Multi‐element effects on arsenate accumulation in a geochemical matrix determined using µ‐XRF, µ‐XANES and spatial statistics

Soils regulate the environmental impacts of trace elements, but direct measurements of reaction mechanisms in these complex, multi‐component systems can be challenging. The objective of this work was to develop approaches for assessing effects of co‐localized geochemical matrix elements on the accumulation and chemical speciation of arsenate applied to a soil matrix. Synchrotron X‐ray fluorescence microprobe (µ‐XRF) images collected across 100 µm × 100 µm and 10 µm × 10 µm regions of a naturally weathered soil sand‐grain coating before and after treatment with As(V) solution showed strong positive partial correlations ( r ′ = 0.77 and 0.64, respectively) between accumulated As and soil Fe, with weaker partial correlations ( r ′ > 0.1) between As and Ca, and As and Zn in the larger image. Spatial and non‐spatial regression models revealed a dominant contribution of Fe and minor contributions of Ca and Ti in predicting accumulated As, depending on the size of the sample area analyzed. Time‐of‐flight secondary ion mass spectrometry analysis of an area of the sand grain showed a significant correlation ( r = 0.51) between Fe and Al, so effects of Fe versus Al (hydr)oxides on accumulated As could not be separated. Fitting results from 25 As  K ‐edge microscale X‐ray absorption near‐edge structure (µ‐XANES) spectra collected across a separate 10 µm × 10 µm region showed ∼60% variation in proportions of Fe(III) and Al(III)‐bound As(V) standards, and fits to µ‐XANES spectra collected across the 100 µm × 100 µm region were more variable. Consistent with insights from studies on model systems, the results obtained here indicate a dominance of Fe and possibly Al (hydr)oxides in controlling As(V) accumulation within microsites of the soil matrix analyzed, but the analyses inferred minor augmentation from co‐localized Ti, Ca and possibly Zn.