Abstract

Sorption of heavy metals plays a vital role in controlling environmental pollution. Here, we reviewed the sorption of heavy metals such as Ni, Co, Cu, Zn, V, Pb, Hg, In, As, Cd, Cr, Ga, Cs, Mn, V, Eu, Mo, Th, TI and Cr on metal oxides and clay minerals. The mechanism of association between these ions and the host minerals, and the factors controlling their sorption are discussed in detail. Both chemical and empirical methods of describing sorption mechanism are discussed. The sorption processes depend on the pH, metal concentration, ionic strength, temperature, time, adsorbent dosage, type of ion, surface area, type of adsorbent modification and nature of adsorbent. The review confirmed that both metal oxides and clay have capability of sequestering heavy metals, however, combination of both metal oxides and modified clay have enhanced capability of removing heavy metals from aqueous solution. These inorganic adsorbents have the regeneration and recycling potentials and can be used to remediate and sequester economic metals for commercial purposes, however, this needs future investigation.

Keywords: sorption, adsorption, heavy metals, remediation, oxides, clay minerals

## 1. Introduction

Heavy metal pollution is an environmental problem that has harmful effect on both aquatic and terrestrial environment. These metals are released to the environment through activities such as mining, electroplating and manufacturing of paper and pesticides in form of mine tailings or effluents [1]. They have ability to complex with minerals to form inorganic ligands with variable solubility and acid–base potentials, thus, making their remediation from contaminated land/soil a major concern [2, 3].

Pollution arising from heavy metals poses serious health problems to both human beings and animals [1]. For example, low concentration of cadmium in human being can lead to 'Itai-Itai' disease, lungs cancer, and kidney and liver problems [4, 5]. Ingestion of water contaminated with high level of Cr can cause ulcer, skin inflammation and pulmonary congestion [6, 7]. Health problems associated with ingestion or inhaling of nickel dust are diarrhoea, renal oedema, nausea, chest pain, pulmonary fibrosis and gastrointestinal ache. Excess of lead in the environment can lead to mental retardation, kidney and nervous disease, cancer [8, 9]. The Cu2+ is the most toxic form of copper [10], and health problems

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*Advanced Sorption Process Applications*

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1994;**28**:1171-1188. DOI: 10.1016/0045-6535(94)90335-2 associated with copper are anaemia, headache, kidney damage and death [11, 12]. Large amount of zinc in human body can result to anaemia, fever, vomiting and skin inflammation [13]. Mercury ingestion can lead to damaging of reproductive and respiratory system, kidney and brain [14]. Also, health problem associated with arsenic are gastrointestinal problems, cancer, diabetes, and liver tumours [15]. Large concentration of Mn causes retardation of growth, infertility, fever, eye blindness and muscles [15]. Consumption of water with high concentration of cobalt can lead to heart problems, asthma, damage of thyroid, pneumonia, hair and weight loss, nausea and vomiting, [16]. Depending on the route and length of exposure to vanadium, it can lead to immunological, genetoxic, developmental, reproductive and systemic problems and even death [17]. Exposure to indium via respiration can lead to lung disease, kidney and lungs damage [18, 19]. Long term exposure of gallium may damage the kidney, liver, lungs and bone marrow [20]. Health effects of molybdenum are headache, weight loss, joint or muscle ache, diarrhoea, kidney and liver problems [21]. Exposure to radioactive caesium can lead to cells damage, cancer, diarrhoea, vomiting, nausea and mental abnormality in babies [22]. Exposure to thorium workers can cause lung and liver disease, cancer, liver diseases and death at large exposure [23].

[34]. Similarly, synthesising and discovering new novel methods of using modified

To this end, this paper presents the review the sorption of heavy metals such as Ni, Co, Cu, Zn, V, Pb, Hg, In, As, Cd, Cr, Ga, Cs, Mn, Eu, Mo, Th, TI and Cr on both natural and synthetic metal oxides and clay minerals for soil and water remediation. The main aim of this review is to summarise the adsorption mechanism and recent remediation method of single and multicomponent system of these toxic metals using clay minerals and metal oxides. In addition, the advantages of reus-

