Article pubs.acs.org/est
Effect of Ca2+ and Zn2+ on UO2 Dissolution Rates José M. Cerrato,*,† Charles J. Barrows,†,§ Lisa Y. Blue,†,∥ Juan S. Lezama-Pacheco,‡ John R. Bargar,‡ and Daniel E. Giammar† †
Department of Energy, Environmental, and Chemical Engineering, One Brookings Drive, Washington University, St. Louis, Missouri 63130, United States ‡ Stanford Synchrotron Radiation Lightsource, SLAC, 2575 Sand Hill Road, Menlo Park, California 94025, United States S Supporting Information *
ABSTRACT: The dissolution of UO2 in a continuously stirred tank reactor (CSTR) in the presence of Ca2+ and Zn2+ was investigated under experimental conditions relevant to contaminated groundwater systems. Complementary experiments were performed to investigate the effect of adsorption and precipitation reactions on UO2 dissolution. The experiments were performed under anoxic and oxic conditions. Zn2+ had a much greater inhibitory effect on UO2 dissolution than did Ca2+. This inhibition was most substantial under oxic conditions, where the experimental rate of UO2 dissolution was 7 times lower in the presence of Ca2+ and 1450 times lower in the presence of Zn2+ than in water free of divalent cations. EXAFS and solution chemistry analyses of UO2 solids recovered from a Ca experiment suggest that a Ca−U(VI) phase precipitated. The Zn carbonate hydrozincite [Zn5(CO3)2(OH)6] or a structurally similar phase precipitated on the UO2 solids recovered from experiments performed in the presence of Zn. These precipitated Ca and Zn phases can coat the UO2 surface, inhibiting the oxidative dissolution of UO2. Interactions with divalent groundwater cations have implications for the longevity of UO2 and the mobilization of U(VI) from these solids in remediated subsurface environments, waste disposal sites, and natural uranium ores.
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INTRODUCTION Dissolved U(VI) species can be found in contaminated soils and groundwater as a consequence of anthropogenic activities associated with mining, milling, and processing of uranium for nuclear fuel production and weapons manufacture. In situ bioremediation of uranium makes use of microbiologically mediated reduction of U(VI) to produce uraninite (UO2) and other U(IV) species that are orders of magnitude less soluble than U(VI) species.1−3 The dissolution rate of biogenic UO2 in aquifers should be low in order for uranium immobilization to be effective. Information on UO2 dissolution at environmentally relevant chemical conditions (i.e., near-neutral pH, presence of important solutes) is necessary to understand the viability of microbial bioremediation strategies1 and the longterm disposition of spent nuclear fuel in geological repositories.4 Groundwater chemistry profoundly affects the stability of UO2. Dissolved inorganic carbon (DIC) reversibly promoted the dissolution of UO2 at near-neutral pH conditions under reducing and oxidizing conditions.5 Previous studies have also reported orders of magnitude higher dissolution rates with increased dissolved oxygen and decreased pH.6−8 The effects of groundwater cations on UO2 dissolution may also substantially impact uranium immobilization strategies. Ca2+ is naturally abundant in groundwater, and a recent study of UO2 reaction at the Old Rifle, CO, site aquifer found that the solid accumulated substantial calcium [0.2−0.3 mol Ca (mol U)−1].9 The suppression of oxidative dissolution of UO2 in the presence of © 2012 American Chemical Society
