The trisulfur radical ion S ₃ ^•− controls platinum transport by hydrothermal fluids

Abstract

Significance Platinum group elements (PGE) are highly valued by many industrial applications, serve as important geochemical tracers of planetary processes, and are among eagerly sought natural resources of critical trace metals. We show here that aqueous sulfur-bearing fluids play a far more important role than believed in many natural and technological processes in which PGE are involved. This role is essentially ensured by the radical sulfur ion S 3 •− that forms extremely stable and soluble complexes with both Pt II and Pt IV . By enabling enhanced dissolution, transport, and precipitation of Pt, these species may be key players in PGE transfer, fractionation, and accumulation in the Earth’s crust. They may also offer novel opportunities for optimizing ore processing and nanomaterial synthesis.

Stability and structure of platinum sulfide complexes in hydrothermal fluids

“…The rare studies of Pt sulfide type complexes at hydrothermal conditions are both inconsistent and controversial. This is because all of them employed the bulk solubility approach to the PtS(s) and PtS 2 (s) solid phases using hydrothermal reactors (Gammons and Bloom, 1993;Pan and Wood, 1994;Kokh et al, 2017;Filimonova et al, 2021;Pokrovski et al, 2021). Such solubility experiments are intrinsically difficult to perform and interpret both because of the low, often ppb-level, Pt dissolved concentrations posing severe analytical challenges, and the narrow pH and sulfur concentration ranges that may practically be covered by such experiments.…”

“…However, the ppblevel Pt concentrations found might have been subjected to significant uncertainties by the use of a quench technique that often suffers from elevated analytical backgrounds. Finally, Pokrovski et al (2021) measured PtS(s) solubility in sulfate-sulfide bearing solutions containing significant concentrations of the trisulfur radical ion S 3within a narrow T-P and S concentration range (275-300 °C, 500-700 bar, 1-2 m S) using different types of hydrothermal reactors (both quench-and samplingbased) as well as in situ X-ray absorption spectroscopy. Their data show that the presence of S 3enhanced Pt solubility by 3 to 5 log units compared to H 2 S-dominated solutions, due to the formation of mixed Pt-HS --S 3species.…”

“…Their data show that the presence of S 3enhanced Pt solubility by 3 to 5 log units compared to H 2 S-dominated solutions, due to the formation of mixed Pt-HS --S 3species. However, the results of Pokrovski et al (2021) might be dependent on the choice of data for Pt-HS complexes dominant in S 3 --free solutions. This brief overview highlights that far more systematic data are required to constrain the nature and stability of Pt sulfide complexes, which is unlikely to be achieved using solely bulk-solubility approaches.…”

“…Extensive XAS measurements have been performed on various Pt mineral phases and aqueous and surface complexes in both technological and geological materials (e.g., Ayala et al, 2001;Peyrelade, 2005;Crowther, 2007;Gorczyca et al, 2014;Scholten et al, 2018;Tagirov et al, 2019;Filimonova et al, 2019). In contrast, no XAS data are available for sulfurbearing hydrothermal fluids (with a single exception of Pokrovski et al, 2021, to the best of our knowledge), and Pt-HS complexes have never been directly investigated by XAS. Another rapidly emerging theoretical method, first-principles molecular dynamics (FPMD) simulations, represents a valuable complementary approach to both solubility and XAS to investigate the structure and stability of metal complexes.…”

“…A commercially available platinum metal foil (GoodFellow; 100 µm thickness, 99.95% purity) was used in XAS hydrothermal experiments instead of PtS(s) powder to avoid an eventual contamination of aqueous solution XAS spectra by the signal from fine solid particles getting dispersed in the solution. The Pt metal foil gets sulfidized very quickly in contact with the sulfur-bearing fluid by forming a strongly adhering homogeneous layer of PtS(s) in equilibrium with the fluid (Pokrovski et al, 2021). For each experiment, a new piece of Pt foil was used to avoid potential cross-contamination; the piece was washed with hot acid (5 wt% HCl + HNO 3 for 30 min) before each experiment to increase its surface roughness and enable more rapid and efficient sulfidation.…”