Speciation and solubility of palladium in aqueous magmatic- hydrothermal solutions

Combining thermodynamic data at 25°C and 1 bar with correlation algorithms enables prediction of equilibrium dissociation constants for aqueous platinum-group-element (PGE) complexes at high pressures and temperatures. These data were used (1) to calculate the speciation of Pd(II) in NaCl solutions at temperatures to 750°C, pressures to 2 kbar, and values from 3 to 8, and (2) to predict the solubility of palladium at 2 kbar as a function of temperature, oxygen fugacity (f _O2 ), pH, and total chloride concentration (m _T Cl ^- ). Calculations indicate that as pH increases there is a sharp transition from chloride- to hydroxide-complex dominance at all chloride concentrations. This shift occurs at or below neutral pH in dilute solution, but at m _T Cl ^- ≥1.0, Pd(II) chloride complexes account for the majority of dissolved Pd(II) well into alkaline conditions. Solubility of palladium increases with increasing temperature, m _T Cl ^- , and f _O2 . At 700°C, 2 kbar, and the f _O2 set by the FMQ buffer, palladium solubility in a 3.0 m NaCl solution is ~0.3 ppm at a pH of 4, and a one-unit pH variation can change palladium solubility by two orders of magnitude. These results suggest that PGE can be transported as chloride complexes in Cl-rich magmatic-hydrothermal solutions like those inferred from petrologic studies of the Stillwater and Bushveld complexes. -Authors