Environ. Sci. Technol. 2009, 43, 8344–8349
Inhibition Effect of Secondary Phosphate Mineral Precipitation on Uranium Release from Contaminated Sediments Z H E N Q I N G S H I , † C H O N G X U A N L I U , * ,† JOHN M. ZACHARA,† ZHEMING WANG,† AND BAOLIN DENG‡ Pacific Northwest National Laboratory, P.O. Box 999, MSIN K8-96, Richland, Washington 99354, and Department of Civil and Environmental Engineering, University of Missouri-Columbia, Columbia, Missouri 65211
Received July 16, 2009. Revised manuscript received August 31, 2009. Accepted September 2, 2009.
The inhibitory effect of phosphate mineral precipitation on diffusion-limited uranium release was evaluated using a U(VI)contaminated sediment collected from the U.S. Department of Energy Hanford site. The sediment contained U(VI) that was associated with diffusion-limited intragrain regions within its millimeter-sized granitic lithic fragments. The sediment was first treated to promote phosphate mineral precipitation in batch suspensions spiked with 1 and 50 mM aqueous phosphate and calcium in the stoichiometric ratio of the mineral hydroxyapatite. The phosphate-treated sediment was then leached to solubilize contaminant U(VI) in a column system using a synthetic groundwater solution with chemical components representative of Hanford groundwater. Phosphate treatment significantly decreased the extent of U(VI) release from the sediment. Within the experimental duration of about 200 pore volumes, the effluent U(VI) concentrations were consistently lower by over 1 and 2 orders of magnitude after the sediment was treated with 1 and 50 mM of phosphate, respectively. Measurements of solid-phase U(VI) using laser-induced fluorescence spectroscopy, scanning electron microscopy, and chemical extraction of the sediment collectively indicated that the inhibition of U(VI) release from the sediment was caused by (1) U(VI) adsorption to the secondary phosphate precipitates and (2) the transformation of original U(VI) mineral phases to less soluble forms.
Introduction Uranium (U) is a common subsurface and groundwater contaminant at sites of uranium ore mining and nuclear fuel processing. Uranium transport in subsurface sediments is controlled by various geochemical reactions, including aqueous complexation, adsorption, and mineral precipitation and coprecipitation. Environmental ligands such as carbonate and phosphate ions form aqueous complexes with U(VI) (1), which stabilize U(VI) in aqueous solution and affect surface complexation with mineral sorbents (2-4). U(VI) adsorbs to various mineral phases and forms many crystalline compounds that decrease U mobility in groundwater (5-8). * Corresponding author phone: (509) 371-6350; fax: (509) 3716354. † Pacific Northwest National Laboratory. ‡ University of Missouri-Columbia. 8344
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Phosphate treatment has been proposed as a remediation technology to immobilize U(VI) in contaminated sediments (8-10). Phosphate affects U(VI) geochemical reactions by forming (1) aqueous and ternary surface U(VI) complexes (4), (2) poorly soluble uranyl phosphate precipitates (11, 12), and (3) U(VI) adsorbing phosphate minerals such as hydroxyapatite (13, 14). The net effect of phosphate treatment on the immobilization of uranium depends on the relative strength of the aqueous uranyl phosphate complexation and on U(VI) adsorption to and precipitation with phosphate minerals. In this study, we investigated the effect of phosphate treatment on the rate of uranium release from a contaminated, deep vadoze zone sediment collected from beneath a high-level waste tank farm at the U.S. Department of Energy (DOE) Hanford site. The sediment has been extensively characterized in terms of U(VI) speciation (15-17), physical location (17, 18), and desorption and dissolution kinetics (18, 19). In summary, U(VI) exists as uranyl precipitates in the form of Na-boltwoodite in the intragrain fractures of granitic clasts in the sediment. The intragrain U(VI) mineral dissolves with a solubility (∼10 µM) much higher than that of the regulatory standard (0.126 µM) in synthetic Hanford groundwater at circumneutral pH. The rate of U(VI) release from the sediment to pore water is controlled by diffusion within the intragrain region that is affected in complex ways by the intragrain pore network (20). The well-characterized sediment provides an ideal system to evaluate whether and how phosphate treatment affects uranyl release and the diffusive mass transfer rate in the sediment. Contaminated sediment was treated with phosphate in a batch system to promote secondary phosphate mineral precipitation. The treated sediment was then leached in a column system to evaluate the extent to which phosphate precipitation influenced the release of contaminant U(VI) from the sediment. Changes in uranyl speciation and distribution and secondary mineral formation were monitored with laser-induced fluorescence spectroscopy (LIFS), scanning electron microscopy (SEM), chemical extraction, and X-ray diffraction to provide insights to mechanisms by which phosphate inhibited diffusion-limited U(VI) release.
