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Department of Chemistry, Northwestern University, 2145 Sheridan Road, Evanston, Illinois 60208. J. Phys. Chem. A , 2010, 114 (4), .... The complex aqu...
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J. Phys. Chem. A 2010, 114, 1797–1805

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Uranyl Adsorption and Speciation at the Fused Silica/Water Interface Studied by Resonantly Enhanced Second Harmonic Generation and the χ(3) Method Jessica N. Malin and Franz, M. Geiger* Department of Chemistry, Northwestern UniVersity, 2145 Sheridan Road, EVanston, Illinois 60208 ReceiVed: October 3, 2009; ReVised Manuscript ReceiVed: December 14, 2009

We report the first use of resonantly enhanced second harmonic generation (SHG) to study uranyl adsorption at a buried mineral oxide/water interface. Uranyl adsorption is studied in real-time, under flow conditions, and in the presence of environmentally relevant screening electrolyte concentrations. The in situ SHG spectrum of surface-bound uranyl reveals a well-defined resonance at 306 nm. By monitoring the SHG response at 306 nm, adsorption isotherms were collected for uranyl species at the fused silica/water interface at pH 7, and in the presence of aqueous carbonate. The measured adsorption free energies determined by the Langmuir isotherm are consistent with physisorption via hydrogen bonding. The speciation of the surface-active uranyl species at pH 7 was elucidated via a free energy versus interfacial potential analysis, which reveals that the uranyl adsorbates are either neutral or univalent cationic species. Complementary surface charge density data, obtained using the Eisenthal χ(3) technique, reveal that the charge on the ionic uranyl species adsorbing under the experimental conditions are positive. It is proposed that a mixture of neutral and univalent, cationic uranyl species is surface active at pH 7 and in the presence of carbonate ions. Insofar as the experimental conditions model those found in natural groundwater systems, the results of this work are valuable to the prediction and assessment of uranium pollution transport in groundwater and soils. Our thermodynamic results can also serve as important benchmarks for computer simulations of U(VI) transport in heterogeneous geochemical environments. Introduction Understanding the mobility of uranium pollution in soils is key to effective groundwater risk assessment and remediation. Adsorption at the mineral/water interface is a critical process controlling the mobility and ultimate transport of uranium in groundwater systems.1-5 The extent to which uranium adsorption occurs at the solid/water interface depends on the interfacial uranium speciation, and the chemical nature of the surface-active species under given environmental conditions (pH, carbonate concentration, ionic strength, etc.). In this work, we used second harmonic generation (SHG) to study uranium adsorption at an environmentally relevant mineral oxide/water interface at neutral pH and constant electrolyte concentration. We also used SHG to perform a free energy versus potential analysis and surface charge screening experiments to gain insight into the valency and chemical nature of the surface-active uranium species at pH 7 and in the presence of aqueous carbonate. Unlike the nonresonant, single wavelength (1064-532 nm) SHG work of Dossot et al.,6 we study U(VI) using resonantly enhanced SHG spectroscopy similar to our work on the ligand-to-metal charge transfer bands of hexavalent chromium,7-9 which allows us to study uranium adsorption under environmentally relevant conditions. Uranium is introduced into soils and groundwater through natural and anthropogenic sources. The weathering of uraniumcontaining minerals and ores results in the accumulation of low background levels of uranium in most soils.1,5,10,11 Anthropogenic point sources of uranium contamination include nuclear industry wastes, as well as uranium milling and mining activities.10-14 According to the U.S. Environmental Protection Agency * To whom correspondence should be addressed. E-mail.

(USEPA), chronic ingestion of uranium-contaminated drinking water causes kidney damage and also can pose a risk of cancer resulting from R emissions from ingested 235U.11 Due to these risks, the USEPA has established a maximum contaminant level (MCL) of 30 ppb for uranium in drinking water.15 Unfortunately, the soils and groundwater surrounding multiple uranium processing sites and several Department of Energy sites, such as Hanford and Savannah River, contain uranium at concentration levels well above the MCL.12,13,16-19 Under the oxidative conditions of groundwater environments, uranium persists as U(VI).16,19-23 Numerous studies have focused on U(VI) adsorption to a range of natural soil, mineral and clay substrates using a variety of techniques including X-ray adsorption spectroscopies, such as EXAFS and XPS,12,14,20,23-27 batch and column techniques,14,17-19,28 theoretical calculations,13,16 and surface complexation modeling.25,29,30 It is well documented that U(VI) is readily complexed by carbonate and hydroxyl ions in groundwater systems, leading to complicated bulk aqueous uranyl speciation.14,19,21,22,30-32 U(VI) speciation is highly pH dependent, with the largest number of different species being present at neutral pH.14,19,21,28,30,33 Several uranyl species exist in equilibrium at pH 7 and common electrolyte and carbonate concentrations. While the neutral diuranyl dihydroxide species is the most abundant, the divalent or tetravalent uranyl species would dominate at positively charged mineral/water interfaces if pure Coulombic interactions were the driving force for adsorption. The complex aqueous uranyl speciation has led to difficulties in the use of surface complexation modeling for predictive analysis of uranyl interactions with soils.19 Lieser et al. performed uranyl adsorption batch studies on SiO2 and TiO2 as a function of pH.28 Their results revealed that both substrates

