*Water Chemistry*

*Pollution of Water Sources from Agricultural and Industrial Effluents: Special Attention… DOI: http://dx.doi.org/10.5772/intechopen.86921*

**Figure 5.** *Solubility curves for common metals in freshwater with pH [60].*

of metal speciation and redox conditions is hexavalent chromium (Cr(VI)). Many of metals are transition metals (such as cadmium and Cu) or heavy metals (such as lead and silver) that can form metalloids which bond to organic compounds to form lipophilic substances that are often highly toxic. Metals are also lost from solution by precipitation as the pH changes [7, 60, 61]. **Table 4** shows the ranking of these metals according to their toxicity through biological and carcinogenicity tests [62].

#### *2.2.3.1.1 Heavy metal pollutants*

Any metallic element with relatively high density as compared to water and toxic even at low concentrations is termed as "heavy metal" [63, 64]. Among these metals, Cr is one of the top 16 major toxic contaminants that have detrimental effects on human health [64, 65]. Besides Cr, Cu is generally considered as a highly harmful metal at high concentration [66]. Accordingly, the following section discusses and summarizes relevant information about Cr and Cu.

Cr as a metallic element was first discovered and isolated in 1797 by the French chemist Nicolas-Louis Vauquelin [35, 67]. The world resources of Cr exceed 10.9 billion metric tons of shipping-grade chromite [67]. Ferric chromite (FeCr2O4) is the principal Cr ore, found mostly (with 96% of the world's reserves) in South Africa. Minor common sources include chrome ochre (Cr2O3), and crocoite (PbCrO4) are also present [58]. Cr is widely used in engineering and chemical industries because of its durability and esthetic quality. The principal uses for Cr are metallurgical (67%), refractories (18%), and chemical (15%). In addition, Cr exits in three stable forms in the environment with different oxidation states and ionic nature [65]. The physicochemical properties of Cr are presented in **Table 5** [67].

*Water Chemistry*

**50**

**Nanomaterial**

**Type** Nanocomposite

Chitosan/Zeolite/nano ZrO2

**Table 3.**

*Comparison of NO3ˉ removal by different nanomaterials [53].*

—

20 mg/L

3

0.02 g

35 °C

60 min

23.58 mg/g

Doped composite

20 mg/L

6–8

50 mg

293–

30 min

83%

313 K

**Modification**

**Initial NO3ˉ**

**pH**

**Nanomaterial** 

**T°**

**Contact** 

**NO3ˉ removal** 

**efficiency**

**Time**

**dose**

**concentration**


#### **Table 4.**

*Classification of metals according to their toxicity [62].*


#### **Table 5.**

*Physical and chemical properties of Cr [67].*

The oxidation states of Cr can go from −2 to +6; however, only the +6 and +3 oxidation states are commonly encountered in the environment. Cr(III) is highly insoluble, relatively immobile, and most thermodynamically stable [58], while Cr(VI) has high water mobility and solubility and can be easily reduced. Cr(VI) is recognized to be more poisonous (100 times more toxic than Cr(III)), mutagenic, and carcinogenic in nature [68].

#### *2.2.3.1.1.1 Chromium VI sources*

To date, naturally occurring Cr(VI) in groundwater has been detected in the following geologic environments: (i) chromite ore bodies, (ii) arid alluvial basins in the southwest United States, (iii) saline brines in evaporate basins, and (iv)

**53**

*Pollution of Water Sources from Agricultural and Industrial Effluents: Special Attention…*

serpentinite ultramafic terrains [67], while the major industrial source of Cr(VI) emissions are (i) chemical manufacturing industry, (ii) metal finishing industry, (iii) manufacturers of pharmaceuticals, (iv) electrical and aircraft manufacturers, (v) cement-producing plants (as cement contains Cr), and (vi) production of wood, stone, glass, and clay products. Owlad et al. [69] reported the concentration of Cr(VI) in the wastewater of several industries (as in **Table 6**) and estimated the concentration of Cr(VI) in wastewater caused by industries between 0.1 and

In aqueous solution, Cr(VI) may exist in a variety of oxospecies (viz., dichro-

depending on pH of the solution, total Cr concentration, the presence of oxidizing and reducing compounds, the redox potential, and kinetics of the redox reactions [64, 70]. At pH < 1, the predominant species is H2CrO4, while as the pH is raised

concentrations are greater than 1 mM, or it may even dominate when the total Cr(Vl) concentrations are greater than 30 mM. At a pH > 8 only the yellow ion

centration, and H2CrO4 is a very strong acid [70]. The chemical equilibria which are

