes from the bulk solution through the aqueous matrix to the adsorbent surface. The principal solution parameters that control fluoride sequestration onto soil surfaces include the pH, temperature, contact time, fluoride concentration, and co-existing ions. Other contributing factors comprise: adsorbent dosage, adsorbent particle size, and the rate of agitation. The effect of adsorbent dosage and particle size and those of the adsorbate concentration mirrors each other. This is because both adsorbent dosage and particle size and those of the adsorbate concentration control the availability of reacting "particles" that drive the thermodynamic adsorption equilibrium on either side of the adsorption interface. Increase in the adsorbent dosage and in the adsorbate concentration results in high rates of adsorption as a result of more intensified solute fluxes through aqueous media to the soil surfaces. This influence is,

Speciation and aqueous availability of fluoride in water is the function of pH, concentration, and the presence of cations such as: Al3+, Fe3+, Mn2+, Ca2+, and Mg2+ [21]. At low pH values of 4 and less, for example, the molecular HF fluoride species predominates aqueous fluoride speciation in solution. The formation of HF, which favors solubility and aqueous availability of fluoride, increases with decreasing pH of the media [22]. The fluoro-aluminum complexes

multivalent cations such as Fe3+, emerge in the pH range of 4–6 and the concentration of free fluoride ions in this pH range is only 21.35% [23]. At higher pH values, the stability of metallo-

In the same way, the solution pH controls the ionization of reactive surface groups in the colloid soil surfaces and determines the nature and the intensity of the soil surface charge and the adsorption potential at the soil surfaces [25]. Calcareous minerals, for instance, facilitate pH-dependent fluoride solubility according to the mass balance Eq. (1) as follows [26]:

This equation relates calcite and fluorite in the natural soil environments when both salts are in contact with the water. Accordingly, the increase in pH and in the concentrations of HCO<sup>3</sup>

In addition, anionic adsorption onto soil adsorbents can proceed through specific or nonspecific adsorption. The former is based on ligand-exchange reactions where the anions dis-

forces and mainly depends on the pH of zero net charge (pHpzc) of the adsorbent soil surface. Above pHzpc, the soil surface assumes positive charge, whereas below net positive surface charge persists [27]. The specific adsorption of fluoride by metal oxyhydroxide surface sites, for example, occurs by ligand exchange according to Eqs. (2) and (3) for protonated and non-

, at pH values of 8–9, where all forms of aluminum species form the alumi-

O from the soil surfaces, whereas the latter involves electrostatic coulombic

and other metallo-fluoro complex species involving other

species [24].

, predominate. All fluorides exist

(1)

−

however, extensively discussed elsewhere in the literature [15].

**3.1. Effects of pH**

that include AlF2+, AlF2

]−

as free anions, F−

nate, [Al(OH)4

place OH−

and H2

protonated sites, respectively, as follows:

+

34 Soil pH for Nutrient Availability and Crop Performance

, and AlF<sup>3</sup>

fluoro complexes decreases and the free fluoride anions, F−

increases water fluoride concentrations and vice versa.

0

, complexes in the presence of excess OH−

However, the solution pH of maximum fluoride adsorption varies from one type of soil adsorbent to the other. For iron-enriched laterites [27–29], kaolinites [22, 30–33] and, to a limited extent, for certain hydroxyapatites [34, 35], the maximum fluoride adsorption capacities occur in acidic media at pH values of 5 or less. Fluoride uptake in low pH (3–5) can be attributed to the formation of weak hydrofluoric acid [27]. It, therefore, shows that the adsorbent surfaces for these minerals have affinity for HF aqueous species.

The maximum fluoride adsorption capacities for montmorillonite clays [22, 36, 37], aluminum (hydrox)oxide minerals [38–44] and calcareous minerals [11, 12] are, however, restricted to pH values of 5–6. Montmorillonites, Mx[(Mg, Al, Fe)2 (OH)2 (Si4 O10)].nH2 O, are a group of expanding smectite clays comprising octahedral sheets of alumina sandwiched between two tetrahedral sheets of silica. The tripartite sheets are then loosely held together by weak oxygen-oxygen and oxygen-cation bonds [22]. The principal fluoride binding sides in montmorillonites are the cationic Fe3+, Al3+ and Si4+ centers. At pH of 4 and less, the capacity of montmorillonites to sorb large amounts of fluoride is greatly compromised due to their disruptive dissolution of the mineral structure with release of Fe3+, Al3+ and SiO<sup>2</sup> . A major part of fluoride in a montmorillonite-water system exists in the form of aqueous iron and aluminum complexes, and only a small fraction is able to sorb onto the clay surface.

