The Effect of pH and Time on the Extractability and Speciation of

May 22, 2012 - The effect of pH and contact time on uranium extractability from quartz surfaces was investigated using either acidic or carbonate (CAR...
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The Effect of pH and Time on the Extractability and Speciation of Uranium(VI) Sorbed to SiO2 Eugene S. Ilton,* Zheming Wang, Jean-François Boily, Odeta Qafoku, Kevin M. Rosso, and Steven C. Smith Pacific Northwest National Laboratory, 902 Battelle Blvd., MSIN: K8-96, Richland, Washington, 99352 S Supporting Information *

ABSTRACT: The effect of pH and contact time on uranium extractability from quartz surfaces was investigated using either acidic or carbonate (CARB) extraction solutions, timedelayed spikes of different U isotopes (238U and 233U), and liquid helium temperature timeresolved laser-induced fluorescence spectroscopy (TRLFS). Quartz powders were reacted with 238U(VI) bearing solutions equilibrated with atmospheric CO2 at pH 6, 7, and 8. After 42 days, the suspensions were spiked with 233U(VI) and reacted for an additional 7 days. Sorbed U was then extracted with either dilute nitric acid or CARB. For the CARB, but not the acid, extraction there was a systematic decrease in extraction efficiency for both isotopes from pH 6 to 8, which was mimicked by less desorption of 238U, after the 233U spike, from pH 6 to 8. The efficiency of 233U extraction was consistently greater than that of 238U, indicating a strong temporal component to the strength of U association with the surface that was accentuated with increasing pH. TRLFS revealed a strong correlation between CARB extraction efficiency and sorbed U speciation as a function of pH and time. Collectively, the observations show that aging and pH are critical factors in determining the form and strength of uranium-silica interactions.

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INTRODUCTION Uranium is the most common and ubiquitous radionuclide contaminant in subsurface systems associated with U.S. Department of Energy sites where nuclear materials were processed and stored.1 Under oxidizing conditions where U(VI) predominates, transport of U in the subsurface can be attenuated by precipitation of uranyl-bearing phases or by sorption. Consequently, there has been much research on characterizing both the solubility of U(VI) phases, for example, see review by Gorman-Lewis et al.,2 and the effect of variable conditions (e.g., pH, ionic strength, cations, and anions) on the sorption of U(VI) by common soil and sedimentary minerals, including quartz and silica gels.3−13 In most cases, U(VI) sorption by silica has been characterized over short time periods (up to a week), with little attention to the effect of long-term aging on reversibility. Nonetheless, some studies have highlighted kinetic sorption effects either attributed to different sorption sites14−16 or mass transport limitations due to intrinsic porosity.7 Further, high pH bicarbonate/carbonate extraction solutions (CARB) are often used to assess the labile fraction of uranium in sediments17 and experimental systems.18 In this regard, recent work has shown that CARB extraction efficiency can depend on aging, even in simple model Fe(III)-oxyhydroxide systems at circum-neutral pH where solubility is at a near-minimum.19 Such time-dependent processes could have a strong long-term impact on the transport of uranium in the subsurface as well as the utility of CARB extractions to determine uranium chemistry. © 2012 American Chemical Society

In this contribution, we probed the effect of aging and pH on uranyl sorbed to quartz powder, under atmospheric PCO2 and at target pH 6, 7, and 8, using isotopic labeling, liquid helium temperature time-resolved laser-induced fluorescence spectroscopy (TRLFS), and bulk extraction methods (CARB and dilute nitric acid). As discussed later, the acid extraction was chosen to test whether any CARB extracted U was occluded or incorporated in the SiO2 structure. The primary objective was to determine the efficacy of CARB for extracting sorbed U as a function of long-term U/quartz contact time and pH, and correlate any effects to possible changes in TRLFS determined uranyl surface speciation.