Materials and Methods Contaminated Sediment. The sediment was collected 36 m below the ground surface from borehole 299-E33-45 in the Hanford BX tank farm. The sediment contains 0.47 µmol g-1 of uranium that exists as a uranyl silicate in the form of Naboltwoodite in the sediment intragrain microfractures. Further information on the sediment and uranyl properties can be found in refs 15-21. Electrolyte. Hanford synthetic groundwater (SGW) (Table S1 of the Supporting Information) was used as the electrolyte solution for column experiments. A phosphate treatment solution was prepared by adding NaH2PO4/Na2HPO4 in SGW at desired phosphate concentrations. The pH of the phosphate-amended solution was adjusted using 1 M NaOH to maintain a value of 8.27. Phosphate Treatment. The sediment was mixed with phosphate-amended SGW at a solid/solution ratio of 200 g/L in two 250 mL plastic bottles. One bottle contained 1 mM and the other contained 50 mM phosphate. To promote secondary phosphate mineral precipitation, 1.7 and 85 mM calcium was provided in bottles containing 1 and 50 mM phosphate, respectively. The Ca concentration was calculated on the basis of the stoichiometric ratio between Ca and P for the mineral hydroxyapatite [Ca5(PO3)3(OH)], which was 10.1021/es9021359 CCC: $40.75
2009 American Chemical Society
Published on Web 10/05/2009
calculated to be the most supersaturated species in the suspensions. The sediment suspensions were gently shaken, and pH was adjusted to maintain SGW pH. At select times (1, 4, and 8 h, and 1, 2, and 5 d), a 3 mL sample was taken from each suspension and centrifuged at 2000 rpm for 10 min. The total removed suspension volume was less than 10% from each bottle. The supernatants in the sample tubes were filtered (0.2 µm) for chemical analysis after discarding the first few drops of filtrate. At the end of phosphate treatment, the suspensions were centrifuged,and the sediments were collected for subsequent column leaching and solid-phase analysis. The sediment samples (0.5 g) before and after phosphate treatment were extracted with 20 mLof bicarbonate solution (0.2 M NaHCO3) to determine the change in bicarbonate-extractable U(VI) in the sediment. The extraction suspension was shaken (100 rpm for 1 h) and filtered (0.2 µm) for U(VI) analysis. U(VI) Desorption. The phosphate-treated sediment was leached with SGW in a column system (2.4 cm inner diameter and 5.5 cm length with a bulk density of 1.76 g/cm3) to evaluate the effect of phosphate treatment on U(VI) desorption. The SGW solution was injected from the bottom of vertically aligned columns with a flow rate of approximately 0.2 mL min-1 using a HPLC pump. The effluent was collected every 12 min using a fraction collector. The intermittent stop-flow(SF) technique, during which advective flow was stopped, was employed with variable SF durations (48-200 h) to evaluate whether U(VI) release was at equilibrium or kinetically controlled. At the end of the experiment, the sediments were sampled for solid-phase analysis. Solution Analysis. U(VI) concentration in the aqueous samples from the phosphate treatment suspensions and column effluents was measured with a kinetic phosphorescent analyzer (Chemcheck Instrument, Inc., Richland, WA) with a detection limit of 1 nmol/L. Major ions Ca, Mg, K, Na, Si, and P were measured by ICP-OES (PerkinElmer, Optima 2100 DV) with a detection limit of 50 µg/L for Ca, Mg, K, and Na, and 5 µg/L for P. Total dissolved inorganic carbon was measured with a Dohrman carbon analyzer, DC-80 with a detection limit of 10 µmol/L. Suspension and effluent pH were measured with a pH electrode (Thermo Orion ROSS). Laser-Induced Florescence Spectroscopic (LIFS) Analysis. Sediment samples before and after phosphate treatment and after column leaching were collected and analyzed using LIFS to identify U(VI) speciation in the sediment. LIFS instrumental setup and procedures for solid-phase U(VI) measurements have been described elsewhere (16, 22). Scanning Electron Microscopic (SEM) Analysis. Sediment samples before and after phosphate treatment, and after column leaching were analyzed by SEM to identify physical locations and mineral associations of uranyl phases and newly formed phosphate precipitates. The SEM samples were imbedded in epoxy, wafered, affixed to a quartz slide, and polished. Thin sections were then carbon-coated and examined using SEM. Compositional information was collected using energy-dispersive X-ray spectroscopy (EDS).
