Mitigation and Remedy of Groundwater Arsenic Menace in India

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Mitigation and Remedy of Groundwater Arsenic Menace in India : A Vision Document1.2.2 Adsorption/ DesorptionTwo categories of processes largely control arsenic mobility in aquifers: (i) adsorptionand desorption reactions, and (ii) solid-phase precipitation and dissolution reactions. Attachmentof arsenic to an iron oxide surface is an example of an adsorption reaction. The reverse of thisreaction i.e., detachment of arsenic from such a surface, is an example of desorption. Arsenicadsorption and desorption reactions are influenced by changes in pH, occurrence of redox(reduction/oxidation) reactions, presence of competing anions, and solid-phase structural changesat the atomic level.Arsenate and arsenite adsorb to surfaces of a variety of aquifer materials, including ironoxides, aluminum oxides, and clay minerals. Adsorption and desorption reactions betweenarsenate and iron-oxide surfaces are important controlling reactions because arsenate adsorbsstrongly to iron-oxide surfaces in acidic and near-neutral-pH water. However, desorption ofarsenate from iron-oxide surfaces becomes favored as pH values become alkaline. As a resultof the pH dependence of arsenic adsorption, changes in groundwater pH can promoteadsorption or desorption of arsenic. Similarly, redox reactions can control aqueous arsenicconcentrations by their effects on arsenic speciation and hence, arsenic adsorption anddesorption. Arsenic adsorption can also be affected by the presence of competing ions.Structural changes in solid phases at the atomic level also affect arsenic adsorption anddesorption.1.2.3 Precipitation and DissolutionThe various solid phases (minerals, amorphous oxides, volcanic glass, and organiccarbon) of aquifer material can exist in a variety of thermodynamic states. Solid-phaseprecipitation is the formation of a solid phase from components present in aqueous solution.Precipitation of the mineral calcite, from calcium and carbonate present in groundwater, is anexample of solid-phase precipitation. Dissolution of volcanic glass within an aquifer is anexample of solid-phase dissolution. At any given time, some aquifer solid phases undergodissolution, whereas others precipitate from solution. Arsenic contained within solid phases,either as a primary structural component or an impurity in any of a variety of solid phases, isreleased to groundwater when those solid phases dissolve. Similarly, arsenic is removed fromgroundwater when solid phases containing arsenic precipitate from aqueous solution. As anexample, arsenic often co-precipitates with iron oxide; iron oxide, in such case, may act as anarsenic source (case of dissolution) or a sink (case of precipitation) for groundwater. Solid-phasedissolution contributes not only arsenic contained within that phase, but also any arsenic adsorbedto the solid-phase surface. The process of release of adsorbed arsenic, as a result of solid-phasedissolution, is distinct from the process of desorption from stable solid phases. Solid-phaseprecipitation and dissolution reactions are controlled by solution chemistry, including pH, redoxstate, and chemical composition.NIH & CGWB 5
Arsenic: Source, Occurrence and Geochemistry1.2.4 Arsenic SpeciationSpeciation of an element in water sample means determination of the concentration ofdifferent physico-chemical forms of the element which together make up its total concentration inthe sample. The speciation of arsenic in environmental material is of interest because of the differentlevels of toxicity exhibited by various species. Species illustrate the various oxidation states thatarsenic commonly exhibits (-3, 0, +3, +5) and the resulting complexities of itschemistry in the environment. Nearly two dozen arsenic species are present in the environmentaland biological systems. Arsenic in groundwater is present in various species like, H 3AsO 3, H 2AsO 3,HAsO 3, H 3AsO 4, H 2AsO 4, and HAsO 4. Arsenic species are generally present as arsenate [As(V)]or arsenite [As(III)] for Eh conditions prevalent in most groundwater. Both As(V) and As(III) formprotonates oxyanions in aqueous solutions and the degree of protonation depends on pH.Differences in their toxicity, biochemical and environmental behaviors require the determinationof these individual arsenic species.1.2.5 Influence of pHAlteration in the state of arsenic oxidation is usually influenced by Eh and pH. The mostsuitable pH for arsenic dissolution is low acidic (pH 12.1.2.6 Influence of competing ionsOther intrinsic factors of a system which can also exert marked influence on theconcentration and speciation of arsenic include: solution composition, competing ions especially,the ratio of Phosphorous to As, and Selenium to As, nature and composition of solid phasespresent, reaction kinetics and flow regime.1.2.7 Biological transformationArsenic undergoes a series of biological transformations in the aquatic environment,yielding a large number of compounds, especially organo-arsenicals. Certain reactions, such asoxidation of As(III) to As(V), may occur both in the presence and absence of microorganisms,whereas other reactions such as methylation, are not thermodynamically favorable in water andcan occur only in the presence of organisms, which indicates that many aquatic organisms arecapable of accumulating arsenic and may catalyse the oxidation of As(III) to As(V). Biologicaltransformation is significantly important in marine ecology.6NIH & CGWB
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Mitigation and Remedy of Groundwater Arsenic Menace in India

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