XAFS Determination of As(V) Associated with Fe(III) Oxyhydroxides in Weathered Mine Tailings and Contaminated Soil from California, U.S.A.

Abstract
The speciation of Arsenic (As) in soils and natural waters is critical in determining its environmental fate. As(III) is the most mobile and toxic of the inorganic arsenic species, but is readily oxidized to As(V), which although still quite toxic is far less mobile. Adsorption to sediment particles may remove As(V) from contaminated water, or the precipitation of arsenic minerals such as scorodite (FeAsO₄·2H₂O) may control the equilibrium aqueous concentration. We have employed XAFS spectroscopy to examine the solid-phase speciation of arsenic in mine tailings samples and a contaminated soil from California. Quantitative speciation of As was determined using XANES fitting methods and EXAFS analysis ; in all samples, As(V) predominates. In an oxidized tailings (no residual sulfides), we find As(V) adsorbed/coprecipitated on Fe(III) oxyhydroxides. In less oxidized tailings containing residual sulfides, there is evidence for scorodite a a similar Fe(III) arsenate and a reduced arsenic phase (arsenopyrite or arseniferous pyrite). In a soil contaminated by smelter waste, we detect Mg₃(AsO₄)₂·8 H₂O (hoernesite), and suggest the presence of an additional weakly bound As(V) species. XAFS spectroscopy is one of the few techniques for direct determination of arsenic speciation in complex matrices such as soils and mine tailings.