Surface Passivation Limited UO2 Oxidative Dissolution in the

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Surface Passivation Limited UO2 Oxidative Dissolution in the Presence of FeS Yuqiang Bi and Kim F. Hayes* Department of Civil and Environmental Engineering, University of Michigan, Ann Arbor, Michigan 48109, United States S Supporting Information *

ABSTRACT: Iron sulfide minerals produced during in situ bioremediation of U can serve as an oxygen scavenger to retard uraninite (UO2) oxidation upon oxygen intrusion. Under persistent oxygen supply, however, iron sulfides become oxidized and depleted, giving rise to elevated dissolved oxygen (DO) levels and remobilization of U(IV). The present study investigated the mechanism that regulates UO2 oxidative dissolution rate in a flow-through system when oxygen breakthrough occurred as a function of mackinawite (FeS) and carbonate concentrations. The formation and evolution of surface layers on UO2 were characterized using XAS and XPS. During FeS inhibition period, the continuous supply of carbonate and calcium in the influent effectively complexed and removed oxidized U(VI) to preserve an intermediate U4O9 surface. When the FeS became depleted by oxidization, a transient, rapid dissolution of UO2 was observed along with DO breakthrough in the reactor. This rate was greater than during the preceding FeS inhibition period and control experiments in the absence of FeS. With increasing DO, the rate slowed and the rate-limiting step shifted from surface oxidation to U(VI) detachment as U(VI) passivation layers developed. In contrast, increasing the carbonate concentrations facilitated detachment of surface-associated U(VI) complexes and impeded the formation of U(VI) passivation layer. This study demonstrates the critical role of U(VI) surface layer formation versus U(VI) detachment in controlling UO2 oxidative dissolution rate during periods of variable oxygen presence under simulated groundwater conditions.

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coating may slow UO2 oxidative dissolution,13 its formation has not been quantitatively related to the dissolution rate. In a previous batch study, when FeS was being depleted, UO2 dissolved at a rate two times faster than in the absence of FeS under a continuous supply of oxygen.9 The accelerated dissolution of UO2 may have been due to variable development of surface passivation in the presence of FeS. However, because surface characterization of UO2 was not performed, the presence or impact of a passivation layer on the UO2 dissolution rate was not confirmed. In groundwater environments, in addition to oxygen, geochemical factors that may affect UO2 surface passivation and oxidative dissolution rates include pH, carbonate, and alkaline earth cations.16−19 In a typical groundwater pH range of 6.0−8.5, a surface coating is more likely to form on UO2 than at more acidic pH because a proton-promoted dissolution pathway is minimized.14 In the presence of complexing agents of U(VI), however, surface passivation may be inhibited as a result of enhanced U(VI) detachment from the UO2 surface.18 A number of field and laboratory studies have shown that carbonate accelerates UO2 oxidative dissolution since it forms stable

INTRODUCTION In situ treatment of uranium (U)-contaminated soils and groundwater often relies on promoting the reduction of soluble U(VI) to sparingly soluble U(IV) solids through chemical and biological processes.1−4 Under sulfate reducing conditions, the reduction of sulfate and Fe(III) may also lead to the formation of mackinawite (FeS),5 which can serve as an additional electron source for U(VI) immobilization and redox buffer for maintaining reducing conditions.6−8 Because FeS forms in close proximity to uraninite (UO2) in bioreduced zone,6 FeS is likely to prolong the stability of U(IV) solids in contaminated subsurface against reoxidation until its reducing capacity is exhausted. Recent studies demonstrated that effective oxygen scavenging by FeS can inhibit UO2 reoxidation and slow its oxidative dissolution rate in artificial groundwater solutions when oxygen intrusion occurs.9,10 Because oxygen preferentially reacted with FeS, UO2 remained predominantly as U(IV) during a prolonged “FeS inhibition period”, within which surface oxidation was likely the rate-limiting step.10 As FeS becomes oxidized and depleted, however, elevated DO levels may result in fast surface oxidation and buildup of oxidized surface products.11−14 A conceptual mechanistic model was proposed for UO2 oxidative dissolution, which involved an intermediate U(V) species and the formation of U4O9 below a metaschoepite-like surface layer, depending on oxygen concentration.15 Whereas the newly formed surface © XXXX American Chemical Society

Received: August 25, 2014 Revised: October 12, 2014 Accepted: October 16, 2014

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dx.doi.org/10.1021/es5041392 | Environ. Sci. Technol. XXXX, XXX, XXX−XXX

Environmental Science & Technology

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uranyl−carbonate complexes in solution.15,20−22 Recent research has further demonstrated that common alkaline earth cations in groundwater, such as Ca2+ and Mg2+, can also promote the UO2 dissolution rate through the favorable formation of uranyl− carbonato−Ca or −Mg complexes.23,24 It remains unclear, however, how the combined effect of these geochemical parameters influences the formation of passivation layers on UO2 and the oxidative dissolution during oxygen intrusion. In the present study, we examined the UO2 oxidative dissolution in the presence of oxygen after FeS was nearly completely oxidized, and characterized surface layers that form on UO2 by X-ray absorption spectroscopy (XAS) and X-ray photoelectron spectroscopy (XPS). The passivation effect of the surface layer and rate controlling mechanisms were demonstrated at changing DO concentrations under simulated groundwater conditions. The results of this study contribute to the understanding of the long-term stability of reduced UO2 during periods of persistent oxygen intrusion in the subsurface after active biostimulation.

