[en] The goal of this research was to study the reoxidation of biologically precipitated U(IV). Several experiments were performed and are summarized below. These experiments include: (1) a long-term (∼200 days) U(VI) reduction experiment under low sulfate conditions in order to study in detail changes in iron phases and biomass and determine how they affect/buffer reoxidation; (2) a short term (∼70 days) experiment where we tracked the uranium profile via XANES prior to reoxidation and during reoxidation in order to determine the U speciation; (3) a short term experiment where we compare the oxidation of U(IV) by oxygen and nitrate in the absence of FeS; and (4) a short term experiment where we compare the oxidation of U(IV) by oxygen and nitrate in the presence of FeS precipitates.