Molybdenum stable isotope fractionation during the precipitation of powellite (CaMoO4) and wulfenite (PbMoO4)

Abstract

Laboratory batch experiments were conducted to study Mo isotope fractionation during the precipitation of powellite (CaMoO4) and wulfenite (PbMoO4). These minerals act as important sinks of dissolved Mo, especially in alkaline-pH mine drainage. Their precipitation might be traceable through Mo isotope analyses, provided that it induces Mo isotope fractionation. Experiments involved the presence/absence of dissolved sulfate and inorganic carbon (DIC) to better mimic the aqueous chemistry of mine drainage, as these constituents are characteristic of mine waters.Molybdate minerals preferentially incorporated light Mo isotopes from aqueous solution with fractionation patterns that are linked to the saturation state. The Rayleigh fractionation model was most suitable to describe powellite precipitation under supersaturated conditions. The average Rayleigh isotope separation factor for powellite precipitation was −0.82 ± 0.12‰, with no significant effect from the presence/absence of sulfate and DIC. Upon approaching chemical equilibrium, bidirectional exchange of molybdate between dissolved Mo and powellite led to a pronounced release of isotopically light Mo to solution, which overrode the Rayleigh fractionation pattern formed at early time. Molybdenum isotope data during wulfenite precipitation could reasonably be fitted using the Rayleigh and equilibrium fractionation models, which yielded isotope separation factors of −0.25 ± 0.10‰ and −0.75 ± 0.07‰, respectively. These results contribute to a growing knowledge of Mo isotope fractionation during processes of dissolved Mo removal (e.g., adsorption and precipitation), thus far all of which show that an enrichment in heavy Mo isotopes in solution is indicative of Mo attenuation.