↔ H<sup>+</sup> + CrO4

↔ Cr2 O7

Ingesting large quantity of Cr(VI), either by human or animal, can be corrosive to the skin and eyes and causes stomach upsets and ulcers, convulsions, kidney and liver damage [72, 73]. Allergic reactions consisting of severe redness and swelling of the skin have been noted. Some evidence on animal studies show that Cr(VI) compounds can cause cancer in various tissues due to the low water insolubility [35, 67, 72]. Indeed, due to the fact that Cr(VI) is unstable in the body, easy to diffuse across the membrane, and is reduced intracellularly, it can provide very reactive pentavalent Cr and trivalent Cr which can alter DNA. Ingestion of 1.0–5.0 g of Cr(VI) as chromate results in severe acute gastrointestinal disorders, hemorrhagic diathesis, and convulsions [74]. In 1998, Dartsch et al. [75] noticed that 5 mmol/L

Furthermore, Cr(VI) compounds at high concentration are extremely toxic to plants and retard their growth. Cr and Cr(VI) compounds can cause severe phytotoxicity which may result in reduction of seed germination, nutrient imbalance, degradation of pigments, decrease of antioxidant and enzyme concentration, and oxidative stress [76]. Indeed, Rout et al. [77] demonstrated a 25% reduction in seed germination in the presence of 200 μM Cr(VI). According to Davies et al. [78] Cr

<sup>2</sup><sup>−</sup> anions prevail.

<sup>2</sup><sup>−</sup> dominates in acidic solution at Cr(VI) concentration above

<sup>2</sup><sup>−</sup> dominates in basic solutions independently of Cr(VI) con-

<sup>2</sup><sup>−</sup>), H2CrO4, and acid chromates (HCrO4

<sup>−</sup>))

<sup>−</sup> dominates when the

<sup>2</sup><sup>−</sup> becomes significant when

2− (5)

2− + H2O (6)

↔ H<sup>+</sup> + HCrO4 (4)

*DOI: http://dx.doi.org/10.5772/intechopen.86921*

200 mg/L.

mates (Cr2O7

CrO2

*2.2.3.1.1.2 Speciation of Cr(VI)*

from pH 2 to 6, the HCrO4

<sup>−</sup> exists. Cr2O7

0.01 M, while CrO4

<sup>2</sup><sup>−</sup>), chromates (CrO4

<sup>−</sup> and Cr2O7

H2 CrO4 +

> H CrO4 −

> > −

According to Palmer and Puls [71] under pH 6.5, HCrO4

Cr(VI) concentrations are low (<30 mM), but Cr2O7

involved for Cr(VI) dissociation are as in Eqs. (4)–(6):

*2.2.3.1.1.3 Cr(VI) effects on human health and environment*

2 HCrO4

Cr(VI) resulted in 50% cell death.

*Pollution of Water Sources from Agricultural and Industrial Effluents: Special Attention… DOI: http://dx.doi.org/10.5772/intechopen.86921*

serpentinite ultramafic terrains [67], while the major industrial source of Cr(VI) emissions are (i) chemical manufacturing industry, (ii) metal finishing industry, (iii) manufacturers of pharmaceuticals, (iv) electrical and aircraft manufacturers, (v) cement-producing plants (as cement contains Cr), and (vi) production of wood, stone, glass, and clay products. Owlad et al. [69] reported the concentration of Cr(VI) in the wastewater of several industries (as in **Table 6**) and estimated the concentration of Cr(VI) in wastewater caused by industries between 0.1 and 200 mg/L.

#### *2.2.3.1.1.2 Speciation of Cr(VI)*

*Water Chemistry*

Carcinogenicity

Toxicity

**Table 4.**

Density (g/cm3

**Bioaccumulative character Chemical elements**

Non-carcinogenic Cr, Ni, Cd, Zn, Cu

Relatively toxic elements Pb, Ni, As, Cu, Cr(VI)

Atomic number 24 Atomic mass 51.996 Atomic radius (pm) 185 Main oxidation state(s) +2, +3, +6

Electronegativity (pauling) 1.66

Melting/boiling point (°C) 1.907/2.671 Isotopes 4 stable + 17 unstable Acid/base of oxide Strong acid State (st 27 °C, 1 atm) Solid Metallic character Metal Element group(s) Transition element Affinity Lithophile

Very bioaccumulative Pb, Zn, Cu Relatively bioaccumulative Hg, Ni, Cd Slightly bioaccumulative As, Cr

Strong carcinogenicity As, Pb

Group of highly toxic elements Cd, Hg

*Classification of metals according to their toxicity [62].*

**Physical and chemical properties of Cr**

Slightly toxic elements Cr(III), Zn

**52**

**Table 5.**

and carcinogenic in nature [68].

