Zinc complexation in aqueous sulfide solutions: Determination of the stoichiometry and stability of complexes via ZnS(cr) solubility measurements at 100 °C and 150 bars

The solubility of ZnS(cr) was measured at 100 °C, 150 bars in sulfide solutions as a function of sulfur concentration (m(Stotal) = 0.02–0.15) and acidity (pHt = 2–11). The experiments were conducted using a Ti flow-through hydrothermal reactor enabling the sampling of large volumes of solutions at experimental conditions, with the subsequent concentration and determination of trace quantities of Zn. Prior to the experiments, a long-term in situ conditioning of the solid phase was performed in order to attain the reproducible Zn concentrations (i.e. solubilities). The ZnS(cr) solubility product was monitored in the course of the experiment. The following species were found to account for Zn speciation in solution: Zn2+ (pHt < 3), Zn(HS)20 (pHt 3–4.5), Zn(HS)3- (pHt 5–8), and ZnS(HS)− (pHt > 8) (pHt predominance regions are given for m(Stotal) = 0.1). Solubility data collected in this study at pHt > 3 were combined with the ZnS(cr) solubility product determined at lower pH to yield the following equilibrium constants (t = 100 °C, P = 150 bars):Zn2++2HS-=Zn(HS)20(aq)logβ2=9.52±0.06,Zn2++3HS-=Zn(HS)3-logβ3=12.39±0.06,Zn2++2HS-=ZnS(HS)-+H+logβ11=2.92±0.06.The solubilities of ZnS(cr), determined beyond the Zn2+ predominance region (at pHt > 2.5), are lower than the sphalerite solubilities previously reported in the literature, resulting in lower values of the formation constants for the Zn–S–HS complexes. The results of this study indicate that in natural sulfide-rich fluids the maximum Zn concentrations are attained at pH =  pK1(H2S), where Zn(HS)3- predominates.