Kinetics of Dimethylated Thioarsenicals and the Formation of Highly Toxic Dimethylmonothioarsinic Acid in Environment.

Dimethylmonothioarsinic acid (DMMTA) is a highly toxic, thiolated analogue of dimethylarsinic acid (DMA). In comparison, a further thiolated analogue, dimethyldithioarsinic acid (DMDTA), and DMA both exhibit lower toxicity. To understand the environmental conditions responsible for forming DMMTA, the kinetics of DMA thiolation are examined. The thiolation of DMA is pH-dependent and consists of two consecutive first-order reactions under excess sulfide conditions. The first thiolation of DMA to form DMMTA is faster than the second one to DMDTA. DMMTA is therefore an intermediate. The first reaction is first-order in HS at pH 6.0 and 20 °C; therefore, the overall reaction is second-order and the rate coefficient in this condition is 0.0780 M s. The rate coefficient significantly decreases at pH 8.0, indicating that HS(aq) triggers the thiolation of DMA. The second reaction rate is significantly decreased at pH 2.5; therefore, reaction under strongly acidic conditions leads to accumulation of highly toxic DMMTA in the early stages of thiolation. The transformation of DMDTA to DMMTA is catalyzed in the presence of ferric iron. Formation of DMMTA should be considered when assessing risk posed by arsenic under sulfidic or sulfate reducing conditions.