previously characterized to consist of goethite, lepidocrocite, and elemental sulfur.9 Similar flow-through experiments were also conducted using ∼9 mM synthetic two-line ferrihydrite as a potential oxidant for UO2. Analyses. Total dissolved iron, uranium, and calcium were determined in effluent samples by ICP-MS (PerkinElmer ELAN DRC-e). Dissolved Fe(II) was determined photometrically by the ferrozine method at 562 nm using a UV/vis spectrophotometer (Varian). The dissolved Fe(II) equaled total dissolved Fe measured by ICP-MS within 5% error. Because aqueous sulfide, thiosulfate, and sulfate were previously determined to be negligible during the abiotic oxidation reaction,9,10 these sulfur species were not measured in the present study. In selected pH 7.1 experiments, solid suspensions were collected at various time points during oxidation to determine weakly bound U. Suspensions of oxidized FeS and UO2 were extracted for 21 h by a 0.5 M anoxic NaHCO3 solution at pH ∼7.8 in an anaerobic chamber to prevent U exposure to atmospheric oxygen.27 The filtered solutions were then measured for total dissolved U concentration by ICP-MS, which indicated a negligible amount of adsorbed U(VI) and monomeric U(IV) in all extraction experiments (10 μM in Figure 1a. When 4.8 mM FeS was being depleted, dissolved U slowly increased to a level (3.6 μM) that was even lower than in the control experiment (4.0 μM) in the absence of FeS. The reduced peak concentrations of U indicate that UO2 dissolved at slower rates under the lower carbonate concentration after DO breakthrough (Table 1). The dissolution rate of UO2 decreased from 4.2 × 10−8 to 2.1 × 10−8 mol·g−1·s−1 (18.7 mM initial FeS) and from 2.9 × 10−8 to 1.6 × 10−8 mol·g−1·s−1 (4.8 mM initial FeS) when DIC concentration dropped from 11.8 to 2.7 mM. In contrast, in the absence of FeS, the UO2 dissolution rate was not significantly affected by the reduced carbonate concentration (1.9 × 10−8 vs 2.0 × 10−8 mol· g−1·s−1). The rate-limiting influence of carbonate on UO2 oxidative dissolution was previously documented,15,18 and exhibited a log−linear relationship with HCO3− concentration until a threshold [HCO3−] value (>0.01 M). By facilitating the detachment of surface U(VI) complexes, UO2 dissolution rate can be promoted by a greater carbonate concentration. After exceeding the threshold, however, UO2 dissolution rate became invariant of carbonate concentration.15 The current and a recent study10 suggest that the dissolution rate−carbonate relationship should also depend on DO concentration. When DO levels are very low (1.5 mg/L) (see following Discussion). Effect of Carbonate Supply on UO2 Dissolution. To investigate the effect of carbonate on UO2 dissolution rate, flowthrough experiments were conducted at a lower DIC concentration. Using a 5% CO2/4% O2 gas mixture and 0.8 mM NaHCO3, DIC concentration decreased from 11.8 to 2.7 mM while the DO concentration remained at 1.8 mg/L in the E



Article

Figure 4. Deconvolution of representative U 4f XPS spectra of CSTR samples collected from pH 7.1 experiments in the presence of 4.8 mM FeS solids (experiment 2a). (a) Initial UO2; (b) 41 τ, effluent DO = 0.6 mg/L DO; (c) 56 τ, effluent DO = 0.9 mg/L DO; (d) 88 τ, effluent DO = 1.8 mg/L DO. All experiments were carried out under influent DO of 1.8 mg/L artificial groundwater conditions.

∼1.8 mg/L, the dissolution rate appears independent of carbonate concentration again, as shown in the control experiments. Comparable steady-state U profiles were quickly established in the absence of FeS at the two different carbonate concentrations (Figure 2a and SI Figure S2), suggesting that a threshold carbonate concentration was achieved. The results show that the accelerating effect of carbonate on UO2 dissolution is indirectly related to the presence of FeS and is impacted by the changing DO levels during FeS depletion. Characterization of UO2 Surface Passivation. To elucidate the rate-controlling mechanism of UO2 dissolution during FeS depletion, XAS and XPS analyses were performed on samples collected during rapid U release after DO breakthrough. For XAS, two solid samples were taken from the CSTR in the presence of 18.7 mM FeS (experiment 1a): one shortly after the inhibition period ended (∼165 τ), the other after the U peak concentration (∼220 τ). The samples were analyzed for U LIIIedge XANES and EXAFS to detect changes in solid phase U speciation that would indicate an increasing presence of U(VI) as

