Speciation of nickel (II) chloride complexes in hydrothermal fluids: In situ XAS study

In situ XAS data of Ni(II) chloride solutions with various salinities (0–7.68 m) were collected to investigate the stoichiometry and geometry of Ni(II) chloride complexes from room temperature up to 369 °C at 400 bar, and to 434 °C at 600 bar. Increasing temperature and/or salinity results in a change in the coordination of the Ni(II) chlorocomplexes from octahedral to (distorted) tetrahedral. Octahedral species predominate within the whole salinity range at room temperature and up to ~ 200 °C, and tetrahedral species become significant beyond this temperature. At 369 °C and 400 bar, octahedral species remain in equilibrium with tetrahedral species, and the number of chloride ligands in the first coordination shell depends on the Cl:Ni molar ratio. EXAFS refinements and ab initio XANES calculations show that the highest order chlorocomplex present over the investigated pressure, temperature and composition ranges is the distorted tetrahedral complex [NiCl3(H2O)]−, which predominates in the highest Cl concentration (7.68 m) solution at 434 °C and 600 bar. A quantitative thermodynamic analysis of the XANES and EXAFS data reveals that the XAS data are consistent with the speciation models derived from recent UV-Vis spectrophotometric measurements (25 °C–250 °C, 100 bar; Liu et al., 2012b) and the high temperature, high pressure solubility experiments of Lin and Popp (1984) and Fahlquist and Popp (1989), if the NiCl2(aq) species exists in both octahedral ([NiCl2(H2O)4](aq)) and tetrahedral ([NiCl2(H2O)2](aq)) forms, with the ratio of octahedral to tetrahedral decreasing at high temperature (> 200 °C). The new XAS data show that the octahedral to tetrahedral transition in Ni(II) chloride complexes occurs at higher temperature and/or salinity than the corresponding Co(II) complexes (Liu et al., 2011).