Uranium(IV) adsorption by natural organic matter in anoxic sediments
Significance Uranium is an important fuel source and pollutant, and its chemical form determines its reactivity in the environment. However, information on the speciation of tetravalent U [U(IV)] in sediments is lacking. The research presented herein takes a holistic view of U(IV) speciation in a natural material containing microbial cells, organic matter, and minerals. Our work demonstrates unequivocally that U(IV) adsorbs to natural organic matter under anoxic, field-relevant conditions. Furthermore, we put forward a conceptual model that provides a framework for future studies of U biogeochemistry in which postreduction surface complexation processes can be used to predict U(IV) behavior. Our work has ramifications for remediation of U-contaminated sites and also informs in situ mining practices. Uranium is an important carbon-free fuel source and environmental contaminant that accumulates in the tetravalent state, U(IV), in anoxic sediments, such as ore deposits, marine basins, and contaminated aquifers. However, little is known about the speciation of U(IV) in low-temperature geochemical environments, inhibiting the development of a conceptual model of U behavior. Until recently, U(IV) was assumed to exist predominantly as the sparingly soluble mineral uraninite (UO 2+x ) in anoxic sediments; however, studies now show that this is not often the case. Yet a model of U(IV) speciation in the absence of mineral formation under field-relevant conditions has not yet been developed. Uranium(IV) speciation controls its reactivity, particularly its susceptibility to oxidative mobilization, impacting its distribution and toxicity. Here we show adsorption to organic carbon and organic carbon-coated clays dominate U(IV) speciation in an organic-rich natural substrate under field-relevant conditions. Whereas previous research assumed that U(IV) speciation is dictated by the mode of reduction (i.e., whether reduction is mediated by microbes or by inorganic reductants), our results demonstrate that mineral formation can be diminished in favor of adsorption, regardless of reduction pathway. Projections of U transport and bioavailability, and thus its threat to human and ecosystem health, must consider U(IV) adsorption to organic matter within the sediment environment.
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