Abstract
Toxicity of arsenic is significantly variable depending on its speciation and it is important to understand the migration behaviors of this speciation, where diffusion can control transport mechanism in the impermeable layer such as in the pore waters of sediments and rocks. Diffusion coefficients of arsenite, arsenate, methylarsonic acid (MMA), and phenylarsonic acid (PAA) as a function of pH were determined in this study for the first time, which contributes to the better understanding of transport of various arsenic speciation in the environment. By aid of ab initio MO calculations and Monte Carlo simulations, this study sheds light on the origin of pH dependence on the diffusion coefficients for the arsenic compounds. For the neutral speciation at low pH, the diffusion is dominated not only by the molecular size but also by charge distribution in the molecule (degree of polarization). On the other hand, for the dissociated speciation at high pH, the diffusion is dominated by charged oxyanion due to high association of water molecules regardless their functional groups. This effect is common for all the arsenic species, which causes that the differences in their diffusion coefficients become smaller as the pH increases, especially for arsenate, MMA, and PAA.
