Microscale Accumulation and Speciation of Arsenic in a Soil Matrix in Relation to Its Microsite Composition.

Soils regulate the fate of potentially toxic trace elements in the environment, but models of trace-element retention and mobilization are confounded by the complexity of these multicomponent systems. Our objective was to determine whether interactions between multiple inorganic components affect arsenic retention and speciation within microsites of a soil matrix. Synchrotron X-ray fluorescence microprobe maps (1 µm² spot size) collected before and after treating an oxide-coated quartz sand grain with 0.1 mM As(V) solution showed spatial patterns of As accumulation in relation to soil-matrix element distributions, and As K-edge micro-XANES analyses indicated As speciation. Of the ten elements of varying abundances mapped in the grain coating, accumulated As showed the strongest positive correlations with Fe (r = 0.88) and Ti (r = 0.43), but the As – Ti correlation was positively conditioned on Fe (conditional correlation = 0.13). Linear combination fitting of five micro-XANES spectra from a 100 x 100 µm map region showed 30% variation in proportions of two standards of As(V) bonded with Fe(III), indicating localized variations in As speciation. No discernable spectral differences across one-hundred micro-XANES spectra from a 10 x 10 µm region containing varying proportions of Fe, Ti, Zn, Cu, and Ca would suggest homogeneous species despite heterogeneous elemental compositions. Our results would then suggest that Fe(III)  dominated the accumulation and speciation of As(V) in the grain coating, with minor effects of other elements analyzed, particularly Ti.