oxidation proceeded. The XANES results of U solids showed that the 165 τ sample was predominantly present as U(IV) (∼90%) as indicated by its unchanged absorption edge positions from the U(IV) standard (SI Figure S4). When significant dissolution of UO2 occurred at 220 τ, the absorption edge slightly shifted from U(IV) standard to a higher energy. A linear combination fitting (LCF) of the XANES revealed that the U(IV) component in the bulk solid decreased from 90% to ∼60% of U as oxidative dissolution proceeded from 165 to 220 τ (SI Table S1). The results agree with previous studies, which demonstrated a lack of accumulation of U(VI) species during an inhibition period.9,10 Only when DO concentration increased to a higher level after breakthrough (>1.5 mg/L), partial oxidation of UO2 occurred. It should be noted that the XANES results do not rule out the possible presence of U(V), given the similar L-edge energy positions of U(IV) and U(V).30 A qualitative comparison of the EXAFS spectra shows similar characteristics between 165 and 220 τ samples (Figure 3), although the spectra amplitudes were significantly reduced F



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Table 2. Mole Percentage of U(IV) and U(VI) as Determined by XPS on CSTR Samples Collected from Selected Flow-through Experiments Containing 0.48 mM Synthetic UO2 and Influent DO of 1.8 mg/L selected exp. ID

[FeS] (mM)

[DIC]a (mM)

reaction time (τ)

U(IV) (%)

U(V) (%)

U(VI) (%)

effluent DO at the time of collection (mg/L)

2a 2a 2a 2a 6a 6a 7a

4.8 4.8 4.8 4.8 4.8 4.8 0

11.8 11.8 11.8 11.8 2.7 2.7 2.7

0 41 56 88 31 58 32

53 44 45 4 49 91% at 58 τ when DO rose to ∼1.6 mg/L. Whereas Ca2UO2(CO3)3(aq) complex remained the dominant U(VI) species (calculated by Visual MINTEQ), the 2.7 mM DIC was less effective in removing U(VI) from the surface. With increasing DO, a thin surface layer on UO2 appears to evolve from a mixed U(IV)/U(V)/U(VI) oxide to a completely oxidized U(VI) phase. Similarly, in the control experiment without FeS the UO2 surface quickly passivated to produce a U(VI)-dominant coating within 30 τ at ∼1.8 mg/L influent DO (SI Figure S5a). The combined results of XAS and XPS indicate that during the FeS inhibition period, the U(VI) species present after UO2 synthesis was effectively removed by 11.8 mM DIC, leaving an intermediate U4O9 surface layer on a bulk core of UO2.0. During FeS depletion and DO breakthrough, a relatively fast oxidation may cause the accumulation of U(VI) on surface, which develops into to a U(VI)-rich layer with increasing DO concentration. These results are consistent with the previous reporting of a layered structure forming on the surface of oxidized UO2 particles, with a U(VI) outer layer, a U4O9 intermediate layer, and a UO2.00 core.15 Unlike the metaschoepite-like U(VI) layer suggested by Ulrich et al.,15 however, a uranyl carbonate mineral phase, i.e., liebigite, was possibly produced at the relatively high calcium concentration (2 mM) in this study. Enhanced UO2 Dissolution in the Absence of U(VI) Passivation Layer. Oxidative dissolution of UO2 by oxygen is proposed to occur through a sequence of reaction steps, including oxygen adsorption, surface oxidation, and subsequent release of U(VI) products.11,31 Because UO2 surface passivation was largely prevented in the presence of FeS and 11.8 mM DIC during the FeS inhibition period, the UO2 dissolution rate was believed to be controlled by a surface oxidation step.10 The absence of U(VI) accumulation as determined by XAS and XPS supports the conclusion that the U(VI) surface detachment step is rapid, preventing the buildup of a U(VI) coating when DO is low (1.5 mg/L to induce surface U(VI) at 11.8 mM DIC. Therefore, the faster and more complete U(VI) detachment that occurs at higher carbonate concentration requires a higher oxygen level for surface passivation. Once the surface becomes passivated, however, UO2 particles appear to dissolve more slowly and carbonate concentration has less impact on UO2 dissolution rate. As shown in control experiments at the two different DIC concentrations (2.7 vs 11.8 mM), UO2 dissolved at similar rates (1.9 × 10−8 vs 2.0 × 10−8 mol·g−1·s−1) when a U(VI) coating rapidly developed on the

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ASSOCIATED CONTENT

S Supporting Information *

Additional methods, figures, and tables of experimental results. This material is available free of charge via the Internet at http:// pubs.acs.org. H



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AUTHOR INFORMATION

Corresponding Author

*Tel: 734-763-9661; fax: 734-763-2275; e-mail: ford@umich. edu. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported by Subsurface Biogeochemical Research Program in the Office of Science (BER), U.S. Department of Energy, Grant DE-FG02-09ER64803. We thank Sandra Fernando for analytical help on XPS. Part of this research was carried out at SSRL. Use of the Stanford Synchrotron Radiation Lightsource, SLAC National Accelerator Laboratory, is supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under Contract DE-AC02-76SF00515.

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Surface Passivation Limited UO2 Oxidative Dissolution in the

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