An XAS study of speciation and thermodynamic properties of aqueous zinc bromide complexes at 25–150 °C

The speciation and thermodynamic properties of aqueous zinc halide complexes are important in controlling the mobility of zinc in natural and man-made hydrothermal brines. To assess the role of bromine on zinc transport in bromine-bearing low temperature hydrothermal fluids (e.g., basinal brines), the speciation and thermodynamic properties of aqueous zinc bromide complexes have been investigated at 25–150 °C and water saturated pressure using synchrotron X-ray absorption spectroscopy.Analysis of the EXAFS (Extended X-ray Absorption Fine Structure) and XANES (X-ray Absorption Near Edge Structure) data showed that the octahedral Zn2 + complex predominates in a 0.1 m ZnBr2 solution at 25 °C, while a high order tetrahedral complex (ZnBr42 -) is predominant in a solution with 0.1 m ZnBr2 and 6.2 m NaBr over the entire temperature range (25, 100 and 150 °C). XANES data showed an increasing stability of tetrahedral species (especially ZnBr2(aq) and ZnBr42 -) as a function of increasing temperature and/or bromide concentration. The XANES data have been used to retrieve the dissociation constants for these two tetrahedral species, which enables modelling of the role of zinc bromide complexes in zinc transport in hydrothermal fluids such as metal-rich basinal brines. Thermodynamic calculations based on the new data suggest that, compared to chloride complexes, Zn(II) bromide complexes will not play a significant role in Zn transport in typical basinal brines.

► A systematic increase of tetrahedral species as a function of temperature and bromide concentration.
► Tetrahedral ZnBr42 - complex is predominant in a high Br and high temperature solutions.
► The formation constants for Zn–Br complexes enable modelling of zinc transport in hydrothermal bromide fluids.
► Zn–Cl complexes are more important than Zn–Br complexes in zinc transport in metal-rich basinal brines