Mitigation and Remedy of Groundwater Arsenic Menace in India

Sources and Causes of Groundwater Arsenic Contamination in Ganga-Brahmaputra PlainsFerric hydroxide is soluble in acidic aqueous environment, but it is precipitated in alkalineand reducing conditions at low Eh. Thus, if the acidity of the solution decreases (pH increases),colloidal precipitation of ferric hydroxide takes place. Some As (V) and As (III) ions beingabsorbed on the particles of Fe (OH) 3, may be co-precipitated with the latter. This reducesarsenic content of water. However, precipitation of As (V) and As (III) is not simultaneousbecause As (III) is 5 to 10 times more soluble than As (V) and its stability in aqueous solutionincreases with the alkalinity of water and reducing character of the environment. Thus, evenafter colloidal precipitation of As (V) ions with ferric hydroxide, the aqueous solution may containAs (III) ions in large amount. In mildly acid to neutral solution (pH < 7) or even in mildlyalkaline solution under oxygenated condition at Eh > 0, breakdown of ferric arsenate and ferric-2arsenite by hydrolysis can produce As (V) bearing arseneous acid (HAsO 4) and As(III)bearing arsenious acid (H 3AsO 3) respectively, together with ferric hydroxide in both cases. Therelevant equations are:(2H 2 O+O)FeAs 3+ – 2–O 4 ↔ HAsO 4 + Fe(OH) 33(H 2 O)FeAs 3+ O 3 ↔ H 3 As 3+ O 3 + Fe(OH) 3Arseneous acid (HAs 5+ O 42-) is the commonest of arsenate compounds in naturalwater as aqueous solution. In a mildly reducing environment, HAsO 42-is converted into As (III)-bearing arsenious acid (H 2AsO 31-) and in a strongly reducing condition into arsenious acid(H 3AsO 3). The change can be shown by the following equations:3H2–HAsO 41–→ H 2 AsO 3 + H 2 O,2H 22–HAsO 4 → H 3 AsO 3 + H 2 O.In the absence of As (III) in the source material (FeAsO 4), As(V)-bearing arseneous acid(HAsO 42-) can be formed by the hydrolysis of FeAsO 4in a mildly alkaline and oxygenatedenvironment and ferric hydroxide is produced at the same time.50NIH & CGWB