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

Abstract

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, 200, and 300 °C). It is found that monosulfide (i.e., Ag(HS)) and disulfide 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-coordinated at 300 °C (i.e., Ag(H2S)2(H2O)+). All of the other mono- and disulfide species (Ag(HS)(H2O)0, Ag(HS)(OH)-, Ag(HS)(H2S)0, Ag(HS)2-, and AgS(HS)2-) have 2-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 disulfide species distributions versus pH have been derived. We found that for monosulfide 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 disulfide 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.