Complexation of copper in acetate-rich low-temperature hydrothermal fluids: Evidence from ab initio molecular dynamics simulations

Acetate ions are widely distributed in various geo-fluids and can be enriched in metamorphic brines under saturated vapor pressure in the temperature range of 80 to 200 °C. To examine the potential role of acetate in transporting metals, we conducted a series of ab initio molecular dynamics (MD) simulations to investigate the complexation of Cu+ with Cl−, HS− and acetate ions. All the ab initio MD simulations were conducted at the temperature of 150 °C and pressures of 10 bar or 1000 bar. The ionic compositions of aqueous solutions for the simulations include four groups: (1) Cu+ and CH3COO−; (2) Cu+, CH3COO− and Cl−; (3) Cu+, CH3COO−, Cl− and HS−; and (4) Cu+, CH3COO− and HS−. The simulation results demonstrated some important regularities for complexation of copper with acetate. The static computation results suggest that Cu+ forms linear complexes with one or two acetate ions, rather than with one acetate ion in a nearly symmetric bidentate structure. The stoichiometry of the complexes, which can be represented by [Cu(CH3COO)(H2O)], [(CH3COO)2Cu]− and [Cu(CH3COO)Cl]−, depends on the fluid composition, environmental pressure and solvated structures of the acetate ligands in these complexes. Compared with Cl−, the acetate ion is a ligand of higher affinity for Cu+, and the solvated structure of acetate ligands can prevent Cl− from approaching Cu+. The presence of HS− also inhibits the formation of a Cu-Cl bond, thereby enhancing the stability of the Cu-OH2 bond in [Cu(CH3COO)(H2O)]. We also investigated the free energy surfaces of the ligand exchange reactions, CuCH3COO2-+2Cl-=CuCl2-+2CH3COO-andCuCH3COO2-+2HS-=CuHS2-+2CH3COO-. Combined with the results from ab initio MD simulations, we conclude that the order of affinity of Cu+ to form complexes in these conditions is HS− > CH3COO− > Cl− > H2O. These conclusions provide important evidence for evaluating the role of acetate ligands in transporting Cu during low temperature mineralization.