The interactions between metallic ions and mineral surfaces in any environment allow the sorption of ions to the solid surface, thereby decreasing the concentration of ions in the aqueous phases below the solubility limits of the solid phase [35]. Determination of partitioning of heavy metals among the solid phase and accurately predicting their mobility in natural environment requires adequate knowledge of the chemical process such as ion exchange, adsorption, surface precipitation, coprecipitation and mineralisation, that govern sorption mechanism [36]. For example, cobalt sorbs to goethite through surface complexation, surface

precipitate and structural incorporation depending on the concentration and sorp-

groups or hydroxyl groups of solute ligands [38]. These functional groups are dominant in natural environments as oxides of Al, Fe and Si and in aqueous solution. The surface charge of metallic oxy-hydroxides, phosphate and carbonates is formed through ionisation of surface groups [39]. The pH of surface oxide and oxyhydroxides changes due to adsorption of protons or ions from solution [40]. Thus, the surface charge set up potential differences between the sorbent and sorbate, thereby influencing the approach of ions towards the surfaces [40]. Consequently,

surface hydroxyl groups are responsible for complexation on metal oxides, oxyhydroxides, hydroxides and aluminosilicates. The type of surface functional group influences the sequence of the adsorption reaction, variation in adsorption solution chemistry, electrical properties of the interface and the adsorption ability [41]. The reaction of metals and oxygen atoms at the surface results in the formation of hydroxyl groups (OH�), which are taken as part of the surface instead of solution. The surface hydroxyl is amphoteric (i.e. they can either accept a proton or

Surfaces of substances (oxides, organic and inorganic) have surface functional

Adsorption of metal to the surface takes place via substitution of surface protons

ð1Þ

ð2Þ

ð3Þ

clay for environmental remediation is required metals [25].

Sorption of Heavy Metals on Clay Minerals and Oxides: A Review

DOI: http://dx.doi.org/10.5772/intechopen.80989

ability these adsorbents after remediation are discussed.

2. Adsorption mechanism and models

2.1 Adsorption mechanism

tion duration [37].

donate a proton) as shown below:

) to form inner sphere complexes:

(H<sup>+</sup>

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Clay minerals and metal oxides are formed from weathering of primary minerals [24]. These minerals have high surface area and can absorb heavy metals from aqueous and natural environments [25]. Clay minerals, bentonite, sepiolite and palygorskite have been used extensively to remove heavy metals from wastewater and agricultural soil, and the mechanisms of remediation are sorption, precipitation and liming [26]. Heavy metal polluted water is mostly remediated using ion-exchange, adsorption and mechanism [27]. Even though, clay minerals have adsorption affinity for heavy metals, e.g. Cu and Zn, their roles are not significant when compared to iron oxides (e.g. goethite), manganese oxides (e.g. birnessite) and organic materials on a unit mass basis [28]. Comparison of the adsorption potentials of deep sea minerals such as clay minerals, Mn-oxides, Fe-oxides, calcite and apatite reveal that Mn and Fe oxides and oxyhydroxides are crucial phases for scavenging heavy metals [29] Lead readily sorbs on all phases but has greatest affinity for carbonate fluorapatite, however, Caesium (Cs) has affinity for illite [29]. In sea water, hydrated cations such as Mn2+, Co2+, Ni2+, Zn2+, Cu2+, Ba2+, MnCl<sup>+</sup> and PbCl<sup>+</sup> have strong affinity for Mn-oxide while HVO4 <sup>2</sup>, MoO4 <sup>2</sup>, PbCO3 and HAsO4 <sup>2</sup> preferentially adsorb onto Fe oxyhydroxide [29].

Sorption techniques have the ability to remove these contaminants via adsorption onto sorbents. However, the type of sorbent to be used for remediation depends on the sorption capacity, sorption pH and distribution coefficient of the contaminant [30]. The review on the sorption of Ni(II) adsorbents indicates that bisorbents are the most effective for removal of the ion from aqueous solution [31]. In addition, Pb and Cd be successfully immobilised with wide range of low cost materials such as metal oxides, animal manure, apatite, lime, biosolids, and biochar [32]. However, the review on the adsorption capabilities of low-cost sorbents such as agricultural waste, household wastes, industrial by-products and sludge indicates that even though these material has great adsorption potentials their particle size, surface area, contact time, initial concentration of ion, adoption dose are not stated by the researchers [33]. This poses great limitation for the use of these adsorbents for remediation. Further review of the sorption of nickel, copper, lead, cadmium, caesium, chromium, europium and thorium on both natural and synthetic materials reveal that more studies on multi-component sorption of these metals are required [34]. Therefore, synthetizing effective and low cost adsorbent that can be recycled for removal of contaminants from aqueous environment will be of high interest

#### Sorption of Heavy Metals on Clay Minerals and Oxides: A Review DOI: http://dx.doi.org/10.5772/intechopen.80989

[34]. Similarly, synthesising and discovering new novel methods of using modified clay for environmental remediation is required metals [25].

To this end, this paper presents the review the sorption of heavy metals such as Ni, Co, Cu, Zn, V, Pb, Hg, In, As, Cd, Cr, Ga, Cs, Mn, Eu, Mo, Th, TI and Cr on both natural and synthetic metal oxides and clay minerals for soil and water remediation. The main aim of this review is to summarise the adsorption mechanism and recent remediation method of single and multicomponent system of these toxic metals using clay minerals and metal oxides. In addition, the advantages of reusability these adsorbents after remediation are discussed.
