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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.