New insights into surface behavior of dimethylated arsenicals on montmorillonite using X-ray absorption spectroscopy

Abstract

Although recent studies have revealed the occurrence of dimethylated arsenicals, little is known about their behavior in environment. This study investigates the adsorption behavior of dimethylarsinic acid (DMAV), dimethyldithioarsinic acid (DMDTA(V)), and dimethylmonothioarsinic acid (DMMTA(V)) on montmorillonite. Complicated transformations among arsenicals under normal environmental conditions were also considered. Our results clearly demonstrate that DMDTA(V) was oxidized to DMMTA(V), which was relatively stable but partially transformed to DMA(V) when exposed to air during adsorption. The transformed DMA(V) exhibited high adsorption affinities for mont-morillonite, while DMMTA(V )and DMDTA(V) were not appreciably retained by montmorillonite for 48 h. This is the first study to provide insights into DMDTA(V )oxidation under environmental conditions. X-ray absorption near edge structure and extended X-ray absorption fine structure studies confirmed that most of the adsorbed arsenicals on montmo-rillonite were DMA(V). The significantly different bonding characteristics of each adsorbed DMA(V) provide direct evidence for the transformation of DMAV(V )from DMDTA(V ) and DMMTA(V). Our study suggests the importance of incorporating the DMMTAV in the realistic risk management for soil environments because it is highly toxic, easily trans-formed from DMDTAV, and stable in the environment.