Cadmium Behavior in Sulfur-bearing Aqueous Environments: Insight from CdS Solubility Measurements at 25–80°C

The present experimental study explores the effects of temperature and sulfur in Cd aqueous geochemistry under reduced conditions. Greenockite CdS solubility is measured in H 2 O–H 2 S–HClO 4 –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 Δ_fG_298.15^0 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) + H 2 S _(aq)^0 = Cd(HS) _2(aq)^0 , 10 –3.87 ± 0.10 for CdS (s) + H 2 S _(aq)^0 + HS – = Cd(HS) _3^ - , and 10 –3.53 ± 0.20 for CdS (s) + H 2 S _(aq)^0 + 2HS – = Cd(HS) _4^2 - . Modeling of Cd behavior at 3–200°C shows that Cd–HS species are more important than previously believed. The fraction of Cd(HS) _n^2 - n ( n = 1–4) complexes increases with m H 2 S and decreases with T . Thus, in euxinic marine environments with m H 2 S ≥ 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 m H 2 S. As a function of these parameters, sulfur has the main control on Cd geochemistry being the main factor of Cd precipitation at low m H 2 S and favoring Cd mobilization at high m H 2 S.