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 °C at a constant pressure of 600 bar. Qualitative analysis of XANES spectra shows that octahedral species predominate in solution at 35 °C, while tetrahedral species become increasingly important with increasing temperature. Ab initio 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 ∼2). EXAFS analyses further revealed the bonding distances for the octahedral Co(H₂O)₆²⁺ (^octCo–O = 2.075(19) Å), tetrahedral CoCl₄²⁻ (^tetCo–Cl = 2.252(19) Å) and tetrahedral CoCl₂(H₂O)_2(aq) (^tetCo–O = 2.038(54) Å and ^tetCo–Cl = 2.210(56) Å). 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 (∼2 m NaCl) at temperatures of 250 °C and higher, and both cooling and dilution processes can cause deposition of cobalt from hydrothermal fluids.