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 transport and distribution of arsenic in the environment is complex due to the manychemical forms in which it may be present and because there is continuous cycling of differentforms of arsenic through air, soil and water. Arsenic dissolved in water can be present in severaldifferent forms. In well-oxygenated water and sediments, nearly all arsenic is present in thestable form of arsenate (V). Some arsenite (III) and arsenate (V) forms are less stable and areinterchangeable, depending on the chemical and biological conditions. Some chemical forms ofarsenic adhere strongly to clay and organic matter, which can affect their behavior in the environment.Weathered rock and soil, containing arsenic, may be transported by wind or watererosion. Arsenic releases into the atmosphere by industrial processes or volcanic activity andattaches to particles that are dispersed by the wind and fall back to the ground.Long term exposure to arsenic in drinking water has variety of health concerns includingseveral types of cancers, cardiovascular diseases, diabetes, and neurological effects. Manycountries, including Bangladesh, India, Taiwan, Mongolia, Vietnam, Argentina, Chile, Mexico,Ghana and the United States, exposed to arsenic have problems because the sources of arsenicare primarily natural rather than anthropogenic or geothermal. Inorganic arsenic of geologicalorigin has been recognized as the main form of arsenic in groundwater.Different Countries have set different standards of arsenic content for drinking-waterquality. WHO's norms for drinking-water quality go back to 1958; in that year, the internationalStandard for drinking-water was established at 200 µg/L as an allowable concentration for As.In 1963 the standard was re-evaluated and reduced to 50 µg/L. The WHO guidelines have beenrevised during the recent past and the permissible limits have been reduced from 50 µg/L to 10µg/L (10 ppb) in year 1993 due to adverse health reports arising from different parts of the worldwhere arsenic is causing severe health problems. India has set its maximum permissible limit forarsenic to 50 µg/L. The standards set by different countries for As for drinking-water qualityare given in Table-1.2.Table 1.2: Standards of Arsenic in potable water set by different Countries.Country/InstitutionUS-EPA, USAAustraliaCanadaBIS, IndiaLatest, WHO’s norm (1993)Allowable level of Arsenic in drinking water10 µg/L or 10 ppb7µg/L or 7 ppb25 µg/L or 25 ppb50 µg/L or 50 ppb10 µg/L or 10 ppbNIH & CGWB 3
Arsenic: Source, Occurrence and Geochemistry1.1 Occurrences in GroundwaterArsenic is introduced into soil and groundwater during weathering of rocks and mineralsfollowed by subsequent leaching and runoff. It can also be introduced into soil and groundwaterfrom anthropogenic sources. These sources are localized and therefore, important in some geologicsettings; biogenic sources can be predominant in marine ecosystem, whereas natural sourcesare primarily from the parent (or rock) material from which they are derived and therefore, aremain causes of concern. There is another mode of occurrence of arsenic, namely Organoarsenic,which is mostly less toxic than both As(III) and As(V) and therefore, less harmful than other twoforms. In groundwater, inorganic arsenic commonly exists as arsenate [As(V)] and arsenite[As(III)] . Inter-conversion of As(V) and As(III) takes place by oxidation of As(III) to As(V)and reduction of As(V) to As(III). High concentrations of arsenic tend to occur in sulphideminerals and metal oxides, especially iron oxides. Many factors control arsenic concentrationand transport in groundwater. An understanding of factors controlling the distribution of arsenicin groundwater requires knowledge of arsenic sources and of processes controlling arsenicmobility. This knowledgebase would help in guiding monitoring and remediation efforts.1.2 Geochemistry of ArsenicGeochemistry of arsenic is controlled by many factors, that include:• Red-ox potential• Adsorption/ desorption• Precipitation/dissolution• Arsenic speciation• pH• Biological transformationIn general, most naturally occurring arsenic compounds are insoluble in water.1.2.1 Red-ox potentialRedox potential, symbolically termed as Eh is also known as reduction potential, meansthe tendency of a chemical species to acquire electrons and thereby to reach to a reduced state.Arsenic is a redox-sensitive element. This means that arsenic may gain or lose electrons in redoxreactions. As a result, it may be present in a variety of redox states. Arsenate generallypredominates under oxidizing conditions, while arsenite predominates when conditions becomesufficiently reducing. Under the pH conditions of most groundwater, arsenate is present as thenegatively charged oxyanions H 2AsO 4-or HAsO 42-, whereas arsenite is present as theuncharged species H 3AsO 3. Natural geochemical and biological processes play critical role incontrolling the fate and transformation of arsenic in the subsurface. Arsenite is thermodynamicallyunstable in aerobic environments and oxidizes to As(V). Presence of other oxides such asFeO, Fe 2O 3, MnO 2and even clay minerals is capable of oxidizing As(III).4NIH & CGWB
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5.2.2 Co-precipitationMitigation an
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5.8 SummaryMitigation and Remedy of
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Mitigation and Remedy of Groundwater Arsenic Menace in India

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