Arsenic Speciation and Redox Chemistry in Acid Mine Drainage

摘要

Increasing concern about arsenic in drinking water of the United States has prompted lowering the arsenic drinking-water maximum contaminant level (MCL) to 10 ?g/L. Arsenic concentrations greater than the MCL in ground- and surface-water samples can be the result of natural phenomena, but often are influenced by anthropogenic activities, such as mining. Arsenic speciation is a factor in risk assessment because of the varying toxicity of different arsenic species. Remediation of arsenic in acid mine drainage (AMD) often is highly dependent on arsenic speciation because of the different sorptive and mobility properties of inorganic arsenic species. Because of the redox conditions of AMD, arsenic speciation often is in near thermodynamic equilibrium with the dominant iron redox couple; this result leads to a wide range of arsenite to arsenate ratios in AMD. Instances of disequilibrium are more common in ground- and surface-water matrices. Results from the authors' recent work show the relation between arsenic speciation and iron chemistry in AMD is controlled by Eh and pH, yet this result does not always hold in other natural matrices. Further work has been done involving photochemical oxidation of arsenite to arsenate, which could be implemented in a passive treatment system for AMD. The complex nature of AMD also requires the development of robust and reliable speciation methods and preservation procedures for studies of arsenic speciation. A laboratory high-performance liquid chromatographic speciation method has been employed with a new preservation technique using cation complexation with EDTA and storage in opaque bottles to remove photooxidation as a factor in interspecies conversions. Extension of this method is currently underway for selenium speciation in similar natural-water matrices.