Comparative dissolution kinetics of biogenic and chemogenic uraninite under oxidizing conditions in the presence of carbonate

Abstract

The long-term stability of biogenic uraninite with respect to oxidative dissolution is pivotal to the success of in situ bioreduction strategies for the subsurface remediation of uranium legacies. Batch and flow-through dissolution experiments were conducted along with spectroscopic analyses to compare biogenic uraninite nanoparticles obtained from Shewanella oneidensis MR-1 and chemogenic UO 2.00 with respect to their equilibrium solubility, dissolution mechanisms, and dissolution kinetics in water of varied oxygen and carbonate concentrations. Both materials exhibited a similar intrinsic solubility of $10 À8 M under reducing conditions. The two materials had comparable dissolution rates under anoxic as well as oxidizing conditions, consistent with structural bulk homology of biogenic and stoichiometric uraninite. Carbonate reversibly promoted uraninite dissolution under both moderately oxidizing and reducing conditions, and the biogenic material yielded higher surface areanormalized 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 a stable U(V)-bearing intermediate phase 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-associated U(VI) increased with dissolved oxygen concentration. Simultaneously, X-ray absorption spectra showed conversion of the bulk from UO 2.0 to UO 2+x . In equilibrium with air, combined spectroscopic results support the formation of a near-surface layer of approximate composition UO 2.25 (U 4 O 9 ) coated by an outer layer of U(VI). This result is in accordance with flowthrough dissolution experiments that indicate control of the dissolution rate of surface-oxidized uraninite by the solubility of metaschoepite under the tested conditions. Although U(V) has been observed in electrochemical studies on the dissolution of spent nuclear fuel, this is the first investigation that demonstrates the formation of a stable U(V) intermediate phase on the surface of submicron-sized uraninite particles suspended in aqueous solutions.

Structural Similarities between Biogenic Uraninites Produced by Phylogenetically and Metabolically Diverse Bacteria

“…XANES spectra indicated that the immobilized precipitates were consistent with 100 ( 10% UO 2.00 for five out of six strains. As described in the Supporting Information, a best fit to XANES demonstrates that the A. dehalogenans product contains higher valent U, possibly surface U(V) atoms (26), corresponding to a composition of UO 2.075 ( Figure SI-2 …”

“…As such, it provides justification for geochemical models for bacterial products based on a single set of rate and thermodynamic constants (26,30).…”

Products of abiotic U(VI) reduction by biogenic magnetite and vivianite

“…While the reactivity of biogenic uraninite has been studied and documented (Ulrich et al, 2008;Ulrich et al, 2009), the reactivity and stability of monomeric U(IV) in the environment is unknown. The results presented in this paper suggest that there is a wealth of U(IV) chemistry not fully understood in these systems, and that there may be complex mixtures of U(IV) products in the field.…”

XPS Study of Ion Irradiated and Unirradiated UO2 Thin Films

“…17,50,55,56,59,60,61,62 A special attention was paid to the study of mechanisms of structure formation, which leads to widening of main peaks and appearance of extra structure in the spectra. 50,59,63 The XPS spectra of some oxides exhibit typical shake-up satellites of about ~25% intensity of the basic peaks 41,51,60,64 The calculated spectroscopic factors f Anf reflecting the fractions of the basic peak XPS intensities with the deduction of shake-up satellite intensities for the U 4f and U 5f peaks are: f U4f =0.83 and f U5f =0.86. 65,66 Shake-up satellites are located from the basic peaks toward the higher BE by ∆E sat (U4f): ~7 eV (U 4+ ); ~8 eV (U 5+ ); ~4 eV and ~10 eV (U 6+ ).…”

“…14, 15,16 The work on the study of the electronic structure of uranium and its alloys [17][18][19][20][21] and oxides UO 2+x (x≤0 and x≥0) employed theoretical calculation results, [22][23][24][25][26][27][28][29][30][31][32][33][34] photoelectron spectroscopy (PES) [35][36][37][38][39][40][41] and X-ray photoelectron spectroscopy (XPS) widely. 38,[42][43][44][45][46][47][48][49][50][51][52][53][54][55] These techniques were used to study films on various substrates. [38][39][40][56][57][58] Determination of uranium oxidation state and sample's ionic composition employs the U 4f doublet split due to the spin-orbit interaction by ∆E sl (U4f)=10.8 eV.…”