Conversely, certain soil sorbents, which include pumice [45, 46]; palygorskites [47]; and particular ferric oxide minerals such as hematite [48, 49] are able to sorb high amounts of fluoride over an entire range of pH values from 2 to about 8. Furthermore, fluoride adsorption onto natural and metal-exchanged zeolites [50] and onto a class of carbonaceous adsorbents including lignite [51, 52] and coal [52, 53] appear to be quite pH-independent and high amounts of fluoride adsorption based on this class of adsorbents occur over the wide range of pH values of 4–12.

In general, therefore, montmorillonites normally tend to solubilize in low pH media and get poisoned by excessive OH− ions in alkaline media. For this reason, montmorillonites usually have narrow fluoride sorption edge within the neutral pH values. Like for montmorillonites, the usual pH for effective fluoride removal from water using metal-enhanced palygorskite is usually in the range of 2–8. Fluoride adsorption onto metal-exchanged zeolites and onto certain synthetic hydroxyapatites is, however, relatively independent of pH, and the adsorbents are able to take up high fluoride adsorption over a wide choice of pH values of 4–10. Aluminum oxide minerals usually have a narrower fluoride sorption edge in the pH range of 5.5–6.5 as is maximum fluoride adsorption onto Ca-based minerals, which occurs within the pH values of 5–6. On the other hand, high fluoride uptakes by hematite occur over a wider range of acidic pH values of 2–7. In the same way, optimum fluoride removal using carbonaceous adsorbents can be achieved at room temperature in the pH range of 4–12.

Differences in pH of maximum fluoride uptake for various soil systems arise principally from the differences in the surface chemistry of the mineral adsorbents, which control the affinity of soil surfaces towards different fluoride species in soil surfaces. It can be assumed that soils that have high fluoride adsorptions in strongly acidic media of pH 5 or less have higher affinity for molecular HF species, which are dominant in this range of medium pH. The HF particles adsorb by forming continuous hydrogen bonds with electronegative centers in the soil surfaces. Certain soils that preferentially sorb fluoride in the near-neutral acidic pH values of 5–6 have affinity for F− species, and the mode of fluoride adsorption is mainly complexation with positive cationic centers in the soil colloid structure, which include Al3+, Fe3+ and Si4+ among others. Soil adsorbent that sorb high fluoride levels over a wide range of pH values contains heterogeneous surfaces, which have attraction to several different fluoride species in solution.

than porous media with intraparticle sorptive sites. This is because in the latter case, the adsorbate particles have to be transported by diffusion into the inner adsorbent structures in order to access the reactive adsorbent sites. Fluoride adsorption onto pulverized crystalline

Fluoride Adsorption onto Soil Adsorbents: The Role of pH and Other Solution Parameters

entire mineral surface [66, 67]. Water defluoridation using calcareous materials is, as a result, normally characterized by fast adsorption rates and the adsorption equilibrium lies within

In less crystalline adsorbents such as lignite, more than 90% fluoride adsorption occurs within the initial 10 min. It, however, takes up to 150 min to saturate the less exposed sites inside the adsorbent structure with the latter 10% of the process [51, 52]. Such trends are also observed in the case of fluoride adsorption onto coal with shorter equilibration periods of 60–90 min for the latter phase of adsorption [52, 53], which shows that coal is more crystalline and less

Equilibration periods required for fluoride adsorption onto pumice have been reported to lie within the range of 20–30 min but pumice adsorbents have not generally been associated with the two-phase adsorption phenomenon. This indicates the presence of limited porosity in the mineral structure of these adsorbents [46, 56]. Although some authors have linked fluoride adsorption onto natural montmorillonites to rapid sorption rates associated with the short adsorption equilibrium periods of just 20–30 min [22, 37], several natural montmorillonites [36, 54] and Fe(III)-modified montmorillonites [55] appear to have consistent fluoride adsorption equilibrium time intervals in the range of 110–180 min. In the same way, a number of mineral adsorbents including metal-intercalated palygorskites [47] and certain aluminum oxide minerals [39, 44] appear to have equilibrium intervals within the same range of periods.

