Dimethylmonothioarsinic acid and dimethyldithioarsinic acid in the environment: Sorption characteristics on 2-line ferrihydrite and acute toxicity to Daphnia magna.

Dimethylmonothioarsinic acid (DMMTA(V)) and dimethyldithioarsinic acid (DMDTA(V)), which are commonly found in landfill leachate and pore water of rice paddy soil, have attracted considerable attention for their high toxicity. This study aims to evaluate the behavior and potential risks of DMMTA(V) and DMDTA(V) in the environment by conducting an equilibrium sorption test using 2-line ferrihydrite and acute toxicity (immobilization) test using Daphnia magna. The overall maximum sorption capacity (qm) derived by the Langmuir isotherm model showed an increase in the order of inorganic arsenate (As(V)) > dimethylarsinic acid (DMA(V)) > DMMTA(V) > DMDTA(V), which was likely due to steric hindrance due to the presence of methyl and thiol groups. Moreover, DMMTA(V) and DMDTA(V) showed no or negligible change in qm with pH change, while qm decreased rapidly with increasing pH in As(V) and DMA(V). The 50% inhibition concentrations (IC50) for As(V), DMA(V), DMMTA(V), and DMDTA(V), which were measured after 48 h exposure to D. magna, were 9.5, > 30, 1.7, and 6.5 mg/L, respectively. Overall, the results demonstrated that DMMTA(V) and DMDTA(V) have high mobility and eco-toxicity in the environment and that methylated thioarsenicals should be accurately monitored and controlled.