Arsenic(III) Complexation and Oxidation Reactions on Soil

The complexation and oxidation reactions of arsenic(III) were investigated in three soils using standard batch reactions, a stirred reactor, and X-ray absorption spectroscopy. The objective was to identity soil components responsible for As(III) oxidation and the surface sites which bind the As(V) product. Speciation of As(III) and As(V) was determined using HPLC coupled with hydride generation atomic absorption spectrometry. Certain soils and soil minerals such as manganese oxides caused rapid oxidation of As(III) and formation of strongly adsorbed As(V) surface complexes. The coordination environment of As(V) in soil was determined to be predominantly an Fe oxide surface complex. Based on results from extended X-ray absorption fine structure (EXAFS) spectroscopy, the As-Fe inter-atomic distance of 3.38 angstroms was indicative of an inner-sphere, bidentate surface complex. This paper discusses the kinetics and mechanism of As(III) oxidation in soil and the use of EXAFS as a probe of the As(V) coordination environment.