The role of sulfur in palladium transport and fractionation from platinum by hydrothermal fluids

Abstract

The solubility of palladium sulfide (PdS_(s)) has been measured in H₂S/HS^– aqueous solutions across wide ranges of sulfur concentrations (0.5–1.2 molal) and pH (5–8) at temperatures from 50 to 300 °C and pressures from 90 to 600 bar, using a hydrothermal flexible-cell reactor allowing controlled fluid injection and sampling. Combined with thermodynamic modeling and analysis of available literature data, our results demonstrate that palladium tetrahydrosulfide, Pd^II(HS)₄^2–, is the dominant complex in sulfide-rich moderate-temperature hydrothermal fluids. The equilibrium constants of PdS_(s) dissolution reaction, PdS_(s) + 3 H₂S⁰_(aq) = Pd(HS)₄^2– + 2H⁺ (K_s4,Pd), generated in this study can be described by the function log₁₀K_s4,Pd = (196.4 ± 60.0) – (11384 ± 3567)/T(K) – (71.24 ± 19.52) × log₁₀T(K), valid over the temperature range 25–300 °C and from saturated vapor pressure to 600 bar. Our results, combined with recent analogous PtS_(s) solubility data, enabled the derivation of a self-consistent set of thermodynamic properties of Pd(HS)₄^2– and Pt(HS)₄^2– in the framework of the HKF model. Solubility predictions of PdS_(s) using these properties yield maximum solubility values of 1 ppb Pd, as the tetrahydrosulfide complex, in H₂S-bearing hydrothermal fluids (>0.01 m S) at moderate temperatures (50–350 °C) and near-neutral pH (6–7), whereas chloride complexes are predominant at acidic pH (<4). The Pd/Pt atomic ratio in typical H₂S-bearing hydrothermal fluids at 300 °C in equilibrium with PdS_(s) and PtS_(s) varies from > 10⁴ at pH < 2 to 10^–2 at pH > 3, corresponding to the change from chloride- to sulfide-dominated speciation for both metals. However, the absolute solubilities of both chloride and sulfide complexes are too small over the whole pH range to significantly contribute to Pd vs. Pt fractionations observed in hydrothermal environments. Among different factors that may lead to such fractionations, the role of polysulfide sulfur species, including trisulfur radical ions, should be considered in future studies.