Results and Discussions Phosphate Treatment. Figure 1 shows the changes in aqueous P, U(VI), Ca, and inorganic C concentrations as a function of time during batch treatment of the sediment with 1 and 50 mM phosphate. The solution phosphate concentration decreased 55% and 99.9% within the first hour of P and Ca addition in the 1 and 50 mM phosphate suspensions, respectively. This was followed by a gradual decrease in phosphate from 1 to 40 h, with stabilization occurring near 0.1 mM in the 1 mM P suspension, and below detection in the 50 mM P suspension. Aqueous Ca and U(VI) concentrations also decreased rapidly within the first hour, followed by slow changes with time. The carbonate con-
FIGURE 1. Temporal evolution of aqueous P, U(VI), Ca, and inorganic C concentrations in the sediment suspensions containing 1 or 50 mM phosphate. centrations slightly increased with time in the 1 mM phosphate suspension. In the 50 mM phosphate suspension, however, the carbonate concentration decreased to below detection within the first hour and then slightly increased with time. Speciation calculations using MINTEQ (23) with an updated literature database suggested that hydroxyapatite was the most supersaturated phase in the suspensions with an initial saturation index (SI) of 15.3 and 23.6 in the 1 and 50 mM phosphate suspensions, respectively. In the 50 mM phosphate suspension, calcite was also initially supersaturated with a SI of 1.34 driven by the added high Ca concentration. No known U(VI) minerals were supersaturated with total aqueous U(VI) concentrations ranging from 0.01 to 1 µM, which bracketed the U(VI) concentrations observed in this study in both batch solutions and column effluents during and after the phosphate treatment. Mass balance calculations indicated that in the 50 mM P suspension, about 93% added Ca and almost all added phosphate were removed from the solutions, and about 82% added Ca and 88% added phosphate were removed from the 1 mM phosphate after 5 days. The removed Ca and P from the aqueous solutions were approximately consistent with the formation of mineral hydroxyapatite with a slightly lower stoichiometric ratio of Ca to P (1.57 to 1) than the theoretical value (1.67 to 1) in both suspensions. The lower removal ratio of Ca to P was attributed to (1) the partial formation of amorphous calcium phosphate precipitate, which has a Ca to P stoichiometric ratio of 1.4-1.5 to 1 as a precursor to hydroxyapatite (24, 25) and (2) the replacement of Ca by other cations such as Na, Mg, and K in the Ca-P precipitates (12). The dissolution of calcite may also have contributed to the apparent lower removal ratio of Ca to P as indicated by the small increase in carbonate with time in the 1 mM phosphate suspension. In the 50 mM phosphate suspension, however, calcite may have precipitated as indicated by the rapid decrease in aqueous carbonate after addition of Ca and P (Figure 1). Calcite precipitation would increase the apparent removal ratio of Ca/P by about 3%, if all of the removed initial aqueous carbonate (1.3 mM) was assumed to produce calcite. The rapid formation of white precipitate was visually observed after the addition of P and Ca, especially in the 50 mM phosphate suspension. X-ray diffraction (XRD) analysis of the phosphate-treated sediments, however, was unable to identify crystalline Ca-P minerals (data not shown), suggesting that the precipitates were amorphous and/or below the nominal X-ray diffraction detection limit. If all of the removed P was converted to hydroxyapatite, the secondary precipitates would represent 0.06 and 3.6 wt % of the sediment treated with 1 and 50 mM phosphate, respectively. VOL. 43, NO. 21, 2009 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 2. Effluent concentrations of U(VI), phosphate, and calcium released from the column containing the sediment before and after treatment with 1 and 50 mM phosphate. Effluent U(VI) concentration peaks resulted from the stop-flow events with their time durations marked. Bicarbonate-extractable U(VI) increased from 7% in the original sediment to 20% and 21% of total U(VI) in the sediments with 1 and 50 mM phosphate treatments, respectively, indicating that intragrain U(VI), which existed as Na-boltwoodite precipitates in the original sediment, partially dissolved and diffused out of intragrain regions. The low aqueous concentrations of U(VI) in the suspensions (Figure 1) indicated that the solubilized U(VI) was adsorbed and/or reprecipitated. The near equivalent bicarbonate-extractable U(VI) in the 1 and 50 mM phosphate-treated sediments implied that the two different phosphate concentrations caused similar changes to U(VI) lability. The residual aqueous U(VI) concentration that slightly increased with time after 20 h in the 1 mM phosphate treatment case apparently resulted from the complexation of U(VI) with aqueous carbonate (Figure 1). This was supported by the corresponding increase in inorganic carbonate with time in the 1 mM phosphate treatment case (Figure 1). Inhibition of U(VI) Release. The effluent U(VI) concentrations from the phosphate-treated sediments were significantly lower than those without phosphate treatment (Figure 2A). The sediment treated with a higher phosphate concentration released less U(VI). The effluent U(VI) concentrations for the 50 mM phosphate-treated sediment were consistently below the EPA regulatory standard, 0.13 µmol/ L, within the experimental duration [200 pore volume (PV)] except for the first PV, when the effluent U(VI) concentration was higher than that at the end of the phosphate treatment (Figure 1). The SF events led to a rebound in effluent U(VI) concentrations, which were, however, still below 0.13 µmol/L regardless of SF duration. The effluent U(VI) concentrations from the 1 mM phosphate-treated sediment were also lower 8346
