<strong>Mitigation</strong> <strong>and</strong> <strong>Remedy</strong> <strong>of</strong> <strong>Groundwater</strong> <strong>Arsenic</strong> <strong>Menace</strong> <strong>in</strong> <strong>India</strong> : A Vision DocumentAdsorptionActivated alum<strong>in</strong>a 88 43Activated alum<strong>in</strong>a 103 51Ion exchange 92 45Ion exchange 103 51Manganese green s<strong>and</strong> 110 6.8-37.7 64-94Membrane separationReverse osmosis 91 1 99Electrodialysis 85 23 73Reverse osmosis 260 2.6 99Coagulation/micro-filtration 50 906.3 Future <strong>of</strong> Remediation Approaches6.3.1 In-situ Treatment <strong>of</strong> <strong>Arsenic</strong> <strong>in</strong> Aquifer by Remov<strong>in</strong>g Dissolved IronThere is wide-scale report <strong>of</strong> the presence <strong>of</strong> dissolved iron, <strong>in</strong> arsenic contam<strong>in</strong>atedgroundwater <strong>in</strong> many countries, <strong>and</strong> <strong>of</strong> co-precipitation <strong>of</strong> iron <strong>and</strong> arsenic under oxidiz<strong>in</strong>gcondition. In the <strong>in</strong>vestigated region <strong>of</strong> Nadia District, West Bengal, it has been found that Ascorrelates with Fe <strong>in</strong> groundwater both positively <strong>and</strong> negatively, depend<strong>in</strong>g upon the condition.This raises the hope <strong>of</strong> a plausible way <strong>of</strong> <strong>in</strong> situ remediation <strong>of</strong> the problem <strong>of</strong> As contam<strong>in</strong>ationby removal <strong>of</strong> Fe from groundwater before withdrawal.In situ Fe removal has proved to be a viable technique for dim<strong>in</strong>ish<strong>in</strong>g Fe concentration<strong>in</strong> groundwater. The technique <strong>in</strong>volves a cyclic <strong>in</strong>jection <strong>of</strong> oxygenated water, <strong>in</strong> which Fe <strong>and</strong>Mn concentrations are lower than <strong>in</strong> the native groundwater. It is applied <strong>in</strong> a number <strong>of</strong>European countries <strong>and</strong> <strong>in</strong> the US (Hallberg <strong>and</strong> Mart<strong>in</strong>ell, 1976; Rott et al. 1978; Booch <strong>and</strong>Barovich, 1981; Van Beek, 1983; Rott <strong>and</strong> Lamberta, 1993; Meyerh<strong>of</strong>f, 1996). The reaction<strong>in</strong>volves the displacement <strong>of</strong> ferrous iron exchange <strong>and</strong> sorption sites <strong>and</strong> subsequent oxidationby oxygen. Clogg<strong>in</strong>g has not been observed <strong>and</strong> appears to be unimportant, by virtue <strong>of</strong>self-regulatory nature <strong>of</strong> Fe 2 exchange <strong>and</strong> sorption mechanism. (Appelo et al., 1999)Increas<strong>in</strong>g oxygen concentration <strong>in</strong> <strong>in</strong>jected water is useless, because efficiency is limited byexchangeable Fe2, capable <strong>of</strong> consum<strong>in</strong>g the oxidant dur<strong>in</strong>g the <strong>in</strong>jection stage.Quantification <strong>of</strong> the reaction mechanism allows assessment <strong>of</strong> operational conditions.The gross mechanism <strong>of</strong> <strong>in</strong> situ oxidation appears to be simple as a given amount <strong>of</strong> oxidant is<strong>in</strong>jected <strong>and</strong> is consumed by reduced substances <strong>in</strong> the aquifer. The problem is how the dissolvedoxidant, such as O 2, <strong>in</strong> <strong>in</strong>jected water reaches the dissolved reductant, Fe 2 , <strong>in</strong> groundwater, whileNIH & CGWB 115
