The speciation and transport of palladium in hydrothermal fluids: Experimental modeling and thermodynamic constraints

The solubility of PdO(cr) was measured in NaOH (to 0.1m, mol/kg H2O) solutions at 400 degrees C, 1 kbar, and the solubility of Pd-(cr) was determined at 400-500 degrees C, 1 kbar in acidic chloride solutions (to 1.5m NaCl) buffered with respect to hydrogen. The Pd electrode potential E-o(PdCl42-)/Pd-(cr) for the reaction PdCl42- + 2e(-) = Pd-(cr) + 4Cl(-) was determined at 50 and 70 degrees C in 1m chloride solutions. These data, together with reliable literature values, were used for calculation of the standard thermodynamic properties and the formation constants for Pd-OH, Pd-Cl, and Pd-S-HS complexes within the framework of the revised Helgeson-Kirkham-Flowers model. It was found that PdCl3- and PdCl42- become the most important Pd complexes in high temperature (t > 300 degrees C), chloride-rich fluids, and PdCl42- predominates at m(Cl) > 0.1. The stability of Pd-Cl complexes increases sharply with increase in temperature. The near-neutral chloride-sulfide solutions (1m NaCl, < 0.1m S-tot) can transport Pd at ppm concentration levels at t >= 600 degrees C, whereas decrease in temperature and increase in pH can lead to effective deposition of Pd minerals. The stability of Pd-S-HS complexes (Pd(HS)(2)degrees, Pd(HS)(3)(-) and PdS(HS)(2-)) decreases with increase in temperature. Therefore, the role of these complexes in hydrothermal transport of palladium is restricted to the low temperature solutions (t < 100 degrees C) and sulfur can be considered an efficient depositing agent for Pd. The calculated HKF Equation of State parameters were used to predict thermodynamic properties of Pd2+, Pd-OH, Pd-Cl, and Pd-S-HS complexes to 700 degrees C, 2 kbar. These parameters are incorporated into the FreeGs web-enabled database (http://www-b.ga.gov.au/minerals/research/methodology/geofluids/thermo/calculator/search.jsp) that can be used for geochemical application of thermodynamic data obtained in the present study. (C) 2013 Elsevier Ltd. All rights reserved.