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MATERIALS AND METHODS Materials. Batch adsorption experiments used fine ground quartz under the brand name Min-U-Sil 30 (U.S. Silica Company, Mill Creek, Oklahoma). The material was pretreated similar to the procedure in ref 14. Min-U-Sil 30 was heated at 550 °C for 60 h to decompose potential organic matter, refluxed twice for 4 and 1 h in 4 M HNO3, washed twice with hot deionized water (DI-water), and rinsed several times with DI-water adjusted to pH 10. The resulting solid was then rinsed several times with DI water until the supernatant conductivity was 18 MΩ cm or lower. The material was then oven-dried at Received: Revised: Accepted: Published: 6604

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40 °C for 3−4 days. Specific surface area was determined by the N2 BET method to be 1.63 (±0.02) m2/g. The specific surface area is higher than Min-U-Sil 30 used in Kohler et al.14 but lower than that used by Huber and Lützenkirchen.6 Scanning electron microscopy (Figure 1) of the treated material indicated that fines, consisting of particles less than 0.1 μm, were adhered to the larger grains.

increased the suspension volumes and Na/NO3 concentrations by up to 0.5% and 0.3%, respectively. The suspensions attained apparent equilibrium with atmospheric CO2, quickly at pH 6−7 and longer at pH 8, as demonstrated by pH stability. Whereas one can spike solutions with appropriate concentrations of carbonate salts to facilitate equilibration with atmospheric CO2, the long pre-equilibration time was helpful for minimizing changes in Si(aq), as well as surface states, during the course of the experiments themselves. Just prior to the addition of uranyl, 8 mL of the suspension was filtered through a 0.2 μm syringe filter and analyzed for dissolved inorganic carbon (IC) and Si. 132 μL of 238U was then added to the suspensions as UO2F2 (International Bio-Analytical Industries, Inc., Boca Raton, Fl) to yield 0.17 μM 238U(aq) and equilibrated for 42 days. Addition of 238U lowered the pH by about 0.1−0.2 units which required titration back to the target pH, producing negligible increases in Na concentrations and suspension volume. The suspensions were continuously bubbled with prehumidified air for the duration of the experiments. 1 mL aliquots of suspension filtrates (0.2 μm syringe filters) were obtained 1, 7, and 41 days following the 238U spike and acidified in order to monitor U(aq) as a function of time. The filtrates obtained 41 days following the addition of 238U were also analyzed for Si. Additional aliquots of each suspension were obtained over the same time period and centrifuged. Following centrifugation both the solids (wet paste) and the supernatant (passed through 0.2 μm syringe filters) were transferred to separate glass cuvettes in preparation for analysis by time-resolved cryogenic (cooled with liquid helium to 6 ± 2 K) laser-induced fluorescence spectroscopy (TRLFS). Forty-two days following the addition of 238 U the suspensions were spiked with 233U [UO2(NO3)2; CRM 111A, New Brunswick National laboratory] to yield 0.037 μM 233U. Two mL of the suspensions were filtered at timed intervals following the 233U spike to monitor the concentrations of 238U and 233U in the aqueous phase. At 7 days, portions of the suspension were mixed with 0.01 M nitric acid or a CARB solution (0.014 mol/L NaHCO3 + 0.0028 mol/L Na2CO3, pH

Figure 1. SEM of pretreated Min-U-Sil 30 prior to reaction with U.

Experiments. Three 200 mL suspensions containing 27 g/ L of precleaned Min-U-Sil 30 and 32 mM NaNO3 were placed in 250 mL Teflon bottles and the pH adjusted to target values 6, 7, and 8. The electrode was only in contact with suspensions during the pH measurements. Filtered prehumidified air was bubbled through each suspension for nearly seven months with pH-adjustments to 6.0, 7.0, or 8.0 using μL amounts of concentrated NaOH or HNO3 (0.1 M) as needed. This

Table 1. Solution Analyses (mol/L) and U Surface Load (Atoms/nm2)a daysb

pH

238

233

U

U

U(tot)

−8

1 7 41 42 + 7 days

6.05 6.02−5.99 6.03 5.62

1.57 6.25 5.79 1.93

× × × ×

10 10−9 10−9 10−8

1 7 41 42 + 7 days

7.04 6.97−7.03 7.02 7.02

1.19 3.61 3.40 1.04

× × × ×

10−8 10−9 10−9 10−8

1 7 41 42 + 7 days

7.84−8.04 7.81−8.04 7.9 7.8

1.03 4.30 3.01 6.11

× × × ×

10−8 10−9 10−9 10−9

4.29 × 10−9

2.36 × 10

1.68 × 10

−9

−9

ICc

Si −8

−4

1.57 6.25 5.79 2.36

× × × ×

10 10−9 10−9 10−8

6.20 × 10

1.19 3.61 3.40 1.28

× × × ×

10−8 10−9 10−9 10−8

7.30 × 10−4

1.03 4.30 3.01 7.79

× × × ×

10−8 10−9 10−9 10−9

8.20 × 10−4

6.80 × 10−4

7.40 × 10−4

8.80 × 10−4

U(ads)

