Due to its widespread use in many industrial processes, chromium (Cr) has become the second most common metal contaminant in the United States. Under environmentally relevant circumneutral pH conditions, trivalent Cr(III) hosted in solid phases can be complexed and solubilized by many types of organic molecules and subsequently oxidized by manganese oxides to the toxic and mobile Cr(VI). Significant isotope fractionation has been shown to occur during initial oxidation and subsequent reduction processes. Thus, Cr isotope systematics offer a quantitative means of assessing the long term transport and fate of chromium in the natural environment in the presence of microbial activities. Furthermore, the Cr isotopes system has been proposed as an atmospheric paleobarometer suitable for tracking the earliest evolution of oxygenic photosynthesis. Our focus is on determining whether significant Cr isotope fractionations can be induced without Cr(III) oxidation. We examined the dissolution of mixed Cr(III)-Fe(III) (oxy)hydroxides, common subsurface sinks for chromium, in the presence of common microbial exudates, including metal chelating ligand siderophores and organic acids. Our preliminary experimental results indicate that it is indeed possible to generate significant Cr isotope fractionation during solubilization of solid-phase Cr(III). Results show that Cr(III) solubilized by DFOB (a common type of microbial siderophore) with or without oxalate is isotopically lighter (-0.5±0.08‰) than the starting solid-phase Cr(III) (-0.07±0.08‰). The observation of significant isotope fractionation during Cr(III)-ligand solubilization, and its directionality, has important implications for the of application of the Cr isotope system in the interpretation of Cr biogeochemical cycling and its use as a paleoredox proxy.

Isotope fractionation induced by ligand-promoted mobilization of Cr(III)

251st National Meeting (2016)

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