Structures and Acidity Constants of Silver-Sulfide Complexes in Hydrothermal Fluids: A First Principles Molecular Dynamics Study.

In order to quantify the speciation and structures of silver-sulfide complexes in aqueous solutions, we have carried out systematic first principles molecular dynamics (FPMD) simulations at three temperatures (25°C, 200°C, and 300°C). It is found that mono-sulfide (i.e., Ag(HS)) and di-sulfide species (i.e., Ag(HS)2-) are the major silver-sulfide species over a wide T-P range, while Ag(HS)32- can hold stably only at ambient temperatures and Ag(HS)43- does not exist even at the ambient conditions. Ag(H2S)+ has a tetrahedral structure up to 300°C (i.e., Ag(H2S)(H2O)3+). Ag(H2S)2+ remains 4-coordinated to 200°C (i.e., Ag(H2S)2(H2O)2+) but it transforms to 3-coordinate at 300°C (i.e., Ag(H2S)2(H2O)+). All of the other mono- and di-sulfide species (Ag(HS)(H2O)0, Ag(HS)(OH)-, Ag(HS)(H2S)0, Ag(HS)2- and AgS(HS)2-) have two-fold linear structures. For their solvation structures, the H2S ligands donate weak H-bonds to water O; the HS- ligands accept weak H-bonds from water H; the dangling S2- form strong H-bonds with H of water molecules and the OH- ligands can form strong H-bonds as donors and weak H-bonds as acceptors. We further calculated the acidity constants (i.e. pKas) of Ag(H2S)+ and Ag(H2S)2+ complexes using FPMD based vertical energy gap method. Based on the calculated pKas, the mono- and di-sulfide species distributions versus pH have been derived. We found that for mono-sulfide species, Ag(HS)(H2O)0 is the major species in near neutral pH, while Ag(H2S)(H2O)3+ and Ag(HS)(OH)- exist in the acid and alkaline pH range at T<=200°C, respectively. At 300°C, both Ag(HS)(OH)- and Ag(HS)(H2O)0 are dominant in the neutral pH range, and Ag(H2S)(H2O)2+ only exists in acidic solutions. For di-sulfide species, Ag(HS)2- is dominative in near neutral pH condition at the three temperatures; Ag(HS)(H2S)0 stays in mild acidic pH range only at 25°C; AgS(HS)2- and Ag(H2S)2(H2O)2+ (Ag(H2S)2(H2O)+ at 300°C) are trivial at the three conditions. The results of structures and acidity constants provide quantitative and microscopic basis for understanding the behavior of silver complexes in hydrothermal fluids.