Ca has been attributed to the possible formation of lowsolubility Ca−U(VI) phases at the UO2 surface.4 Zn2+ can also be introduced to groundwater through anthropogenic activities and from geogenic sources10 and can occur as a cocontamiant with U, such as at the Durango, CO, legacy ore processing site.11 Singer et al. showed that Zn2+ uptake to UO2 at pH 6.5 is dominated by bidentate edge-sharing inner-sphere surface complexes.12 Both Ca2+ and Zn2+ are nonredox active divalent cations and should not directly oxidize or reduce U in the solids. Zn2+ has a stronger sorption affinity to metal oxides than Ca2+.13,14 Potential mechanisms by which dissolved Ca2+ and Zn2+ can affect UO2 dissolution rates are (a) enhancement of UO2 dissolution caused by the formation of ternary Ca− U(VI)−carbonate aqueous complexes15 and (b) inhibition of UO2 dissolution by adsorbed or precipitated phases that block UO2 surface sites from oxidants that promote oxidative dissolution.4,16 Furthermore, the formation of secondary U(VI) phases with Ca/Zn at the surface may alter the electronic structure and limit further oxidation of the underlying UO2 or sequester any U(VI) that is produced and prevent its release to solution. The primary objective of this study was to evaluate the effect of Ca2+ and Zn2+ on the kinetics of UO2 dissolution under Received: Revised: Accepted: Published: 2731
October 22, 2011 December 14, 2011 February 3, 2012 February 3, 2012 dx.doi.org/10.1021/es203751t | Environ. Sci. Technol. 2012, 46, 2731−2737
Environmental Science & Technology
Article
Table 1. Experimental UO2 Dissolution Rates Normalized to Mass (Rm) and Surface Area (Rn) Obtained under Various Combinations of Oxidizing Conditions and Water Compositions experimental conditiona 2+
5 mM Ca 5 mM Ca2+ 5 mM Ca2+ 0.1 mM Zn2+ 0.1 mM Zn2+ 0.1 mM Zn2+ ultrapureb ultrapureb ultrapureb a
pH 7.8 8.2 8.2 7.5 7.5 7.5 8.6 8.6 8.0
flow, mL min‑1 1.06 1.67 1.75 1.97 1.90 1.77 1.97 1.98 1.95
DO, mg L‑1
Rm, mol g‑1 s‑1 −11
3.5 × 10 8.5 × 10−11 4.1 × 10−9 1.7 × 10−11 2.1 × 10−11 2.0 × 10−11 4.9 × 10−11 3.3 × 10−9 2.9 × 10−8
0 0.4 8.2 0 0.4 8.2 0 0.4 8.2
Rn, mol m‑2 s‑1
[U]ss, μM
5.9 × 10−12 1.4 × 10−11 7.0 × 10−10 2.9 × 10−12 3.6 × 10−12 3.4 × 10−12 8.6 × 10−12 5.6 × 10−10 4.8 × 10−9
0.0267 0.0354 1.6700 0.0063 0.0085 0.0089 0.0147 1.2000 8.6400
All experiments were performed in the presence of 1 mM DIC. bData from Ulrich et al.5
influent were collected after reaction under anoxic conditions for further characterization by scanning electron microscopy (SEM) and X-ray absorption spectroscopy (XAS). A sample representative of UO2 reacted at anoxic conditions in the presence of Zn2+ was prepared in a parallel batch experiment to generate solids for SEM and XAS analyses. Experimental dissolution rates were determined using eqs 1 and 2:
different oxidizing conditions. A secondary objective was to investigate sorption and precipitation reactions of Ca2+ and Zn2+ on the UO2 surface, which may impact UO2 dissolution. The overall goal of this research was to advance understanding of the effects of nonredox active sorbent cations of contrasting coordination chemistry on UO2 stability under environmentally relevant conditions.
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METHODS Materials. All reagents used in this study were certified analytical grade or better and used without further purification. Synthetic uraninite (syn-UO2) was used for this study; this material will be called “uraninite (UO2)” throughout this paper. The protocol used for synthesis and preparation of UO2 was previously described in detail by Ulrich et al.17 Briefly, studtite was precipitated by reaction of uranyl nitrate with hydrogen peroxide. The precipitated studtite was dialyzed against ultrapure water and dried. The dried solid was reduced to uraninite by heating to 400 °C with H2(g). This synthesis yielded crystalline UO2, as confirmed by X-ray diffraction analysis. UO2 preparation and aliquot transfer were performed in an anaerobic chamber (Coy Laboratory Products Inc.) containing a gas mixture of 95% (v/v) N2(g) and 5% (v/v) H2(g) circulated through a Pd catalyst and Si gel. UO2 Dissolution Experiments in the Presence of Ca2+ and Zn2+. Continuously stirred tank reactors (CSTRs) were used to measure UO2 dissolution rates at conditions relevant to groundwater systems.5,17 The volume of the CSTRs was 12.6 mL; influents were provided with a peristaltic pump, and flow rates for these experiments are presented in Table 1. Influent solutions consisted of 5 mM Ca as CaCl2 or 0.1 mM Zn as ZnCl2 (Acros Organics) plus 1 mM dissolved inorganic carbon (DIC) added as NaHCO3; 5 mM HEPES buffer was used to maintain the pH in the range from 7.5 to 8.2. A 0.025 μm membrane (mixed cellulose ester, Millipore) was placed at the top of each reactor to retain the solids within the reactor and while producing a filtered effluent for sample collection. Reactors were loaded with 1 g L−1 UO2 and influent solution was fed under anoxic conditions for approximately five residence times to equilibrate the UO2 and desorb U(VI) initially present due to impurities in the solid. Subsequently, the reactors were operated for 18 h under increasing dissolved oxygen (DO) content (6 h per condition): 0 mg L−1 DO [water sparged with 5% (v/v) H2/95% (v/v) N2 in presence of Pd catalyst], 0.4 mg L−1 DO [sparged with 1% (v/v) O2/99% (v/v) N2], and 8.2 mg L−1 (sparged with air). Influent and effluent samples were regularly collected for U and pH analyses. In one experiment, solids from a reactor with a Ca-containing
Rm =
Q [U]ss V [solid]
Rn =
Q [U]ss V × SSA × [solid] −1
(1)
(2)
−1
where Rm (mol g min ) is the dissolution rate normalized to mass, Rn (mol g−1 m−2) is the dissolution rate normalized to surface area, [U]ss (mol L−1) is the effluent U concentration at steady state, Q (L min−1) is the flow rate, V (L) is the reactor volume, [solid] (g L−1) is the solid concentration in the reactor, and SSA (m2) is the specific surface area. Adsorption Experiments of Ca2+ and Zn2+ onto UO2. Batch experiments were performed in an anaerobic chamber using 50 mL polyethylene reactors loaded with 0.25 g L−1 UO2. The reactors were operated in duplicate at 25 °C and mixed by end-over-end rotation at 10 rpm. Each reactor contained a background aqueous solution adjusted to pH 7.5 at a target ionic strength of 0.01 M after the addition of 9.97 mM NaCl and 250 μM HEPES buffer. No DIC was added in these experiments to avoid solid precipitation, as predicted by chemical equilibrium calculations. Preliminary adsorption experiments performed under anoxic conditions using an initial Ca2+ and Zn2+ concentration of 250 μM added as CaCl2 and ZnCl2, respectively, suggested that Ca2+ and Zn2+ sorption onto UO2 reached equilibrium within 24 h (data not shown). Equilibrium adsorption of Ca and Zn to UO2 was examined over a range of total divalent cation loadings through incremental additions of CaCl2 or ZnCl2 to increase the total Ca and Zn concentrations in UO2 suspensions. After each incremental addition was allowed to equilibrate for 24 h, a 5 mL sample of the supernatant from the reactor was collected for measurement of dissolved Ca or Zn, and a 5 mL aliquot of CaCl2 or ZnCl2 solution was added to increase the total Ca or Zn concentration for the next increment. All samples were filtered through 0.1 μm membranes (polycarbonate, Millipore). In addition to studying adsorption using the incremental addition of cations to UO2 suspensions, selected experiments were performed with single additions of relatively low concentrations of Ca and Zn to produce solids for which the 2732
dx.doi.org/10.1021/es203751t | Environ. Sci. Technol. 2012, 46, 2731−2737
Environmental Science & Technology
Article
The [U]eq under oxic conditions was calculated using a Ksp of 5.39 for schoepite24 based on the following reaction:
cation association with UO2 could be unambiguously attributed to adsorption. The UO2 solids from these experiments were collected for further SEM and XAS characterization. These samples will be called “Ca adsorbed to UO2” and “Zn adsorbed to UO2” from this point forward. Solution- and Solid-Phase Analysis. Filtered samples were acidified with 2% HNO3. Dissolved concentrations of U, Ca, and Zn were then measured using inductively coupled plasma mass spectrometry (ICP-MS, Agilent 7500ce). Influent dissolved oxygen was monitored in the CSTR experiments using a dissolved oxygen sensor (Microelectrodes, Inc.). The pH was measured using a benchtop pH electrode (Accumet) and meter (Accumet AB15 pH meter). Scanning electron microscopy (SEM) imaging was performed on a JEOL JSM7001 FLV SEM operated at high vacuum mode (
