Uranium (U) contamination of ground and surface waters poses an acute hazard on the ecosystem and human health. Since the discovery of microbial U(VI) reduction, U bioremediation has been explored as a promising and cost-effective method compared to traditional treatments. The speciation of the bioreduction product was originally stated to be uraninite (UO2) which is a recalcitrance crystalline U(IV) species. On the other hand, recent studies demonstrated that non-crystalline species (NCU4) are the dominant product of bioreduction. Since NCU4 species are more labile than UO2, new concerns are associated with the long-term stability of these U(IV) products. In fact, the effectiveness of bioremediation depends on the resistance of NCU4 species to oxidation and remobilization into solution. In this regard, it was previously hypothesized that aging might transform NCU4 to UO2 that would enhance the resistance of U(IV) to oxidation and release into the aqueous phase. We investigated this hypothesis by incubating NCU4 species immobilized in natural sediments under anoxic conditions for a period of 12 months. We systematically probe the speciation of U in the sediment using X-ray absorption spectroscopy. Under the investigated conditions, NCU4 does not age to UO2. Thus it is likely that NCU4 produced during bioremediation persists in the subsurface even for an extended period of time if anoxia is maintained. Therefore the re-mediated site remains vulnerable to events that bring oxygen into the reduced zone. In fact, NCU4 species are rapidly oxidized upon exposure to oxygen. Furthermore, we demonstrated that under certain conditions (i.e., high dissolved oxygen (DO) concentration), the presence of FeS accelerates the oxidation and re-mobilization of NCU4 via the production of reactive oxygen species. Since ROS production during FeS oxidation depends on the concentration of DO and the speciation of Fe(II) in the sediments, at low DO concentrations, low amounts of ROS were detected, and the enhancement of U(IV) oxidation by FeS be-came negligible. Moreover, this thesis investigates the isotopic fractionation (238U/235U) during abiotic reduction by magnetite (Fe3O4), a Fe(II) bearing mineral capable of reducing U(VI) that is commonly found in bioreduced zones. The results of preliminary experiments confirmed that U reduction by Fe3O4 has opposite fractionation than predicted by the nuclear field shift effect suggesting that a kinetic effect may drive fractionation during reduction. Preliminary XAS speciation of U immobilized on the surface of magnetite report that U(V) may occurs during reduction.