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 CH 3 COO − ; (2) Cu + , CH 3 COO − and Cl − ; (3) Cu + , CH 3 COO − , Cl − and HS − ; and (4) Cu + , CH 3 COO − 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(CH 3 COO)(H 2 O)], [(CH 3 COO) 2 Cu] − and [Cu(CH 3 COO)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-OH 2 bond in [Cu(CH 3 COO)(H 2 O)]. We also investigated the free energy surfaces of the ligand exchange reactions, Cu CH 3 COO 2 - + 2 Cl - = CuCl 2 - + 2 CH 3 COO - and Cu CH 3 COO 2 - + 2 HS - = Cu HS 2 - + 2 CH 3 COO - . 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 − > CH 3 COO − > Cl − > H 2 O. These conclusions provide important evidence for evaluating the role of acetate ligands in transporting Cu during low temperature mineralization. [ABSTRACT FROM AUTHOR]