Mitigation and Remedy of Groundwater Arsenic Menace in India

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Mitigation and Remedy of Groundwater Arsenic Menace in India : A Vision Documentthat impact contaminant concentrations. Increases in pH favor the precipitation of carbonates ofcalcium and iron as well as insoluble metal hydroxides. Decreases in Eh drive reduction ofmetals and metalloids with multiple oxidation states. Finally, an increase in the partial pressure ofhydrogen in subsurface systems supports the activity of various chemotrophic organisms thatuse hydrogen as an energy source, especially sulfate-reducing bacteria and iron-reducing bacteria.Arsenate ions bind tightly to the iron filings, causing the ZVI to be oxidized to ferrousiron, aerobically or anaerobically in the presence of water. The process results in a positivelycharged iron surface that sorbs the arsenate species by electrostatic interactions (Su and Puls,2001). In systems where dissolved sulfate is reduced to sulfide by sulfate-reducing bacteria,arsenic may be removed by precipitation of insoluble arsenic sulfide (As2S3) or co-precipitatedwith iron sulfides (FeS).PRBs can be constructed by excavating a trench of the appropriate width andbackfilling it with a reactive medium. Commercial PRBs are built in two basic configurations: thefunnel-and-gate and the continuous wall. The funnel-and-gate uses impermeable walls, forexample, sheet pilings or slurry walls, as a "funnel" to direct the contaminant plume to a "gate(s)"containing the reactive media, while the continuous wall transects the flow path of the plumewith reactive media (EPA, 1998).PRBs are a passive treatment technology, designed to function for a long time with littleor no energy input. They produce less waste than active remediation, as the contaminants areimmobilized or altered in the subsurface. PRBs can treat ground water with multiple contaminantsand can be effective over a range of concentrations. PRBs require no above ground equipment,except monitoring devices, allowing return of the property to economic use during remediation.PRBs are best applied to shallow, unconfined aquifer systems in unconsolidated deposits, as longas the reactive material is more conductive than the aquifer (EPA, 2001a,b ).The technology relies on the natural movement of ground water; therefore, aquiferswith low hydraulic conductivity can require relatively long periods of time to be remediated. Inaddition, PRBs do not remediate the entire plume, but only the portion of the plume that haspassed through the PRB. Because cleanup of ground water contaminated with arsenic has beenconducted at only few sites, the long-term effectiveness of PRBs for arsenic treatment has notbeen demonstrated fully (EPA, 2001a,b).5.3.2 PhytoremediationPhytoremediation is an in-situ technology applicable to contaminated soil and groundwater. It is designed to use plants to degrade, extract, contain or immobilize contaminants in soil,sediment or ground water (Zhang et al., 2001). Typically, trees with deep roots are applied toground water and other plants are used for shallow soil contamination. This technology tends tohave low capital, operating and maintenance costs relative to other arsenic treatmenttechnologies because it relies on the activity and growth of plants.NIH & CGWB 95
Technological Options and Arsenic Removal TechnologiesThe mechanisms of phytoremediation include phytoextraction (also known asphytoaccumulation, the uptake of contaminants by plant roots and the translocation/accumulation of contaminants into plant shoots and leaves), enhanced rhizospherebiodegradation (takes place in soil or ground water immediately surrounding plant roots),phytodegradation (metabolism of contaminants within plant tissues), and phytostabilization(production of chemical compounds by plants to immobilize contaminants at the interface ofroots and soil). Most applications of phytroremediation for arsenic removal include phytoextractionand phytostabilization.Experimental research into identifying appropriate plant species for phytoremediation isongoing. It is generally applicable only to shallow soil or relatively shallow ground water that canbe reached by plant roots. In addition, the phytoremediating plants may accumulate high levels ofarsenic during the phytoremediation process, and may require additional treatment prior todisposal.The selection of the phytoremediating species depends upon the species' ability to treatthe contaminants and the depth of contamination. Plants with shallow roots (e.g. grasses) areappropriate only for contamination near the surface, typically in shallow soil. Plants with deeperroots, (e.g. trees) may be capable of remediating deeper contaminants in soil or ground waterplumes.Examples of vegetation, used in phytoremediation, include sunflower, Indian mustard,corn, and grasses (such as ryegrass and prairie grasses) (EPA, 2001b). Some plant species,known as hyperaccumulators, absorb and concentrate contaminants within the plant at levelsgreater than the concentration in the surrounding soil or ground water. The ratio of contaminantconcentration in the plant to that in the surrounding soil or ground water is known as thebioconcentration factor. A hyperaccumulating fern (Pteris vittata) has been used in the remediationof arsenic-contaminated soil, waste and water. The fern can tolerate as much as 1,500 ppm ofarsenic in soil, and can have a bioconcentration factor up to 265. The arsenic concentration inthe plant can be as high as 2% (dry weight) (Ma et al., 2001; Zhang et al., 2001).5.3.3 Biological TreatmentAlthough biological treatments have usually been applied to the degradation of organiccontaminants, some innovative techniques have applied biological remediation to the treatmentof arsenic. This technology involves biological activity or micro-organisms that promoteprecipitation/co precipitation of arsenic from water and leaching of arsenic in soil. Biologicalprecipitation/co precipitation processes for water create ambient conditions intended to causearsenic to precipitate/co precipitate or act directly on arsenic species to transform them intospecies that are more amenable to precipitation/co precipitation. The microbes may besuspended in the water or attached to a submerged solid substrate.96NIH & CGWB
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ACKNOWLEDGEMENTThe Government of In
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nearly 33 villages in 4 districts i
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(iii) Whether ex-situ removal techn
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6.4 Experiences during Arsenic Remo
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List of FiguresFigure no. Figure ca
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List of TablesTable no. Table Capti
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Mitigation and Remedy of Groundwater Arsenic Menace in India

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