**Table 3.**

**51**

*2.2.3.1.1 Heavy metal pollutants*

**Figure 5.**

summarizes relevant information about Cr and Cu.

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

physicochemical properties of Cr are presented in **Table 5** [67].

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

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].

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

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

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

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