This signifies that these minerals possess structural porosities that are comparable.

ride immobilization upon hydroxyapatite, hydr(oxide) aluminum and iron minerals.

Natural water systems contain dissolved species across the organic-inorganic chemical continuum. Co-existent ions in water control the adsorption of fluoride by their competitive effect for the sorptive space on the adsorbent soil surfaces and by their influence on the adsorbate flux from the bulk solution to the sorbent surface. Co-ions tend to lower the rates and magnitude of adsorption, but the extent of these influence largely depend on the chemical and geometric dimensions of the ions, relative concentrations and affinities of the individual ions for the adsorbent surface. The influence of interfering ion, however, varies from one soil

As in the case of fluoride adsorption onto calcareous and carbonaceous soil adsorbents, the immobilization of fluoride into adsorbent zeolites [50, 69, 70], hydroxyapatites [60, 63, 64], iron oxide minerals [49] as well as into certain classes of aluminum oxide minerals [41] is characterized by initial rapid phases of adsorption characterized typically by short equilibration intervals of just 10–30 min, which are then followed by prolonged equilibration that could extend to 10–48 h. The final slow phase of equilibration can be ascribed to high structural porosity as in the case of zeolites or to slow valence exchange reaction mechanisms characteristic of fluo-

, over the

37

http://dx.doi.org/10.5772/intechopen.74652

calcareous minerals tends to occur rapidly by surface precipitation of fluorite, CaF2

the range of 30–60 min [11, 68].

porous than lignite.

**3.4. Co-existent ions**

adsorbent to the other.

#### **3.2. Adsorption temperature**

The effect of temperature on fluoride adsorption onto soil surfaces arise from its influence on the adsorption energy balance, on the kinetics of adsorbate particles, and on the chemical activation of reacting species. Higher temperatures enhance increased rates of adsorption by enhancement of faster solute transport from the bulk solution towards the adsorbent surfaces. Higher temperatures also raise the average energy of the particles allowing a higher number of particles to attain necessary activation energy to enable them to react. Very high temperatures may, however, counter the adsorption fluxes leading to reduced rates and magnitude of uptake of the adsorbate by the adsorbents.

As for the effects of solution pH, however, the effect of temperature on fluoride adsorption on popular soil adsorbents is varied. The peak fluoride adsorption by natural montmorillonites [22, 36, 37, 54], Fe(III)-modified montmorillonite [55], pumice [56] and lignite [51, 52] occur within a range of temperatures close to room temperature (298 K). Nevertheless, the highest fluoride uptake by both aniline-modified montmorillonites and pyrole-modified montmorillonites [57] as well as by coal [52, 53] is favored by above room temperatures close to 303 K. It has been found that fluoride-exchange reactions for hydroxyapatites [58–60] and for certain ferric oxide minerals such as hematite [48, 49] can occur over a wide range of temperatures of 298–323 K. Fluoride adsorption onto Mg2+ and Al3+ [47], Fe3+ [61] and ZrO2+ [62] loaded palygorskite minerals; synthetic hydroxyapatites [63, 64]; calcareous minerals [65] and onto magnesia-loaded fly ash cenospheres (MLC) is favored by higher temperatures in the range of 303–323 K. This indicates the existence of endothermic chemical surface reactions. The efficacies of bauxite to sorb fluoride has, however, been found to decrease with increasing temperature indicating the existence of exothermic fluoride immobilization in bauxite surfaces [39, 44].

#### **3.3. Contact time**

The resident time required for equilibration in an adsorption process depends mainly on the adsorbent structure and on the nature of reactions that occur between the adsorbate particles and reactive sites at the adsorbent surface. Adsorbents with compact crystalline structures and characteristically surface exposed reactive sites tend to have rapid rates of adsorption than porous media with intraparticle sorptive sites. This is because in the latter case, the adsorbate particles have to be transported by diffusion into the inner adsorbent structures in order to access the reactive adsorbent sites. Fluoride adsorption onto pulverized crystalline calcareous minerals tends to occur rapidly by surface precipitation of fluorite, CaF2 , over the entire mineral surface [66, 67]. Water defluoridation using calcareous materials is, as a result, normally characterized by fast adsorption rates and the adsorption equilibrium lies within the range of 30–60 min [11, 68].

