Complexation of metal ions in brines: application of electronic spectroscopy in the study of the Cu(II)-LiCl-H2O system between 25 and 90°C

Abstract

The concentration and transport of metals in hydrothermal solutions depend on how metals ions combine with ligands to form complexes, and experimental methods are necessary to identify the important complexes. UV-Vis-NIR spectrophotometry was used to study the formation of Cu(II)-chloride complexes in LiCl brines up to very high chlorinities (18 m LiCl), at temperatures between 25°C and 90°C. The number of Cu(II)-chloride complexes necessary to account for the variation in spectra with varying chloride molality at each temperature was estimated using principal component analysis. The molar absorptivity coefficients and concentrations of each complex were then determined using a “model-free” analysis, which does not require any assumption about the chemistry of the system, other than the number of absorbing species present. Subsequently, the results from the “model-free” analysis were integrated with independent experimental evidence to develop a thermodynamic speciation model, where the logarithms of the equilibrium constants for Cu(II)-chloride formation reactions were fitted to the data using a non-linear least-squares approach. Maps of the residual function were used to estimate uncertainties in the fitted equilibrium constants.The results of this study are similar to published properties of distorted octahedral [CuCl(OH2)5]+ and [CuCl2(OH2)4]0 at all temperatures, but diverge for [CuCl3(OH2)3]− and distorted tetrahedral [CuCl4]2−. Moreover, the data suggest the presence of [CuCl5]3−, probably with D3h point group, at very high salt concentration. This study demonstrates that it is possible to determine apparent thermodynamic equilibrium constants for the formation of complexes of trace amount of metals in highly concentrated brines, such as those associated with many ore deposits. The results are dependent on the choice of activity coefficients for charged and neutral aqueous complexes, but this influence is relatively small compared with the experimental uncertainty. This study shows that Cu2+ chloro-complexes, predominantly [CuCl2(OH2)4]0 and [CuCl4]2−, will play a dominant role in nature where free oxygen is available (near-surface), and where chloride activities are very high (evaporitic basins; hypersaline soils).