Multiscale assessment of methylarsenic reactivity in soil. 1. Sorption and desorption on soils.

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 persistency of these compounds in the environment is of great concern. MMA and DMA sorption and desorption were investigated in soils, varying in mineralogical and organic matter (OM) contents. Sorption studies showed that the MMA sorption capacity and rate were greater than DMA sorption. Al/Fe-oxyhydroxides were the main sorbents in the soils, and the sorption capacity was proportional to the Al/Fe concentration in the soils. Extended X-ray absorption fine structure (EXAFS) studies showed that both MMA/DMA-Fe interatomic distances were around 3.3 Å, which were indicative of bidentate binuclear inner-sphere complex formation. Desorption studies showed that not all of the sorbed MMA or DMA was desorbed due to the strong binding between MMA/DMA and Al/Fe-oxyhydroxide surfaces via possible inner-sphere complex formation. The amount of the desorbed MMA and DMA decreased as the sorption residence time increased. For example, 77% of sorbed MMA was desorbed from the Reybold subsoil after 1 day residence time, while 66% of sorbed MMA was desorbed from the soil after six months of residence time. The decreases in desorption were likely due to As speciation changes from MMA/DMA to inorganic arsenate, which was more strongly bound to the surface.