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than the original sediment (Figure 2A). In contrast to the 50 mM phosphate treatment, the effluent U(VI) concentrations from the 1 mM phosphate-treated sediment consistently increased with time after 60 PV after excluding the SF effects. Mass balance calculations revealed that 22% of the contaminant U(VI) was released from the untreated sediment in 200 PV. In contrast, only 1.46% and 0.07% of the sedimentbound U(VI) was released from the 1 and 50 mM phosphatetreated sediments, respectively, within the comparable duration. Stop-flow events increased the effluent U(VI) concentrations (Figure 2A) regardless of phosphate treatment, consistent with previous findings that U(VI) release from the sediment was controlled by a diffusive mass transfer process in the intragrain regions (18-20). The relatively high effluent U(VI) concentration within the first PV was partially attributed to kinetic mass transfer that occurred during the collection of the phosphate-treated sediments and their packing in columns (∼3 days). However, the amount of U(VI) release during the SF durations and within the first PV was significantly reduced by the phosphate treatment, indicating that U(VI) dissolution and/or desorption was inhibited by phosphate mineral precipitation. The temporal increase of U(VI) release from the 1 mM phosphate-treated sediment after 60 PV signified a decreasing inhibition effect of phosphate treatment with time. This result demonstrated that a relatively large amount of phosphate precipitate was required to sustain the long-term inhibition of U(VI) release from the sediment. Phosphate was slowly but continuously released from the phosphate-treated sediments during the column experiments (Figure 2B). The effluent data for the 50 mM phosphatetreated sediment displayed unexplained scatter within 60 PV. After 60 PV, the effluent phosphate concentrations decreased significantly and slightly rebounded immediately after SF events. Mass balance calculations indicated that less than 0.4% of the phosphate was released from the 50 mM phosphate-treated sediment, while 15% percent of the phosphate was released from the 1 mM phosphate treatment over the course of column leaching (200 PV). Most of the phosphate release (80%) occurred during the first 60 PV. Effluent Ca concentrations were relatively constant (Figure 2C) except during the first 30 PV, where a concentration decrease and rebound to the influent concentration (0.3 mM) was observed. The same trend was observed for Mg and K (Figures S1 and S2 of the Supporting Information). The high initial effluent concentrations of these cations were comparable in magnitude to the final solution compositions in the batch phosphate treatments (Figure 1). The decrease of Ca (Figure 2C), Mg, and K (Figures S1 and S2 of the Supporting Information) within the first few PV apparently resulted from (1) lower concentrations of these cations in the influent than in the initial solutions and (2) ion sorption as a result of aqueous composition evolution during column leaching (e.g., pH and carbonate; Figures S1 and S2 of the Supporting Information). As sorption sites were saturated, their effluent concentrations gradually rebounded to the influent levels. The significant changes between influent and effluent compositions within the first PV may have promoted U(VI) desorption, which together with the mass transfer factor during the sediment collection and column packing as mentioned before resulted in effluent U(VI) concentrations that were higher than those in the aqueous solutions at the end of the batch phosphate treatment (Figure 1). The poor correlation of effluent P with Ca concentrations in the first 60 PV (Figure 2B,C) implied that phosphate release during this period resulted from desorption of surfacecomplexed phases. The correspondent increase in phosphate and calcium after SF events after 60 PV suggested that the slow dissolution of Ca-P minerals became important after
FIGURE 3. (A) U(VI) fluorescence spectra of the sediment (a) untreated, (b) treated with 1 mM phosphate, (c) treated with 1 mM phosphate and leached, (d) treated with 50 mM phosphate, and (e) treated with 50 mM phosphate and leached. (B) Time-resolved fluorescence spectra of the sediment sample treated with 50 mM phosphate and leached: (a) time delay