A Critical Appraisal- Future Risk, Scope to Remediate, Technological Competence, etc.the latter is be<strong>in</strong>g displaced dur<strong>in</strong>g <strong>in</strong>jection. The essence <strong>of</strong> <strong>in</strong> situ treatment is, <strong>in</strong> fact, the ironremoval, <strong>and</strong> hence, arsenic removal, it cont<strong>in</strong>ues even after the complete withdrawal <strong>of</strong> the<strong>in</strong>jected water. The result<strong>in</strong>g ferric iron is highly <strong>in</strong>soluble <strong>and</strong> precipitates as an oxyhydroxide. Ithas been found that clogg<strong>in</strong>g does not occur, even <strong>in</strong> the case <strong>of</strong> systems, operat<strong>in</strong>g for morethan 30 years. The lack <strong>of</strong> clogg<strong>in</strong>g suggests that precipitation takes place at a distance awayfrom the well <strong>and</strong> possibly at vary<strong>in</strong>g locations <strong>in</strong> time.In the aquifer, the fronts spread out due to dispersion <strong>and</strong> the comb<strong>in</strong>ed effects <strong>of</strong>transport <strong>and</strong> reaction. When a new run is started with the <strong>in</strong>jection <strong>of</strong> oxygenated water, theconcentration <strong>of</strong> iron <strong>in</strong>creases gradually <strong>in</strong> the well on pump<strong>in</strong>g, <strong>and</strong> its efficiency depends onthe limit<strong>in</strong>g concentration <strong>of</strong> Fe 2 . With each cycle 1000 cubic meter <strong>of</strong> oxygenated water may be<strong>in</strong>jected <strong>and</strong> 7000 cubic meter pumped out. The ensu<strong>in</strong>g runs show a delayed rise <strong>of</strong> ironconcentration, <strong>in</strong> the pumped water. In other words, the efficiency <strong>in</strong>creases with the number <strong>of</strong>runs. The efficiency can better be improved, by optimiz<strong>in</strong>g the well arrangement, for example, by<strong>in</strong>stall<strong>in</strong>g separate <strong>in</strong>jection <strong>and</strong> pump<strong>in</strong>g wells, to prevent the last part <strong>of</strong> oxygenated water,be<strong>in</strong>g withdrawn without reaction.<strong>Groundwater</strong> pH should be above 6 for <strong>in</strong> situ iron removal, because rapid decrease <strong>of</strong>oxidation rate <strong>of</strong> Fe 2 occurs when pH is below 6. Moreover, if the aquifer conta<strong>in</strong>s lot <strong>of</strong> sulphides,the oxidation acidifies the system. And aquifers should be as homogeneous as possible. And thisshould be without extremely coarse layers to prevent preferential flow <strong>of</strong> <strong>in</strong>jected water, throughthe most permeable parts, which generally have low exchange capacity. Thus, plann<strong>in</strong>g for <strong>in</strong>situ remediation, with <strong>in</strong>jection <strong>of</strong> oxygenated water (pH ~7.5), four times a year, is a plausiblelong last<strong>in</strong>g mitigation technique for decontam<strong>in</strong>ation <strong>of</strong> arsenic <strong>in</strong> groundwater.Researchers <strong>in</strong> the Queens University, Belfast, claimed to have developed a low-costtechnology, which <strong>of</strong>fers chemical-free groundwater arsenic treatment technology to providearsenic-free water to rural communities for dr<strong>in</strong>k<strong>in</strong>g <strong>and</strong> farm<strong>in</strong>g needs. The technology is basedon recharg<strong>in</strong>g a part <strong>of</strong> the groundwater, after aeration, <strong>in</strong>to a subterranean aquifer (permeablerock), which is able to hold water. Increased levels <strong>of</strong> oxygen, <strong>in</strong> the groundwater, slow down thearsenic release from the soil. At higher dissolved oxygen levels, soil micro organisms as well asiron <strong>and</strong> manganese reduce the dissolved arsenic level significantly. Based on this concept, a trialplant <strong>in</strong> Kasimpore near Kolkata was planted <strong>and</strong> its performance was found satisfactory.6.3.2 Limestone-based <strong>Arsenic</strong> Removal MethodsExperiments have been performed, us<strong>in</strong>g Limestone to remove arsenic from water. Thisapproach is supported by previous research, regard<strong>in</strong>g the removal <strong>of</strong> arsenic by the formation<strong>of</strong> calcium arsenate (Bothe <strong>and</strong> Brown, 1999). Mobilization <strong>of</strong> arsenic, from sediment, is mostlikely when the sediment is low <strong>in</strong> iron <strong>and</strong> calcium carbonate (Brannon <strong>and</strong> Patrick, 1987). Areasonable conclusion is that arsenic is immobilized <strong>in</strong> iron <strong>and</strong>/or calcium compounds. Work on116NIH & CGWB
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