Structure, acidity, and metal complexing properties of oxythioarsenites in hydrothermal solutions

In the pursuit of understanding the aqueous chemistry of oxythioarsenites (i.e., H3AsO2S and H3AsOS2) on a microscopic scale, first principles molecular dynamics (FPMD) simulations were performed to investigate the hydration structures, acidity constants, and metal-complexing properties of these species in aqueous solutions at temperatures up to 573 K. The simulations showed that the oxythioarsenite species are stable and have trigonal pyramidal structures in both low- and high-temperature fluids. Their solvated structures were characterized in detail and it was found that the S or H atoms of the SH ligands form very weak H-bonds with the solvating water molecules and the OH ligands form H-bonds with water as both donors and acceptors. Furthermore, the dangling S or O sites act as strong acceptors to form H-bonds. Their acidity constants were calculated by using the FPMD-based vertical energy gap method. Together with our previous work (Liu et al., 2015, Chemical Geology, 411, 192-199), we found that in the arsenite-thioarsenite series (i.e., H3AsO3 − xSx for x varying from 0 to 3), the pKa values of the oxythioarsenite species (H3AsO2S and H3AsOS2) were always located between those of the end members, H3AsO3 and H3AsS3, and decreased with the increasing temperature up to 573 K. Based on the calculated acidity constants, the speciation versus pH distributions were obtained at temperatures ranging from ambient temperature to 573 K. It was found that both H2AsO2S− and H2AsOS2− are the predominant species at near neutral pH for H3AsO2S and H3AsOS2 species, respectively. In addition, the structures and the dissociation free energy of the oxythioarsenite-metal complexes at 573 K indicated that these As-S moieties are effective ligands for complexing with ore-forming metals in geological fluids.