In less crystalline adsorbents such as lignite, more than 90% fluoride adsorption occurs within the initial 10 min. It, however, takes up to 150 min to saturate the less exposed sites inside the adsorbent structure with the latter 10% of the process [51, 52]. Such trends are also observed in the case of fluoride adsorption onto coal with shorter equilibration periods of 60–90 min for the latter phase of adsorption [52, 53], which shows that coal is more crystalline and less porous than lignite.

Equilibration periods required for fluoride adsorption onto pumice have been reported to lie within the range of 20–30 min but pumice adsorbents have not generally been associated with the two-phase adsorption phenomenon. This indicates the presence of limited porosity in the mineral structure of these adsorbents [46, 56]. Although some authors have linked fluoride adsorption onto natural montmorillonites to rapid sorption rates associated with the short adsorption equilibrium periods of just 20–30 min [22, 37], several natural montmorillonites [36, 54] and Fe(III)-modified montmorillonites [55] appear to have consistent fluoride adsorption equilibrium time intervals in the range of 110–180 min. In the same way, a number of mineral adsorbents including metal-intercalated palygorskites [47] and certain aluminum oxide minerals [39, 44] appear to have equilibrium intervals within the same range of periods. This signifies that these minerals possess structural porosities that are comparable.

As in the case of fluoride adsorption onto calcareous and carbonaceous soil adsorbents, the immobilization of fluoride into adsorbent zeolites [50, 69, 70], hydroxyapatites [60, 63, 64], iron oxide minerals [49] as well as into certain classes of aluminum oxide minerals [41] is characterized by initial rapid phases of adsorption characterized typically by short equilibration intervals of just 10–30 min, which are then followed by prolonged equilibration that could extend to 10–48 h. The final slow phase of equilibration can be ascribed to high structural porosity as in the case of zeolites or to slow valence exchange reaction mechanisms characteristic of fluoride immobilization upon hydroxyapatite, hydr(oxide) aluminum and iron minerals.

#### **3.4. Co-existent ions**

Differences in pH of maximum fluoride uptake for various soil systems arise principally from the differences in the surface chemistry of the mineral adsorbents, which control the affinity of soil surfaces towards different fluoride species in soil surfaces. It can be assumed that soils that have high fluoride adsorptions in strongly acidic media of pH 5 or less have higher affinity for molecular HF species, which are dominant in this range of medium pH. The HF particles adsorb by forming continuous hydrogen bonds with electronegative centers in the soil surfaces. Certain soils that preferentially sorb fluoride in the near-neutral acidic pH values of

with positive cationic centers in the soil colloid structure, which include Al3+, Fe3+ and Si4+ among others. Soil adsorbent that sorb high fluoride levels over a wide range of pH values contains heterogeneous surfaces, which have attraction to several different fluoride species

The effect of temperature on fluoride adsorption onto soil surfaces arise from its influence on the adsorption energy balance, on the kinetics of adsorbate particles, and on the chemical activation of reacting species. Higher temperatures enhance increased rates of adsorption by enhancement of faster solute transport from the bulk solution towards the adsorbent surfaces. Higher temperatures also raise the average energy of the particles allowing a higher number of particles to attain necessary activation energy to enable them to react. Very high temperatures may, however, counter the adsorption fluxes leading to reduced rates and magnitude of

As for the effects of solution pH, however, the effect of temperature on fluoride adsorption on popular soil adsorbents is varied. The peak fluoride adsorption by natural montmorillonites [22, 36, 37, 54], Fe(III)-modified montmorillonite [55], pumice [56] and lignite [51, 52] occur within a range of temperatures close to room temperature (298 K). Nevertheless, the highest fluoride uptake by both aniline-modified montmorillonites and pyrole-modified montmorillonites [57] as well as by coal [52, 53] is favored by above room temperatures close to 303 K. It has been found that fluoride-exchange reactions for hydroxyapatites [58–60] and for certain ferric oxide minerals such as hematite [48, 49] can occur over a wide range of temperatures of 298–323 K. Fluoride adsorption onto Mg2+ and Al3+ [47], Fe3+ [61] and ZrO2+ [62] loaded palygorskite minerals; synthetic hydroxyapatites [63, 64]; calcareous minerals [65] and onto magnesia-loaded fly ash cenospheres (MLC) is favored by higher temperatures in the range of 303–323 K. This indicates the existence of endothermic chemical surface reactions. The efficacies of bauxite to sorb fluoride has, however, been found to decrease with increasing temperature indicating the existence of exothermic fluoride immobilization in bauxite surfaces [39, 44].

