Structure and stability of cadmium chloride complexes in hydrothermal fluids

The identity and stability of aqueous species formed by cadmium in H2O–Na/LiCl–HCl–HNO3 solutions were investigated using in situ X-ray absorption spectroscopy (XANES and EXAFS) at 20–450 °C and 1–600 bar, combined with solubility measurements of CdO and CdS solids at 350–400 °C and 300–600 bar. Results show that aqueous Cd speciation is dominated by the cation Cd(H2O)62+ in acidic Cl-free solutions, and by chloride species CdClm(H2O)n2 − m over a wide range of temperatures (20 ≤ T ≤ 450 °C), acidities (1 ≤ pH ≤ 8), and chloride concentrations (0.04 ≤ mCl ≤ 18 mol/kg H2O). EXAFS spectra from chloride solutions show that with increasing T and mCl the average number of Cl atoms increases from 1 to ∼ 3 (± 0.6), accompanied by a decrease of the number of O from 6 to ∼ 1 (± 0.7), in the nearest coordination sphere around Cd. Average Cd–Cl distances in Cd complexes vary in the range 2.41–2.52 (± 0.02) Å, increasing with increasing mCl and decreasing with increasing T, while Cd–O distances remain constant within errors at 2.30 (± 0.05) Å in the whole T and mCl range investigated. This evolution of the Cd atomic environment is accompanied by changes in geometry of the dominant Cd–O/Cl species from octahedral-like to tetrahedral-like with increasing T and mCl, as shown by XANES spectra analyses. These structural data are consistent with the formation of octahedral Cd(H2O)62+ and CdCl(H2O)5+ and tetrahedral CdCl2(H2O)20, CdCl3(H2O)− and CdCl4− species at T ≤ 200–300 °C and mCl ≤ 18. At T > 300 °C, aqueous Cd speciation is dominated by two complexes, CdCl2(H2O)20 and CdCl3(H2O)− in a wide mCl range (0.04–5m). The stability constants of these complexes, derived from solubility measurements at 400 °C and 600 bar are ∼ 6 orders of magnitude higher than available HKF-model predictions based on low-T data. The new data on aqueous Cd complexes may be used for interpretation of Cd/Zn ratios measured in natural hydrothermal saline fluids and volcanic vapors in which the greater stability of Cd chloride species as compared to their Zn analogs is likely to be responsible for the elevated Cd/Zn ratios in comparison to crustal rocks and Cl-poor waters.