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 (Pb) pollution in groundwater is a cumbersome and daunting affair for the global population. The manufacturing plants of lead acid batteries contribute to lead pollution in groundwater. Old pipes that contain lead are direct sources of lead contamination in water. The industrial sources and vehicle exhaust also produce Pb contamination in water. Pb is addressed as a normal constituent of The Earth's crust and it is found into two oxidation states as 0 and 2. It exists in water as Pb2+, PbOH+, and PbHCO3+ at normal pH (Raviraja et al. 2008). The exposure to a small amount of Pb by humans is more lethal than other heavy metal contamination (Hashim et al. 2011).
4.3.3.5 Antimony
Intake of antimony (Sb) in lower quantities is good for human health. But the acute exposure to Sb (more than prescribed limit as given in Table 4.1) is lethal for the human body. The Sb contamination is released in water by anthropogenic activities, heavy commercial activities, and natural activities. Sb has similar chemical and physical properties as arsenic because Sb and As belong to same family in the periodic table (Willis et al. 2011). As arsenic, Sb(III) is also more toxic than Sb(V) (WHO 2003).
4.3.3.6 Zinc
Zinc (Zn) is a 3D block element which is found in the Earth's crust. Generally, Zn is found in +2 oxidation states and it forms compounds with the anions, amino acid, and organic acid. Zn is soluble in water bodies at neutral and acidic pH values. At basic pH, it makes the carbonate and hydroxide compounds, which are insoluble in water. The anthropogenic and natural sources release the Zn contamination in ground as well as surface water. Zn is an essential element for the human body in limited quantities. The exposure to high concentrations of Zn is harmful for many age groups (Hashim et al. 2011).
4.3.3.7 Chromium
Chromium (Cr) belongs to 3D block elements and it exists in three valence states as metallic state Cr(0), Cr(III), and Cr(VI). Cr(III) is insoluble in water; it only forms hydroxide and oxy‐hydroxide and solid solution with iron (Fe), while Cr(VI) is resolvable in water under environmental conditions. Cr occurs in the form of chromate (CrO42−) and dichromate ions (Cr2O72−). Cr(VI) is more lethal than Cr(III). Chromium pollution in water is accumulated by natural sources as well as anthropogenic sources. However, industrial sources as chrome plating, steel production, corrosion inhibition, wood preservative, well drilling, and paint and primer pigments lead to chromium contamination in water. Nevertheless, natural sources like rock ores also lead to chromium contamination in groundwater (Zhitkovich 2011).
4.4 Effects on the Human Body
The various diseases like nausea, vomiting, diarrhoea, cyanosis, cardiac arrhythmia, confusion and hallucinations can arise in the human body owing to short‐ term consumption of arsenic. The long‐term consumption of arsenic through food, water, and other sources is associated with various types of health problems like skin disease (hyperpigmentation), skin cancer, tumours in lungs, bladder, kidneys, and liver, spontaneous abortion, depression, numbness, sleeping disorders, and headaches (Ali et al. 2012; Singh et al. 2015). Poisonous levels diverge between various compounds; for example, monomethyl arsenic acid and inorganic arsenide have higher levels of deadliness than arsenic choline. Acute toxicity is generally higher for inorganic arsenic compounds than for organic arsenic compounds (Hughes 2002). Arsenic can also cause low birth weight. Owing to the serious effect of arsenic on people's health, WHO recommended 10 μgL−1 As in drinking water in 2001 (WHO 2001). The US Environmental Protection Agency and European Commission have also revised the maximum contaminant level (MCL) for arsenic in drinking water to 50–10 μgL−1 (EPA 2002; European Commission Directive 1998).
As discussed in Section 4.3.3.2, Hg is found in three forms as metallic, inorganic, and organic Hg. The exposure of organic mercury is more lethal than inorganic mercury. The organic Hg is consumed through fish, water, and food in the body and it can cause different human health issues. The long‐term consumption of Hg causes various symptoms in the human body like damage to the nervous system and circulatory system, skin problems, and kidney disease. It also causes rheumatoid arthritis, which shows symptoms over a long period (Järup 2003, Azimi et al. 2017; Filter 2020). Owing to their poisonous nature, WHO and BIS reported the acceptable limit of mercury in water as summarized in Table 4.1.
The impact of Cd on human health is more dangerous. When Cd is exposed in very low quantities, it causes nausea with vomiting, headache, and cough.