The difference of diffusion coefficients in water for arsenic compounds at various pH and its dominant factors implied by molecular simulations

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 the transport mechanism in impermeable layers, such as in the pore waters of sediments and rocks. Diffusion coefficients of arsenious acid (arsenite species), arsenic acid (arsenate species), methylarsonic acid (MMA), and phenylarsonic acid (PAA) as a function of pH were determined in this study for the first time. The results would contribute to better understanding of the transport of various arsenic speciations in the environment. By aid of ab initio molecular orbital 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 because of the high association of water molecules regardless of 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.