Mitigation and Remedy of Groundwater Arsenic Menace in India

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Mitigation and Remedy of Groundwater Arsenic Menace in India : A Vision DocumentThe adsorption and desorption reactions, arsenic species, Eh, pH and solid-phasedissolutions and precipitations may vary from aquifer to aquifer that depend upon thegeological settings, geo-chemistry and geo-environmental conditions of the aquifer.Therefore, rigorous geochemical investigation for adequate understanding of arsenicgeochemistry under different hydrogeological and geo-environmental conditions ofaquifers is essentially required for evolving sustainable solutions.1.3 Hypotheses on Mechanism of Arsenic MobilizationWidely accepted mechanisms of arsenic mobilization in groundwater are still to beestablished. However, based on arsenic geochemistry, three hypotheses describing probablemechanisms of As mobilization in groundwater specially, with reference to Holocene aquiferslike in West Bengal and Bangladesh, have been suggested ( Bose and Sharma, 2002). These are:(i)Mobilization of arsenic due to the oxidation of As-bearing pyrite minerals: InsolubleAs-bearing minerals, such as Arsenopyrite (FeAsS), are rapidly oxidized when exposedto atmosphere, realizing soluble As(III), sulfate (SO 42 -), and ferrous iron (Fe 2+ ). Thedissolution of these As-containing minerals is highly dependent on the availability ofoxygen and the rate of oxidation of sulfide. The released As(III) is partially oxidized toAs(V) by microbially mediated reactions. The chemical reaction is given by:FeAsS + 13 Fe 3+ + 8 H 2 O ⎯→ 14 Fe 2+ + SO 42 - + 13 H + + H 3 AsO 4 (aq.)(ii)Dissolution of As-rich iron oxyhydroxides (FeOOH) due to onset of reducing conditionsin the subsurface: Under oxidizing conditions, and in the presence of Fe, inorganicspecies of As are predominantly retained in the solid phase through interaction withFeOOH coatings on soil particles. The onset of reducing conditions in suchenvironments can lead to the dissolution of FeOOH coatings. Fermentation of peat inthe subsurface releases organic molecules (e.g., acetate) to drive reducing dissolution ofFeOOH, resulting in release of Fe 2+ , As 3+ , and As 5+ present on such coatings. Thechemical reaction is given by:8FeOOH - As (s) + CH 3 COOH + 14 H 2 CO 3 ⎯→ 8 Fe 2+ + As (d) +16 HCO 3 - + 12 H 2 Owhere As(s) is sorbed As, and As(d) is dissolved As.(iii)Release of As sorbed to aquifer minerals by competitive exchange with phosphate(H 2PO 4-) ions that migrate into aquifers from the application of fertilizers to subsurfacesoil.The second mechanism involving dissolution of FeOOH under reducing conditions is consideredto be the most probable reason for excessive accumulation of As in groundwater.NIH & CGWB 7
Arsenic: Source, Occurrence and Geochemistry1.4 SummaryArsenic (As) in soil and aquatic ecosystem can occur from: (i) geological (geogenic),(ii) anthropogenic (human activities), and (iii) biological (biogenic) sources. In soil and groundwater,it can be introduced during weathering of rocks and minerals followed by subsequentleaching and runoff or from anthropogenic sources. Biogenic sources are predominant in marineecosystem. The most common Arsenic bearing mineral is Arsenopyrite (FeAsS). In sedimentsand waters, As can be present in the form of arsenite (As(III)) and arsenate (As(V)). As(III) isnormally 60 times more toxic than As(V). In groundwater, arsenic species can be present invarious forms of arsenic acids, such as; H 3AsO 3, H 2AsO 3, HAsO 3, H 3AsO 4, H 2AsO 4, andHAsO 4. The standards of arsenic for drinking-water quality set by WHO and BIS are: 10 µg/Land 50 µg/L, respectively. Many factors control arsenic concentration, distribution and transportin groundwater. The factors, which govern the geochemistry of arsenic, are: red-ox potential,adsorption/desorption, precipitation /dissolution, arsenic speciation, pH, presence and concentrationof competing ions, and biological transformation. These factors vary from aquifer to aquifer thatdepend upon the geological settings, geo-chemistry and eco-environmental conditions.Out of three hypotheses describing (i) oxidation of As-bearing pyrite minerals andsubsequent release of As(III) , (ii) dissolution of As-rich iron oxyhydroxides (FeOOH) andresulting release of Fe(II), As(III), and As(V), and (iii) release of As sorbed to aquifer mineralsby competitive exchange with phosphate ions as probable mechanisms of As mobilizationin groundwater specially with reference to Holocene aquifers, the second hypothesis isbelieved to be the probable reason for excessive accumulation of As in groundwater. Theoxidation model is the one in which As(III) is oxidized to As(V) while in the reduction modelAs(V) is reduced to As(III).Questions thus to be answered are:(i)(ii)(iii)(iv)Whether the available literatures are adequate to resolve the issues of arseniccontamination in groundwater in India or, there is further need for investigation andstudy to strengthen understanding of geochemical processes for different hydrogeologicalconditions?Whether suggested hypotheses of As-mobilization in groundwater hold well for allhydro-geological and geochemical settings or for a select few?Whether occurrences of As in groundwater and their chemical composition containsimilar proposition?What are the controlling factors for As in different geochemical environments?8NIH & CGWB
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Mitigation and Remedy of Groundwater Arsenic Menace in India

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