Arsenic-contaminated groundwater is of concern because it is geographically widespread including Bangladesh, West Bengal, India, Asia, South America and the U.S.. Arsenic distribution in the environment results from both natural weathering processes and anthropogenic activities. Of the several oxidation states of arsenic species, trivalent arsenic (arsenite or As(III)) and pentavalent arsenic (arsenate or As(V)) are predominant in groundwater and surface water. Arsenic speciation in the environment is affected by redox potential and pH. Under reducing conditions and below 9.2 pH, arsenite exists predominantly as neutral species (H3AsO_3~0). Arsenate occurs as oxyanions (H2AsO_4~- and HAsO_4~(2-)) under oxidizing conditions at neutral pH values. However, both arsenite and arsenate species are often present in either oxidized or reduced environments because of their slow kinetic transformations. Worldwide attention is generated because of arsenic poisoning to humans. Exposure to arsenic causes human health problems such as carcinogenesis and non-carcinogenesis at even low concentration (e.g., ppb levels). In January 2006, the U.S. Environmental Protection Agency lowered the arsenic drinking water standard to 10 μg/L. To meet the standard, coagulation, sorption and membrane methods are used in drinking water utilities; however, comprehensive and appropriate methods are not currently available to treat arsenic in drinking water.
展开▼
