Chromium isotope fractionation during oxidation of trivalent chromium by birnessite at near-neutral pH

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A specific type of chromium, hexavalent chromium, is a known carcinogen and common environmental contaminant. Chromium ranks fifth in importance among inorganic contaminants at superfund sites. Hexavalent chromium enters our environment from industrial releases and weathering of certain rocks. The research project advances understanding of chromium geochemistry in modern and past environments by applying a novel approach that uses specialized, highly precise measurements of the stable isotopes of chromium. The objectives of this approach include (1) “fingerprinting” chromium to distinguish and track chromium sources in the environment and (2) determining when toxic, mobile, hexavalent chromium converts to immobile, non-toxic, trivalent chromium. Such reactions occurred early in Earth’s history and the research helps identify how and when ancient earth systems shifted from a low oxygen state to an oxygen-rich environment that fostered development of complex life. The findings will advance practical use of chromium isotope measurements to improve management of chromium contamination in the environment. To promote a diverse science workforce, the project provides environmental geochemical training for graduate and undergraduate students underrepresented in the geosciences. The research team provides geochemical content for outreach to a minority-serving school district, including a middle school girls geosciences summer camp.The research answers two outstanding questions regarding the fundamental drivers of chromium stable isotope variation. Chromium isotope measurements in modern waters and ancient rocks suggest that hexavalent chromium released by oxidative weathering is systematically enriched in heavy isotopes relative to trivalent Cr in the parent rock. This implies that oxidation of chromium favors the heavier isotopes, but the processes behind this phenomenon are paradoxical and not well understood. This presents a fundamental knowledge gap in our understanding of Cr isotope variations in both modern and ancient environments. The project targets two major questions: Does oxidation of solid-phase trivalent chromium at near-neutral pH generate significant chromium isotope shifts, and if so, does the magnitude of isotopic fractionation induced by chromium oxidation depend on geochemical variables such as pH, oxygen levels, and the type of chromium-bearing solids being oxidized? The project is answering these questions via a series of novel laboratory experiments wherein chromium present in three different solid forms, and in natural sediments, is oxidized under a range of relevant conditions by manganese oxides, the dominant chromium oxidant in natural settings.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.