Characterization of ion sorption on metal sulfide minerals using macroscopic and spectroscopic approaches

Sulfide minerals commonly form in anoxic environments, often sorbing trace elements. The mechanism of such sorption is a critical factor in determining their transport and fate in the environment. Consequently, this research examines the sorption of three such elements, arsenic, molybdenum, and zinc on sulfide mineral suspensions and in soils subject to periodic reduction. Arsenite sorption to sphalerite (ZnS), galena (PbS), amorphous iron sulfide (FeS), and pyrite (FeS₂) was investigated as a function of solution composition and using X-ray absorption spectroscopy (XAS). Sorption to each followed a Langmuir isotherm, and increased sharply above pH 5 to 6. Spectroscopic investigation revealed that a large change in arsenic coordination accompanied sorption on sulfide minerals. An As₃S₃(SH)₃-type surface complex was formed on ZnS and PbS, while As(III) was reduced to As(−I) on FeS and FeS₂ and a surface precipitate similar in structure to FeAsS was formed. This surface precipitate was also formed upon arsenite sorption to estuarine soils from Pescadero, California, although it was partially converted to orpiment over time. Importantly, arsenite was irreversibly retained on each of these sulfide minerals except PbS; thus, arsenic will likely be stable in sulfidic, anoxic environments. However, oxidation may release arsenic in these systems.; Other ions react in distinct manners. Molybdate and tetrathiomolybdate are most strongly retained to pyrite under acidic conditions. Molybdate formed a weak bidentate-mononuclear sorption complex on FeS₂ that was reversibly adsorbed while tetrathiomolybdate adsorption was irreversible and it formed a Mo-Fe-S cubane on the FeS₂ surface, in which the Mo is triply coordinated to surface-bound sulfur atoms. Zinc was often sequestered in a contaminated wetland as ZnS, ZnCO₃, ZnO and Zn adsorbed on (hydr)oxides. Zinc speciation was dynamic, responding to changes in overlying water depth. The fractions of ZnS and ZnCO₃ were highest under flooded conditions, while adsorbed Zn species occurred predominantly under oxic conditions (dry or shallow water depths). Therefore, the dynamic nature of sulfidic environments needs to be considered when determining trace element partitioning.