PNNL

Abstract

The long-term stability of biogenic uraninite with respect to oxidation is pivotal to the success of in situ bioreduction strategies for the subsurface remediation of uranium legacies. Given the structural homology between biogenic uraninite nanoparticles obtained from Shewanella oneidensis MR-1 and chemogenic UO2.00, similar surface free energy and reactivity is expected. Batch and flow-through dissolution experiments were conducted along with spectroscopic analyses to compare both materials with respect to their equilibrium solubility, dissolution mechanisms, and dissolution kinetics in water of varied oxygen and carbonate concentrations. Both uraninite materials exhibited a similar intrinsic solubility of ~10-8 M under reducing conditions. The observation of comparable dissolution rates under anoxic as well as oxidizing conditions is consistent with the structural bulk homology of biogenic and stoichiometric uraninite. Carbonate, a ubiquitous groundwater component and strong complexant of U(VI), reversibly promoted the uraninite dissolution not only under moderately oxidizing, but also under reducing conditions, where the biogenic material yielded higher dissolution rates than the chemogenic. This difference is in accordance with the higher proportion of U(V) detected on the biogenic uraninite surface by means of x-ray photoelectron spectroscopy. Reasonable sources of the intermediate U(V) are discussed. The observed increase of the dissolution rates can be explained by carbonate complexation of U(V) facilitating the detachment of U(V) from the uraninite surface. The fraction of surface U(VI) increased with increasing oxygen concentration; this result is consistent with x-ray absorption near-edge spectra showing evidence of higher-valent U in the form of UO2+x (0