8. Carbonate is known to quench fluorescence at RT, but not so much at LHe temperatures. Nonetheless, we saw no evidence of a third component, which is consistent with our experiments performed at pH ≤ 8. The vibronic band positions of surface complex b (Figure 3) are strongly red-shifted relative to complex a, but are only a few nm blue-shifted relative to those of Na-boltwoodite.26 In fact, the spectra for b are very similar to the fluorescence spectrum of Na-boltwoodite. However, no plausible uranyl phase was calculated to be thermodynamically stable. Because TRLFS probes the local bonding environment of U(VI), surface species b likely involves extensive complexation between the uranyl ion and silica, thus the resemblance to a uranyl silicate phase. Note that the saturation calculations are conservative with respect to Na-Boltwoodite; we used log Ksp = 5.82 (solution species/solid phase), whereas a likely better constrained value from recent experiments is 6.04.28 Unfortunately, the spectroscopic data is not definitive with respect the presence or absence of U polymerization. Sylwester et al.12 documented U−U correlations for uranyl adsorbed to colloidal silica around pH 6.5 with 6607

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Figure 5. CARB extraction efficiencies for 238U and 233U as a function of pH. (a) CARB extraction and (b) Nitric acid extractions. Typical analytical errors were on the order of ±2% relative, about the size of the markers at higher fraction of U extracted.

h of reaction, respectively. For the pH 7 sample, CARB extraction efficiencies for both isotopes were considerably lower relative to the pH 6 sample (Figure 5a). Further, 233U was more efficiently extracted compared to 238U. CARB extraction was less efficient yet for the pH 8 sample where the difference between 233U and 238U was appreciably greater than for the pH 6 and 7 samples. Consequently there appears to be both a strong temporal (i.e., 233U versus 238U) and pH dependence on the CARB extraction efficiency. The isotopic exchange and CARB extractions are consistent with more tightly bound and/or less accessible U, at higher pH values and after longer reaction times. However, occlusion of uranyl is an unlikely mechanism for two reasons. First, SiO2 dissolution was significantly less during the nitric acid compared to the CARB treatment (SI Table S3), consistent with previous dissolution data at low and high pH,30,31 yet nitric acid was a far more efficient U extractant (compare Figure 5a and b). Consequently, if the only process was occlusion or structural incorporation of adsorbed uranyl, then CARB should have been more efficient than nitric acid at extracting uranium, which was not the case. Second, all solutions were undersaturated with respect to amorphous silica, so that silica precipitation should not have occurred over the course of the experiments; in fact, as mentioned previously, Si(aq) rose slightly. Consequently, the following discussion focuses on the possibility that uranyl is more tightly bound to the surface at higher pH and after longer reaction times. In this regard, it is notable that the surface complex b is favored at longer aging times and at higher pH where CARB extraction and 233U isotopic exchange are less efficient which suggests that b is more strongly bound to the surface than complex a. This is

Figure 4. Aqueous concentrations of 238U and 233U reacted with MinU-Sil. Typical analytical errors were on the order of ±2% relative. The top horizontal dashed line gives the initial (time 0) concentration 233U. The bottom dashed line gives the 238U concentration just prior to the 233 U spike and after 41 days of reaction. The initial concentrations of 238 U and 233U were 0.17 and 0.037 μM, respectively. Top, middle, and bottom panels are for the pH 6, 7, and 8 experiments, respectively.

equilibration after the 233U spike, was strongly dependent on the pH, where 238U desorption was about 11, 6, and 3.5% at pH 6, 7, and 8, respectively. This is an indication that 238U was more tightly bound to the surface with increasing pH. The CARB extraction efficiencies of 238U and 233U were measured 7 days after the 233U spike. The absolute and relative extraction efficiencies of both isotopes were dependent on the pH of the sorption experiments (Figure 5a). For pH 6, CARB extraction of both isotopes was relatively rapid and nearly identical, with about 50 and 80% U released after 5 min and 24 6608