The resident time required for equilibration in an adsorption process depends mainly on the adsorbent structure and on the nature of reactions that occur between the adsorbate particles and reactive sites at the adsorbent surface. Adsorbents with compact crystalline structures and characteristically surface exposed reactive sites tend to have rapid rates of adsorption

species, and the mode of fluoride adsorption is mainly complexation

5–6 have affinity for F−

36 Soil pH for Nutrient Availability and Crop Performance

**3.2. Adsorption temperature**

uptake of the adsorbate by the adsorbents.

in solution.

**3.3. Contact time**

Natural water systems contain dissolved species across the organic-inorganic chemical continuum. Co-existent ions in water control the adsorption of fluoride by their competitive effect for the sorptive space on the adsorbent soil surfaces and by their influence on the adsorbate flux from the bulk solution to the sorbent surface. Co-ions tend to lower the rates and magnitude of adsorption, but the extent of these influence largely depend on the chemical and geometric dimensions of the ions, relative concentrations and affinities of the individual ions for the adsorbent surface. The influence of interfering ion, however, varies from one soil adsorbent to the other.

The soil adsorbents whose fluoride uptake is most affected by co-existent anions include iron oxide minerals [27–29] and certain carbonaceous mineral adsorbents. The suppression of fluoride immobilization upon ferric oxide minerals in the presence of common anions follows the order: PO<sup>4</sup> <sup>3</sup><sup>−</sup> <sup>&</sup>gt; SO<sup>4</sup> 2− <sup>&</sup>gt; Cl<sup>−</sup> <sup>&</sup>gt; NO3 − [29]. Fluoride adsorption onto zeolites [50], HAps [59, 63, 64], bauxite [39, 40] and calcareous mineral adsorbents [11, 12, 58, 66, 67, 71, 72] is, however, site specific, and it is not normally affected by competing anions in solution. For that reason, the adsorbents are able to sorb relatively high amounts of fluoride independent of co-existing anions such as Cl− NO3 − , SO4 2−, CH<sup>3</sup> COO<sup>−</sup> and PO<sup>4</sup> 3− ions.

[2] Ayoob S, Gupta K. Fluoride in drinking water: A review on the status and stress effects. Critical Reviews in Environmental Science and Technology. 2006;**36**:433-487. DOI:

Fluoride Adsorption onto Soil Adsorbents: The Role of pH and Other Solution Parameters

http://dx.doi.org/10.5772/intechopen.74652

39

[3] Kalisinska E, Natalia IB, Bird ÁBÁ. Fluoride concentrations in the pineal gland, brain and bone of goosander (Mergus merganser) and its prey in Odra River estuary in Poland. Environmental Geochemistry and Health. 2014;**36**:1063-1077. DOI: 10.1007/

[4] Kebede A, Retta N, Abuye C, Whiting SJ, Kassaw M, Zeru T, et al. Dietary fluoride intake and associated skeletal and dental fluorosis in school age children in rural Ethiopian Rift Valley. International Journal of Environmental Research and Public Health. 2016;**13**:756-

[5] Centre for Disease Control and Prevention (CDC). Private well water and fluoride. Priv. Well Water Fluoride. 2005:7-9. http://www.cdc.gov/fluoridation/fact\_sheets/wellwater.

