Cadmium Complexation with Bisulfide

We have used in situ dialysis to measure the solubility of CdS(s) in sulfidic solutions. The solubility product for the reaction CdS(s) + H + ⇄ Cd 2+ + HS - at 25 °C and 1 atm was found to be 10 - 14.82 ± 0.03 for a crystalline product and to be 10 - 14.15 ± 0.06 and 10 - 14.40 ± 0.06 for two precipitates, respectively. We show that the solubility of these three solids at various pH values (4.2−8.6) and sulfide concentrations (10 - 4.3 −10 - 1.3 M) can be reproduced adequately by the following four bisulfide complexes: CdHS +, Cd(HS) 2, Cd(HS) 3 -, and Cd(HS) 4 2 -; the log K n values for the general equation Cd 2+ + n HS - ⇄ Cd(HS) n 2 - n are 7.38 ± 0.68, 14.43 ± 0.01, 16.26 ± 0.58, and 18.43 ± 0.05, respectively. The species CdOHS -, which has been reported previously, does not explain our experimental results. Calculations using estimated concentrations of organic ligands (humic substances and organic thiols) known to be present in natural waters indicate that sulfide complexes largely dominate Cd speciation in natural waters at ∑S(−II) ≥ 10 - 6 M.