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fluorescent signals is not known. However, one should be cautious when comparing TRLFS to EXAFS studies when large differences in uranium sorption loadings occur. That the U sorption complex evolved over time was definitively demonstrated by TRLFS; however, perhaps this occurred in concert with diffusion of uranium into micro- and nanosize pores. Although the pretreated Min-U-Sil surface appears nonporous at the SEM scale, Si(aq) concentrations indicate the presence of amorphous silica. Diffusion of U(VI) into amorphous silica coatings on acid leached feldspars has been documented by Chardon et al.35 Further, Michard et al.7 noted that carbonate solutions were not as effective in extracting uranyl that had diffused into micro- and mesoporous silica compared to acidic solutions which may be relevant to our findings (i.e., compare Figure 5a and 5b). Thus the temporal component to the decrease in CARB extraction efficiency might also reflect diffusion of uranyl into micropores. Another MD study, in fact, suggests that silanol groups lining 2−5 nm pores of amorphous silica reduce the mobility of water.36 Nonetheless, Goyne et al.30 showed that silanol groups behave similarly (i.e., nearly identical pKa values) whether in nm scale pores (∼3.54 nm) associated with porous silica or on the surface of nonporous silica. Consequently, it is likely, at least to a first approximation, that diffusion of uranyl into such pores will not strongly affect its surface speciation, which is consistent with fact that we only resolve two primary fluorescent U species. Further, as argued later, some evidence indicates that U diffusion into pores is not a dominant process. As noted, the nitric acid extractions were consistently more rapid and efficient than the CARB extractions, at least over the 24 h time frame that was studied (Figure 5). Again, this has precedence in the context of diffusion of U into micro and nano scale pores7 which provides a potential reason for some of the difference in U extractability between the two methods. A complementary mechanism might be due to differences in surface charge. One can speculate that under the greater alkaline conditions during CARB treatment further deprotonation of silanol groups would create a more negative surface charge that could yield a significant activation energy barrier to desorption by increasing the electrostatic attraction and repulsion for uranyl and carbonate, respectively. In contrast, protonation of silanol groups under acidic conditions of the nitric acid treatment would lower the negative surface charge and possibly decrease the activation energy required for desorption. Given more than the allotted 24 h, the CARB extraction might well have reached a similar level of U release as in the nitric acid treatment (Figure 5). This would be expected as little to no measurable absorption should occur at the pH values and diluted total uranium concentrations of either extractant; that is, the difference between the nitric acid and CARB treatments should be due to kinetics, not thermodynamics. In fact, given the steeper slopes for CARB extraction of the pH 7 and 8 experiments one cannot exclude the possibility that the release of U would have eventually converged or even been greater for the pH 7 and 8 compared to pH 6 experiments, and approached those of the nitric acid solution. However, the nitric acid extractions appear to indicate that a modest fraction of sorbed U is recalcitrant; around 5−10 and 18% for the pH 7−8 and 6 experiments, respectively. It is possible that this recalcitrant U is structurally incorporated or deeply embedded in a nanopore. The CARB extraction of the pH 6 experiment might also have converged on a recalcitrant ∼18% sorbed U, although confirmation would require a higher

