Solubility of Pt and Pd sulfides and Au metal in aqueous bisulfide solutions |
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| Authors: | P Pan S A Wood |
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| Institution: | (1) Department of Geological Sciences, McGill University, 3450 University Street, H3A 2A7 Montreal, Quebec, Canada;(2) Present address: Research Chemistry Branch, Atomic Energy of Canada Ltd., Whiteshell Laboratories, R0E 1L0 Pinawa, MB, Canada;(3) Present address: Department of Geology and Geological Engineering, University of Idaho, 83843 Moscow, ID, USA |
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| Abstract: | An experimental study of the solubility of Pt and Pd sulfides and Au metal in aqueous bisulfide solutions was conducted at temperatures from 200° to 350 °C and at saturated vapor pressure. A 500-mL Bridgemantype pressure vessel constructed of titanium, and equipped with a motor-driven magnetic stirrer was employed. The pH and the oxidation state were buffered by the coexistence of H2S/HS–/SO
inf4
sup2–
. The pH at temperature was calculated to be in the range 5.91–9.43, and  S was 0.3–2.2 m. Under the experimental conditions, the measured solubility of gold is about two to three orders of magnitude greater than that of either platinum and palladium, and the measured solubility of platinum is, in general, approximately equal to that of palladium, in molal units. The solubilities are found to be in the range: platinum 4–800 ppb, palladium 1–400 ppb, and gold 2–300 ppm. The solubility data can be modeled adequately using the following reactions: Au+H2S+HH–=Au(HS)
2
–
+1/2H2 (K14); PtS+HS–+H+=Pt (HS)
2
0
(K15); PdS+HS–+H+=Pd (HS)
2
0
(K16); PtS2+H2=Pt (HS)
2
0
(K21).With equilibrium constants determined as follows (errors represent two standard deviations): Preliminary measurements of the solubilities of metallic Pt, Pd and Au as hydroxide complexes were also conducted using a second titanium pressure vessel, at temperatures of 200° to 350 °C and vapor saturation pressure, with pH and the oxidation state controlled or buffered by adding known amounts of NaOH and H2 gas. The concentration of NaOH was in the range 0.01–1.3 m, and the partial pressure of H2 at 200 °C was 62–275 bars, initially. Under the temperature and pressure conditions of these experiments, the solubility of platinum in 1 m NaOH solution is less than 100 ppb, that of palladium is less than 10 ppb and that of gold is less than 0.2 ppm; and in 0.01 m NaOH solutions, both Pt and Pd solubilities are less than 1 ppb. These data indicate that the contributions of hydroxide complexes to the total solubilities in the bisulfide runs, where the pH was in the range of 5.9–9.4, are negligible. The concentrations of both Pt and Pd as bisulfide complexes in the Salton Sea geothermal system predicted using the stability constants determined in this work agree very well with those values measured by McKibben et al. (1990). This calculation strongly suggests that the PGE are transported in moderately reducing, near neutral hydrothermal fluids as bisulfide complexes, as is gold. However, the much lower maximum solubility of the PGE relative to gold severely constrains models of re genesis, and may explain the relative rarity of hydrothermal PGE deposits compared to the relative abundance of hydrothermal Au deposits. |
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