Modeling the Reactivity of Arsenate in Relation to Microscale Composition of a Soil Matrix.

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 multi-component systems. Our objective was to quantify spatial relationships among elements within microsites of a soil matrix and arsenate accumulation and speciation. Synchrotron X-ray fluorescence microprobe maps (~1 µm x 1µm spot size) were collected across a 100 µm x 100 µm region before and after treating an oxide-coated quartz sand grain (~ 2 mm grain size) with 0.1 mM As(V) solution. Analyzing spatial patterns of As accumulation in relation to natural spatial patterns of Ca, Cr, Cu, Fe, Mn, Ni, Si, Ti, and Zn, arsenic showed the strongest correlations with Fe, Ti, and Ca (r = 0.85, 0.52, and 0.33, respectively). However, the As-Ti and As-Ca correlation was positive conditioned on rest of the elements present, mainly Fe (partial correlation (r') = 0.05 for As-Ti and r' = 0.11 for As-Ca). Also, analyzing elemental distribution using a spatial likelihood regression model, a significant interactions (α = 0.05) of Fe with Ti and Ca in predicting As accumulation was found. Linear combination fits of As K-edge microscale X-ray- absorption near edge structure (µ-XANES) spectra collected from selected hotspots across the full image, and 100 spectra collected within a 10 µm x 10 µm region of varying elemental composition indicated localized variations in As speciation. Both sets of µ-XANES spectra were fit with varying proportions of XANES spectra from standards of As(III/V) bound to organic matter containing Fe(III), and As(V) adsorbed on boehmite (AlOOH). Aluminum-bonded arsenate standards were dominant in high Fe regions, consistent with a spatial correlation (r = 0.51) between Fe and Al measured with time-of-flight secondary mass spectrometry (TOF-SIMS). In essence, significant interactions of Fe-Ti, and Fe-Ca on arsenate accumulation suggest that multiple co-localized components in soil microsites may affect the reactivity of trace elements in soils.