Multiscale assessment of methylarsenic reactivity in soil. 2. Distribution and speciation in soil.

Methylated forms of arsenic (As), monomethylarsenate (MMA) and dimethylarsenate (DMA), have historically been used as herbicides and pesticides. Because of their large application to agriculture fields and the toxicity of MMA and DMA, the distribution, speciation, and sorption of methylated As to soils requires investigation. Monomethylarsenate and DMA were reacted with a soil up to one year under aerobic and anaerobic conditions. Microsynchrotron based X-ray fluorescence (μ-SXRF) mapping studies showed that MMA and DMA were heterogeneously distributed in the soil and were mainly associated with iron oxyhydroxides, e.g., goethite, in the soil. Micro-X-ray absorption near edge structure (XANES) spectra collected from As hotspots showed MMA and DMA were demethylated to arsenate over one year incubation under aerobic conditions. Monomethylarsenate was methylated to DMA, and DMA was maintained as DMA over a 3 month incubation under anaerobic conditions. Arsenic-iron precipitation, such as the formation of scorodite (FeAsO(4)·2H(2)O), was not observed, indicating that MMA and DMA were mainly associated with Fe-oxyhydroxides as sorption complexes.