supported by interpretation of the fluorescence spectra. For U(VI) coordinated to similar ligands, a red-shift typically indicates stronger U(VI)-ligand bonds that can go hand in hand with increased coordination.25 However, the red shift could signal an increase in the strength of any uranyl-ligand bond, such as would occur during hydrolysis of equatorial waters. Consequently, it is the combination of wet chemistry and spectroscopy that suggests species b is more strongly bonded to surface silanol groups than species a. That surface complex b is highly coordinated by silica is also supported by the fluorescence spectra which resembles that of Na-boltwoodite (see previous Discussion). Consequently, given the prior argument against appreciable structural incorporation, the correlation between the spectroscopy and wet chemistry could be due to the formation of energetically favorable multidentate U-silica surface complexes at higher pH and at longer reaction times. In this regard, recent molecular dynamic (MD) modeling of hydroxylated-uranyl adsorption to low index quartz surfaces is instructive.32 We note that a previous MD study indicated that mono- and dicarbonate uranyl complex formation at the quartz (010) surface adopt comparable adsorption geometries.33 For both the (010) and (101) surfaces, adsorption involves a number of sequential steps, where the progression is monodentate outer sphere complex → removal of oxo-bound waters → monodentate inner sphere complex → bidentate inner sphere complex → loss of ∼2 equatorial waters. Support comes from a short-term (on the order of 10−20 min) second harmonic generation (SHG) study of uranyl adsorption by fused quartz, at pH 7 and in the presence of carbonate, that documented an outer sphere monodentate complex which was easily desorbed.34 Of particular relevance to the present study, bidentate inner sphere complexes were estimated to be 1.9−2.8 kJ/mol more stable in energy than their monodentate counterparts, and the approach to the final, most stable, surface complex is predicated by a significant activation energy32 which implies a temporal element to the strength of the U−O−Si surface complex. Therefore, the MD modeling is consistent with the experimental data in two ways; first, the CARB extraction efficiency depends on the length of the adsorption experiment and, second, the surface complex evolved toward higher coordination with Si. The MD modeling also suggests a reason for the increase in species b with pH. As described by the model, the monodentate species is a precursor of the bidentate species. At higher pH, the probability of having two or more deprotonated adjacent surface sites is increased, which likely facilitates multidentate binding, possibly at steps. The MD model provided context for the previous mechanistic discussion. However, it is not known if the material used in the present study is better represented by lowindex or vicinal surfaces, or more likely some combination thereof. In this regard, the available EXAFS data, which is restricted to high surface area SiO2 colloids with potentially high densities of vicinal as opposed to low index surfaces, suggests that bidentate species can occur at acidic pH.10,12,29 Consequently, the present study could be documenting transitions from monodentate and/or bidentate to multidentate (or stronger bidentate) sorption complexes, but the same principles would apply. Further, there is likely a distribution of different bidentate uranyl species with long U−Si distances favored on different low index surfaces,32 and short U−Si distances as recorded by EXAFS on vicinal surfaces. Whether these different possible surface complexes have distinct 6609

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density of data points or greater extraction time. Nonetheless, only two primary fluorescent uranyl species were resolved. If the recalcitrant uranium fluoresces with sufficient intensity, then it must be strongly overlapped with complex b. Curiously, there was a minor yet consistent reversal in acid compared to CARB extraction efficiency (Figure 5) as a function of the experimental pH and sorption time (i.e., the extraction behavior of the two U isotopes for samples equilibrated at pH 7 and 8). This appears counterintuitive and raises the possibility, as discussed above in terms of total desorption, that differences in the surface charge at high and low pH might also differentially affect the activation energy of desorption for the two species. Unfortunately, there is insufficient data to warrant further discussion. However, the reversal in extraction behavior can be used to argue against a dominant imprint of porosity on CARB extraction efficiency. Specifically, the deeper uranyl diffused into a pore, the less efficient any extraction process should be. Thus the reversal indicates that uranyl diffusion is not a dominant process in this system and that surface complexation more consistently explains the extraction data. Regardless, this study shows that both aging and pH are critical for determining the form and strength of uranium-silica interactions. In particular, long-term contact of uranyl with SiO2 and equilibration at higher pH can dramatically reduce the extraction efficiency of U by CARB solutions which is correlated to the formation of multidentate U-silica surface complexes. Future work should focus on using quartz powders with an ultralow amorphous SiO2 content and systems in equilibrium with different PCO2, as well as using more direct probes of the local structure of uranyl (e.g., EXAFS, although sensitivity limits apply). The collective findings deepen our understanding of uranium-silica interactions, and have implications for both the retention of uranyl by quartz and assessments of the labile uranium fraction in environmental systems.

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REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

Thermodynamic database, saturation indices, aqueous speciation, Si(aq) dissolved during extraction procedures, selected time delayed TRLFS spectrum. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Phone: 509-371-6387; fax: 509 371 6354; e-mail: eugene. [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Funding for this project was provided in part by the U.S. DoE National Nuclear Security Administration, and in part by the Office of Science through the Office of Biological and Environmental Research and the Subsurface Biogeochemistry Research Science Focus Area program at PNNL. A portion of the work was performed at the W.R Wiley Environmental Molecular Science Laboratory at PNNL, a national user facility operated by Battelle on behalf of the U.S. DoE OBER. We thank Bruce Arey, Tom Resch, and Dean Moore for help with SEM, inorganic carbon, and solute analyses, respectively. 6610

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