Cadmium behavior in sulfur-bearing aqueous environments: insight from CdS solubility measurements at 25-80°C

Abstract

The present experimental study explores the effects of temperature and sulfur in Cd aqueous geochemistry under reduced conditions. Greenockite CdS solubility is measured in H2O-H2S-HClO4-NaHS solutions at 25-80°C as a function of pH and sulfur concentration. Based on solubility product measurements in highly acid solutions, the standard thermodynamic properties of greenockite (CdS) are revised, and the recommended value of ∆fG0298.15 for greenockite CdS(s) is -151.5±0.3 kJ mol-1. The stability of greenockite (CdS) is higher than predicted by calculations using previous literature data. At 80°C, the stability constants for Cd-HS complexes are measured for the first time, the values are 10–5.65±1.00 for CdS(s) + H+ = CdHS+, 10–6.00±0.40 for CdS(s) + H2S0(aq) = Cd(HS)20(aq), 10–3.87±0.10 for CdS(s) + H2S0(aq) + HS- = Cd(HS)3-, and 10–3.53±0.20 for CdS(s) + H2S0(aq) + 2HS- = Cd(HS)42-. Modeling of Cd behavior at 3-200°C shows that Cd-HS species are more important than previously believed. The fraction of Cd(HS)n2-n (n = 1-4) complexes increases with mH2S and decreases with T. Thus, in euxinic marine environments with mH2S ≥ 10-5, Cd speciation changes from Cd-Cl to Cd-HS. This speciation change is expected to affect Cd isotope fractionation and should be accounted for when applying Cd isotopic signature as a paleo tracer in marine sediments. The new thermodynamic data are indispensable for modeling Cd behavior in response to pH, T, and mH2S. As a function of these parameters, sulfur has the main control on Cd geochemistry being the main factor of Cd precipitation at low mH2S and favoring Cd mobilization at high mH2S.