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s10653-014-9615-6

htm

#### **4. Conclusions**

Soil adsorbents that have attracted the highest interest as possible adsorbents for the removal of fluoride from water include: aluminosilicates, iron and aluminum (hydr)oxides, apatites, carbonaceous minerals, calcareous soils and zeolites. It is found that the pH is the main solution factor controlling fluoride adsorption onto soil surface. The other contributing parameters include temperature, time of contact and co-existent ions. The montmorillonite clays, generally, solubilize in low pH media and get poisoned by excess OH− ions in alkaline media. They are generally characterized by small fluoride sorption edge within the neutral pH values. The usual pH for efficient fluoride removal from water using metalenhanced palygorskite is in the range of 32–2. Fluoride adsorption onto metal-exchanged zeolites and onto synthetic HAps is, however, independent of pH, and high fluoride adsorption occurs in the pH range of 4–10. Aluminum oxide minerals, on the other hand, usually have a narrow sorption edge in the pH range of 5.5–6.5. In the same way, maximum fluoride adsorptions onto most of the calcareous minerals occur within the pH values of 5–6. High fluoride uptakes by hematite occur over a wide range of pH (2–7) but optimum fluoride removal using carbonaceous adsorbents can be achieved at room temperature in the pH range of 4–12.

#### **Author details**

Enos Wamalwa Wambu\* and Audre Jerop Kurui

\*Address all correspondence to: wambuenos@yahoo.com

Department of Chemistry and Biochemistry, University of Eldoret, Eldoret, Kenya

#### **References**

[1] World Health Organisation. Writing Oral Health Policy: A Manual for Oral Health Managers in the WHO African Region. Brazzavile: WHO Regional Office for Africa; 2005

[2] Ayoob S, Gupta K. Fluoride in drinking water: A review on the status and stress effects. Critical Reviews in Environmental Science and Technology. 2006;**36**:433-487. DOI: 10.1080/10643380600678112

The soil adsorbents whose fluoride uptake is most affected by co-existent anions include iron oxide minerals [27–29] and certain carbonaceous mineral adsorbents. The suppression of fluoride immobilization upon ferric oxide minerals in the presence of common anions follows

bauxite [39, 40] and calcareous mineral adsorbents [11, 12, 58, 66, 67, 71, 72] is, however, site specific, and it is not normally affected by competing anions in solution. For that reason, the adsorbents are able to sorb relatively high amounts of fluoride independent of co-existing

and PO<sup>4</sup>

Soil adsorbents that have attracted the highest interest as possible adsorbents for the removal of fluoride from water include: aluminosilicates, iron and aluminum (hydr)oxides, apatites, carbonaceous minerals, calcareous soils and zeolites. It is found that the pH is the main solution factor controlling fluoride adsorption onto soil surface. The other contributing parameters include temperature, time of contact and co-existent ions. The montmoril-

alkaline media. They are generally characterized by small fluoride sorption edge within the neutral pH values. The usual pH for efficient fluoride removal from water using metalenhanced palygorskite is in the range of 32–2. Fluoride adsorption onto metal-exchanged zeolites and onto synthetic HAps is, however, independent of pH, and high fluoride adsorption occurs in the pH range of 4–10. Aluminum oxide minerals, on the other hand, usually have a narrow sorption edge in the pH range of 5.5–6.5. In the same way, maximum fluoride adsorptions onto most of the calcareous minerals occur within the pH values of 5–6. High fluoride uptakes by hematite occur over a wide range of pH (2–7) but optimum fluoride removal using carbonaceous adsorbents can be achieved at room temperature in the pH

lonite clays, generally, solubilize in low pH media and get poisoned by excess OH−

Department of Chemistry and Biochemistry, University of Eldoret, Eldoret, Kenya

[1] World Health Organisation. Writing Oral Health Policy: A Manual for Oral Health Managers in the WHO African Region. Brazzavile: WHO Regional Office for Africa; 2005

3− ions.

[29]. Fluoride adsorption onto zeolites [50], HAps [59, 63, 64],

ions in

the order: PO<sup>4</sup>

anions such as Cl−

**4. Conclusions**

range of 4–12.

**Author details**

**References**

Enos Wamalwa Wambu\* and Audre Jerop Kurui

\*Address all correspondence to: wambuenos@yahoo.com

<sup>3</sup><sup>−</sup> <sup>&</sup>gt; SO<sup>4</sup>

38 Soil pH for Nutrient Availability and Crop Performance

 NO3 − , SO4

2− <sup>&</sup>gt; Cl<sup>−</sup> <sup>&</sup>gt; NO3

−

2−, CH<sup>3</sup>

COO<sup>−</sup>


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**Section 4**

**Agro-ecology**


**Section 4**
