Other
Scientific paper
Nov 1992
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1992gecoa..56.3881g&link_type=abstract
Geochimica et Cosmochimica Acta, vol. 56, Issue 11, pp.3881-3894
Other
14
Scientific paper
Approximately 150 silica tube experiments were used to determine the solubility of Pt and Pd sulfides in NaCl/H 2 SO 4 solutions at 300°C. Oxidation state was controlled by equilibrium between aqueous sulfate and coexisting solids. Buffering assemblages included Pd + Pd 4 S , Pd 4 S + Pd 16 S 7 , Pd 16 S 7 + PdS , Pt + PtS , and PtS + PtS 2 . Solution pH was controlled by the dissociation of H 2 SO 4 (0.1 or 1.0 m), adjusted for ion pairing. Salinities ranged from 0.01 to 3.0 m NaCl. Under these conditions, measured Pt and Pd solubilities varied over a wide range (<0.1 ppb to >1000 ppm), indicating dissolution as chloride complexes of high ligand number. Detailed interpretation of the results gives equilibrium constants for the following reactions: Pd ( s ) + 2 H + + 4 Cl - + 1/2 O 2 ( g ) = PdCl 4 2- + H 2 O (a1) Pt ( s ) + 2 H + + 3 Cl - + 1/2 O 2 ( g ) = PtCl 3 - + H 2 O , (a2) log K (300° C , P sat ) = +12.65 ± 0.5 (reaction al) and +14.84 ± 0.3 (reaction a2). Our experimentally derived constants are in fair (Pt) to excellent (Pd) agreement with previously published estimates based on extrapolation of low-temperature data. The temperature dependence of reaction (a1) is described by the following polynomial (valid between 25 and 300°C): log K al = 22.53 - 7.398 e - 02 T + 1.433 e -04 T 2 - 1.034 e - 08 T 3 ( T = ° C ). Our study indicates that chloride complexes may contribute significant solubilities (> 1 ppb) of Pt and Pd at low to moderate temperature, but only under conditions which are highly oxidized (hematite stable), highly saline (>3 m NaCl eq ), and/or unusually acidic ( pH < 4). In the presence of aqueous sulfide, Pt-group element (PGE) solubilities as chloride complexes are extremely low (<1 ppt) and are probably less than the total contribution from other aqueous species (e.g., bisulfide, hydroxy, or ammonia complexes). However, the very strong temperature dependence of reaction (a1) suggests that much higher PGE mobility may be expected at supercritical temperatures. PGEs dissolved as chloride complexes may be deposited in response to reduction, increase in pH, dilution, and/or temperature decrease. Of these, reduction is the most effective mechanism and explains the common association of anomalous PGEs with strongly reduced horizons within sedimentary basins.
Bloom Michael S.
Gammons Christopher H.
Yu Yue
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