Silver speciation in chloride-containing hydrothermal solutions from first principles molecular dynamics simulations

First principles molecular dynamics (FPMD) simulations are performed to investigate the speciation of Ag+ in chloride-containing hydrothermal solutions. We derive the stability and microscopic structures of aqua cations and chloro complexes of Ag+ under T–P conditions ranging from ambient to supercritical conditions. Our simulations show that the average hydration number for hydrated Ag+ decreases from 4 to 3 as temperature increases and the first hydration shell is very flexible, yielding rapid water exchange. AgCl2− and AgCl are the major chloro complexes in hydrothermal solutions over a wide T–P range. AgCl2− has a linear structure, which is similar to its counterpart chloro complexes of Au and Cu (i.e., AuCl2− and CuCl2−). AgCl has a trigonal planar structure (AgCl(H2O)2), which is quite different from the mono-chloro complexes of Au and Cu that have 2-fold linear structures (AuCl(H2O) and CuCl(H2O)). AgCl32 − holds weak stability at ambient conditions, but is no longer stable at elevated temperatures. AgCl43 − is not stable even under ambient conditions. AgCl32 − and AgCl43 −, therefore, should not be considered as important species in hydrothermal fluids.

► We study Ag speciation in Cl−-containing solutions with FPMD. ► The hypothesized complex AgCl43 − does not exist even under ambient conditions. ► AgCl32 − exists at ambient condition, but it does not hold at elevated temperatures. ► AgCl2− and AgCl(H2O)2 are major chloro complexes of Ag in hydrothermal solutions. ► The irregular hydration shells of Ag+ evolves from 4 to 3-fold as T increases.