XAS studies of metal speciation in hydrothermal fluids

Abstract

Knowledge of metal speciation and thermodynamic properties underpins our capability to model metal transport and deposition in natural and engineered systems. Using synchrotronbased X-ray Absorption Spectroscopy and high temperature – high pressure experimental techniques, this project aims to elucidate nickel and manganese speciation in hydrothermal chloride solutions, and obtain the thermodynamic properties for predominant species. Ab initio XANES simulation methods were used in this study to provide independent or complementary information about the nature (stoichiometry and geometry) of aqueous complexes. Application of this technique to the calculation of XANES spectra of Mo(VI) complexes in hydrothermal systems confirmed that [MoO₄]²⁻ is stable in neutral and basic solutions over a wide range of temperature and salinity, and chlorinated Mo complexes (e.g., [MoCl₂O₂(H₂O)₂](aq), [MoOCl₄](aq)) exist in strongly acidic solutions. XANES simulations of Te complexes added additional evidence that [Te(OH)₃] and [TeO₃] species predominate in basic and acidic solutions, respectively, and that the deprotonation process to convert [Te(OH)₃] to [TeO₃] is associated with a distance contraction for the Te-O bond. Ni(II) speciation in hydrothermal brines was investigated over a wide range of temperatures (25-434 °C) and fluid compositions (0-7.68 m Cl⁻) at 400 and 600 bar. Quantitative XAS data interpretation revealed that Ni(II) chloroaqua complexes undergo a structural transition from octahedral at room temperature to distorted tetrahedral at elevated temperatures. Both heating and an increase in salinity promote the stability of tetrahedral complexes relative to octahedral complexes. 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 highest order Ni chloride complex identified in this work is not the fully chlorinated [NiCl₄]²⁻ but the tri-chloro mono-aqua complex [NiCl₃(H₂O)]⁻, confirmed by both EXAFS analysis and XANES simulations. A similar coordination change of Mn(II) chloroaqua complexes has been quantitatively identified by analysis of both XANES and EXAFS data collected between 30 to 550 °C at 600 bar, with chlorinity ranging from 0.100 to 10.344 m. Octahedral species predominate at room temperature within the whole salinity range and persist up to ~400 °C in low salinity solutions (mCl < 1 m), and tetrahedral species become significant at temperatures above 300 °C. Compared with Fe(II) chloride complexation, the octahedral to tetrahedral structural transition occurs at higher temperature for Mn(II) complexes. A combination of EXAFS refinements, Density Functional Theory calculations and ab initio XANES simulations confirmed that at elevated temperatures (≥ 400 °C) the highest order chloride complex predominating in highly saline brines (mCl > 3 m, Cl:Mn ratio > 53) is [MnCl₃(H₂O)]⁻ with [MnCl₄]²⁻ being unstable through all T-P-salinity range, while a lower order chlorocomplex, [MnCl₂(H₂O)₂](aq), is the major species in low salinity solutions (mCl < 0.5 m, Cl:Mn ratio < 10). The differences regarding to the stoichiometry and stability of highest order metal chloride complexes identified in this study, [NiCl₃(H₂O)]⁻ and [MnCl₃(H₂O)]⁻, and in previous studies (i.e., [CoCl₄]²⁻ and [FeCl₄]²⁻) may play a role in the fractionation between metals with closely related geochemical properties in hydrothermal systems (e.g., Ni/Co; Mn/Fe). Overall, the combination of XANES and EXAFS data provided us with a molecular level understanding of Ni and Mn speciation in hydrothermal brines and improved our capability for modeling metal mobility in the Earth’s crust.