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

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 spec trophotometry was used to study the formation of Cu(II)-chloride complexes in LiCl brines up to very high chlorinities (18 m LiCl), at temperatures be tween 25 degreesC and 90 degreesC. The number of Cu(II)-chloride complexes necessary to account for the variation in spectra with varying chloride mol ality at each temperature was estimated using principal component analysis. The molar absorptivity coefficients and concentrations of each complex wer e then determined using a "model-free" analysis, which does not require any assumption about the chemistry of the system, other than the number of abs orbing species present. Subsequently, the results from the "model-free" ana lysis were integrated with independent experimental evidence to develop a t hermodynamic speciation model, where the logarithms of the equilibrium cons tants for Cu(II)-chloride formation reactions were fitted to the data using a non-linear least-squares approach. Maps of the residual function were us ed to estimate uncertainties in the fitted equilibrium constants. The resul ts of this study are similar to published properties of distorted octahedra l [CuCl(OH2)(5)](+) and [CuCl2(OH2)(4)](0) at all temperatures, but diverge for [CuCl3(OH2)(3)](-) and distorted tetrahedral [CuCl4](2-). Moreover, th e data suggest the presence of [CuCl5](3-), probably with D-3h point group, at very high salt concentration. This study demonstrates that it is possib le to determine apparent thermodynamic equilibrium constants for the format ion of complexes of trace amount of metals in highly concentrated brines, s uch as those associated with many ore deposits. The results are dependent o n the choice of activity coefficients for charged and neutral aqueous compl exes, but this influence is relatively small compared with the experimental uncertainty. This study shows that Cu2+ chloro-complexes, predominantly [C uCl2(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). Copyright (C) 2001 Elsev ier Science Ltd.