Speciation and thermodynamic properties for cobalt chloride complexes in hydrothermal fluids at 35-440 degrees C and 600 bar: An in-situ XAS study

Aqueous Co(II) chloride complexes play a crucial role in cobalt transport and deposition in ore-forming hydrothermal systems, ore processing plants, and in the corrosion of special Co-bearing alloys. Reactive transport modelling of cobalt in hydrothermal fluids relies on the availability of thermodynamic properties for Co complexes over a wide range of temperature, pressure and salinity. Synchrotron X-ray absorption spectroscopy was used to determine the speciation of cobalt(II) in 0-6 m chloride solutions at temperatures between 35 and 440 degrees C at a constant pressure of 600 bar. Qualitative analysis of XANES spectra shows that octahedral species predominate in solution at 35 degrees C, while tetrahedral species become increasingly important with increasing temperature. Ab MUM XANES calculations and EXAFS analyses suggest that in high temperature solutions the main species at high salinity (Cl:Co >> 2) is CoCl42- while a lower order tetrahedral complex, most likely CoCl2(H2O)(2(aq)), predominates at low salinity (Cl:Co ratios similar to 2). EXAFS analyses further revealed the bonding distances for the octahedral Co(H2O)(6)(2+) (Co-oct-O = 2.075(19) angstrom), tetrahedral CoCl42- (Co-tet-Cl = 2.252(19) angstrom) and tetrahedral CoCl2(H2O)(2(aq)) (Co-tet-O = 2.038(54) angstrom and Co-tet-Cl = 2.210(56) angstrom). An analysis of the Co(II) speciation in sodium bromide solutions shows a similar trend, with tetrahedral bromide complexes becoming predominant at higher temperature/salinity than in the chloride system. EXAFS analysis confirms that the limiting complex at high bromide concentration at high temperature is CoBr42-. Finally, XANES spectra were used to derive the thermodynamic properties for the CoCl42 and CoCl2(H2O)(2(aq)) complexes, enabling thermodynamic modelling of cobalt transport in hydrothermal fluids. Solubility calculations show that tetrahedral CoCl42- is responsible for transport of cobalt in hydrothermal solutions with moderate chloride concentration (similar to 2 m NaCl) at temperatures of 250 degrees C and higher, and both cooling and dilution processes can cause deposition of cobalt from hydrothermal fluids.