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Until recently, the reduction of U(VI) to U(IV) during bioremediation was assumed to produce solely the sparingly soluble mineral uraninite, UO_2(s). However, results from several laboratories reveal other species of U(IV) characterized by the absence of an EXAFS U–U pair correlation (referred to here as noncrystalline U(IV)). Because it lacks the crystalline structure of uraninite, this species is likely to be more labile and susceptible to reoxidation. In the case of single species cultures, analyses of U extended X-ray fine structure (EXAFS) spectra have previously suggested U(IV) coordination to carboxyl, phosphoryl or carbonate groups. In spite of this evidence, little is understood about the species that make up noncrystalline U(IV), their structural chemistry and the nature of the U(IV)–ligand interactions. Here, we use infrared spectroscopy (IR), uranium L_III-edge X-ray absorption spectroscopy (XAS), and phosphorus K …