Monosodium methylarsenate (MSMA) is a selective contact herbicide used for post-emergent control of a very broad spectrum of weeds. In water, MSMA dissociates to monomethylarsonic acid (MMA) and sodium ions (Na+), and it can be metabolized by certain types of soil-dwelling microorganisms to form dimethylarsinic acid (DMA) through methylation, or inorganic arsenate through demethylation. The rate of metabolism decreases with time as a result of increased binding with soil minerals in a biphasic process. Its soil binding potential is proportional to the amorphous iron content of the soil and, to a lesser extent, soil pH - - more specifically, the extent of dissociation (a function of the pKa) - - and clay content. A recent soil column study, combined with a comprehensive analysis of an extensive database of published literature, have provided insight into quantifying and predicting the mechanisms, factors, and sorption kinetics. The sorption chemodynamics of MMA are highly relevant to determine the partitioning of the chemicals into the aqueous phase. The results of the analysis of the biphasic behavior, in the context of multi-factor environmental chemistry, will be presented. This work serves to resolve several of the questions regarding the environmental fate of MSMA.

Biphasic sorption and transformation are key factors in the environmental fate of the herbicide monosodium methylarsenate

2019 Fall National Meeting

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