to human health, as well as animals and marine life, owing to their carcinogenic nature. These include arsenic (As), mercury (Hg), lead (Pb), cadmium (Cd), chromium (Cr), zinc (Zn), antimony (Sb), and nickel (Ni). The exposure to heavy metals by human beings is lethal and toxic (Mohankumar et al. 2016; Talabi and Kayode 2019; Ahamad et al. 2020; Pooja et al. 2020). To address the carcinogenic nature of the elements, various foundations like the WHO, Bureau of Indian Standard (BIS), United States Environmental Protection Agency (US‐EPA), European Commission Environment (ECE), Australian Drinking Water Guidelines (ADWG), and Norma Official Mexicana (NOM)‐127‐SSA1‐1994 prescribe the limit (Fernandez‐Luqueno et al. 2013) of contaminants in water as described in Table 4.1.
4.3.3.1 Arsenic
Arsenic (As), the twentieth‐most abundant element in the Earth’s crust, is a well‐known deadly poison from time immemorial (Mandal and Suzuki 2002). The compounds of arsenic in water are found in both organic and inorganic moieties. The organic moieties of arsenic existing in compound form are monomethylarsonicacid (MMA[V]), dimethylarsinic acid (DMA[V]), monomethylarsonous acid (MMA[III]), dimethylarsinous acid (DMA[III]), thiomethylarsonic acid (Thio MMA), thiodimethylarsinic acid (Thio DMA), arsenobetaine, and arsenocholine (Gomez‐Caminero et al. 2001). Apart from this, the inorganic compounds of As are about 100 times more toxic than the organic ones and are more prevalent in water (Jain and Ali 2000). The major inorganic compounds of arsenic as pentavalent arsenate ions [As(V)] like H3AsO4, H2AsO4−, HAsO42−, and AsO43− are found in surface water and trivalent arsenite ions [As(III)] like H3AsO3, H2AsO3−, and HAsO32− are found in groundwater (Jain and Ali 2000). As(V) is found as an oxidizing agent in surface water. Arsenic exists as H3AsO4 at pH > 2 and in the form like H2AsO4− and HAsO42− at pH 2–11. However, As(III) is found under reducing conditions at low pH. Considering this, As(III) is more toxic than As(V). Although As(III) is more lethal, the metabolism of As(V) plays an important role in its toxicity in the human body. Arsenate has the same structure as the phosphate ion and it can replace the phosphate ion from enzymatic reactions, which occur in the human body (Fan et al. 2018). The mechanism of As(V) in human body is completed in the following steps (Thomas et al. 2001; Hughes 2002; Fan et al. 2018):
Table 4.1 Acceptable limits of heavy metals in ppb in drinking water reported by different foundations.
| Heavy metal | WHO | ECE | USEPA | ADWG | NOM‐127 | BIS |
|---|---|---|---|---|---|---|
| Arsenic | 10 | 10 | 10 | 10 | 25 | 10 |
| Mercury | 6 | 1 | 2 | 1 | 1 | – |
| Cadmium | 3 | 5 | 5 | 2 | 5 | 3 |
| Chromium | 50 | 100 | 50 | 50 | 50 | 50 |
| Antimony | 20 | 5 | 6 | 3 | – | – |
| Lead | 10 | 10 | 15 | 10 | 10 | 10 |
| Nickel | 70 | – | 20 | 20 | – | 20 |
| Zinc | – | – | 500 | 3000 | 5000 | 5000 |
As(V) involves to reduce As(III) in presence of glutathionine (GSH). GSH takes part in reduction reaction as an electron donor.
As(III) takes place oxidative methylation to pentavalent state (As[V]) in presence of S‐adenosylmethyl.
4.3.3.2 Mercury
Mercury (Hg) has received more attention in groundwater owing to their carcinogenic nature. The different path of mercury contributes to groundwater pollution. For instance, coal‐fired plants, smelting, alkali processing, and other industrial actions cause mercury pollution in groundwater. Natural activities are another cause of mercury contamination in groundwater. Apart from this, over the last decades, metallic mercury has been utilized in various fields like medical fields (thermometers, barometers, and other instruments for measuring blood pressure), which are causes of mercury pollution in groundwater, including the consumption of calomel and mercury amalgam to healing teeth (feeling and diuretics) in the field of dentistry contributing to Hg groundwater pollution (Barringer et al. 2013). The utilization of Hg as voltametric sensor to detect the trace metals in water is other reason for the contribution of Hg to water contamination. The inorganic Hg is less toxic than organic mercury. However, inorganic Hg is easily transformed into methyl mercury as organic compound, which is more stable and exposed to fish. Humans consume the organic Hg through the food chain like fish consumption and dental amalgam (Järup 2003; Hashim et al. 2011).
4.3.3.3 Cadmium
Cadmium is symbolized by Cd and belongs to 3D block elements. Cd is introduced as a toxic element and the sources of Cd are rock, coal, and petroleum. Cd is often found in combination with zinc. Cd is found in two forms: metallic form and cationic form (Cd+2) (Smith 1995). Both natural and human sources contribute to the cadmium impurities in groundwater. The industrial activities, like manufacturing of batteries (NiCd), pigments, plastic, and electroplating, directly discharge into the water and contaminate the water bodies. Other activities like mining, seepage of hazardous waste materials from sites, and discharge of waste industrial water cause the increasing concentration of cadmium in water day by day. On the other side, agricultural sources like cadmium‐containing phosphate fertilizers are also producing cadmium contamination in water resources (Ryan et al. 2000; Järup 2003; Hashim et al. 2011).
4.3.3.4 Lead
Lead