*Physical and chemical properties of Cr [67].*

*2.2.3.1.1.1 Chromium VI sources*

The oxidation states of Cr can go from −2 to +6; however, only the +6 and +3 oxidation states are commonly encountered in the environment. Cr(III) is highly insoluble, relatively immobile, and most thermodynamically stable [58], while Cr(VI) has high water mobility and solubility and can be easily reduced. Cr(VI) is recognized to be more poisonous (100 times more toxic than Cr(III)), mutagenic,

Ionic radius (pm) 87–94 (+2), 75.5 (+3), 55–69 (+4), 48.5–71 (+5), 40–58 (+6)

) 7.19

To date, naturally occurring Cr(VI) in groundwater has been detected in the following geologic environments: (i) chromite ore bodies, (ii) arid alluvial basins in the southwest United States, (iii) saline brines in evaporate basins, and (iv)

In aqueous solution, Cr(VI) may exist in a variety of oxospecies (viz., dichromates (Cr2O7 <sup>2</sup><sup>−</sup>), chromates (CrO4 <sup>2</sup><sup>−</sup>), H2CrO4, and acid chromates (HCrO4 <sup>−</sup>)) depending on pH of the solution, total Cr concentration, the presence of oxidizing and reducing compounds, the redox potential, and kinetics of the redox reactions [64, 70]. At pH < 1, the predominant species is H2CrO4, while as the pH is raised from pH 2 to 6, the HCrO4 <sup>−</sup> and Cr2O7 <sup>2</sup><sup>−</sup> anions prevail.

According to Palmer and Puls [71] under pH 6.5, HCrO4 <sup>−</sup> dominates when the Cr(VI) concentrations are low (<30 mM), but Cr2O7 <sup>2</sup><sup>−</sup> becomes significant when concentrations are greater than 1 mM, or it may even dominate when the total Cr(Vl) concentrations are greater than 30 mM. At a pH > 8 only the yellow ion CrO2 <sup>−</sup> exists. Cr2O7 <sup>2</sup><sup>−</sup> dominates in acidic solution at Cr(VI) concentration above 0.01 M, while CrO4 <sup>2</sup><sup>−</sup> dominates in basic solutions independently of Cr(VI) concentration, and H2CrO4 is a very strong acid [70]. The chemical equilibria which are involved for Cr(VI) dissociation are as in Eqs. (4)–(6):

$$\text{H}\_2\text{CrO}\_4^\* \leftrightarrow \text{H}^\* + \text{HCrO}\_4 \tag{4}$$

$$\rm{HCrO\_4^- \leftrightarrow H^+ + CrO\_4^{2-}}\tag{5}$$

$$2\,\text{HCrO}\_4^- \leftrightarrow \text{Cr}\_2\text{O}\_7^{2-} + \text{H}\_2\text{O} \tag{6}$$

#### *2.2.3.1.1.3 Cr(VI) effects on human health and environment*

Ingesting large quantity of Cr(VI), either by human or animal, can be corrosive to the skin and eyes and causes stomach upsets and ulcers, convulsions, kidney and liver damage [72, 73]. Allergic reactions consisting of severe redness and swelling of the skin have been noted. Some evidence on animal studies show that Cr(VI) compounds can cause cancer in various tissues due to the low water insolubility [35, 67, 72]. Indeed, due to the fact that Cr(VI) is unstable in the body, easy to diffuse across the membrane, and is reduced intracellularly, it can provide very reactive pentavalent Cr and trivalent Cr which can alter DNA. Ingestion of 1.0–5.0 g of Cr(VI) as chromate results in severe acute gastrointestinal disorders, hemorrhagic diathesis, and convulsions [74]. In 1998, Dartsch et al. [75] noticed that 5 mmol/L Cr(VI) resulted in 50% cell death.

Furthermore, Cr(VI) compounds at high concentration are extremely toxic to plants and retard their growth. Cr and Cr(VI) compounds can cause severe phytotoxicity which may result in reduction of seed germination, nutrient imbalance, degradation of pigments, decrease of antioxidant and enzyme concentration, and oxidative stress [76]. Indeed, Rout et al. [77] demonstrated a 25% reduction in seed germination in the presence of 200 μM Cr(VI). According to Davies et al. [78] Cr


#### **Table 6.**

*Industrial wastewater containing Cr(VI) [69].*

is mostly toxic to higher plants at 100 μg/kg dry weights. Sinha et al. [79] reported that Cr is toxic for most agronomic plants at a concentration of about 0.5–5.0 mg/L in nutrient media and of 5–100 mg/g under soil condition. The Cr toxicity in plants affects photosynthesis in terms of CO2 fixation, photophosphorylation, electron transport, and enzyme activities as reported by H. Oliveira [76, 80].