10.1021/jp909504n  2010 American Chemical Society Published on Web 01/05/2010

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showed the same pH adsorption edge despite having different isoelectric points, indicating that uranyl adsorption is dictated by the uranyl speciation rather than the electrostatic properties of the adsorbing surface. Such results illustrate the motivation for studies focused on adsorbed uranyl speciation. An additional motivation for this work is rooted in the fact that while chemical binding and surface speciation in soils are governed by mechanisms operating at interfaces, chemical transport models commonly rely on parameters that are derived from bulk measurements.2,34,35 Specifically, the Kd parameter is widely used in model predictions of contaminant transport, i.e., retardation factors,36,37 but commonly overlooks important molecular-level processes that control the binding events. Likewise, surface complexation models are forced to rely on bulk-specific parameters, such as equilibrium dissociation constants and pKa values, when experimental interfacial data is not available.2,38,39 Given that equilibrium constants and pKa values for processes occurring in interfacial environments can be substantially shifted by multiple orders of magnitude from their corresponding bulk values,40-49 the predictive power and accuracy of surface complexation models for assessing contaminant mobility and fate would benefit from the input of thermodynamic parameters that are experimentally determined explicitly for interfacial environments. Yet, the experimental evaluation of the interfacial uranyl speciation is complicated by the fact that many experimental techniques used for interrogating liquid/solid interfaces are limited by long signal integration times and low surface sensitivities.50,51 X-ray studies have yielded exquisite structural informationregardingU(VI)atvariousfluid/solidinterfaces.23,26,27,52-56 Yet, while the experiments can be carried out in the 10-5 to 10-4 M range,52 it is challenging to track adsorption and desorption processes and the interfacial speciation states in space or in real time. X-ray microprobe studies57 are notable exceptions, but imaging the spatial and temporal evolution of U(VI)-surface interactions in real time and under bulk aqueous solution and flow conditions has not yet been achieved. In addition, these experiments are often performed at either acidic or alkaline conditions24,26,31,58,59 or under carbonate-free conditions25,27,30 to simplify the bulk aqueous speciation. Nonlinear optical methods can overcome these challenges and yield experimental surfacespecific thermodynamic, spectroscopic, charge state, speciation, and kinetic data complementary to synchrotron studies. In this work, we quantify the adsorption thermodynamics, assess the kinetics and reversibility of uranyl adsorption and determine the adsorbed charge density for uranyl adsorption at the environmentally relevant silica/water interface at neutral pH and in the presence of aqueous carbonate. Through the use of SHG, we performed all our adsorption studies in real-time, and under flow conditions, using solutions with environmentally relevant background electrolyte concentrations. We analyze our measured uranyl adsorption free energies as a function of interfacial potential, and use SHG to measure the adsorbed surface charge density at pH 7 and in the presence of aqueous carbonate. We emphasize that our adsorption experiments are performed at a model fused silica/water interface, in situ, and under flow conditions. We expect that our results will prove useful in modeling and predicting uranium mobility in groundwater systems. Background Theory. The SHG studies performed in this work utilize the pH- and ligand-dependent aqueous uranyl absorption bands centered at 306 nm as an intrinsic label for adsorbed uranyl species (see Figure 1).22,32,59,60 Second harmonic generation (SHG) is a surface-specific spectroscopy applicable to studying buried interfaces. Within the electric dipole ap-

Malin and Geiger

Figure 1. Averaged (3 trials) SHG spectrum of adsorbed uranyl at the fused silica/water interface, at pH 7, in the presence of 10 mM NaCl, and in the presence of aqueous carbonate (b). For comparison, the SHG spectrum of the fused silica/water interface, at pH 7, in the absence of any uranium is presented as well (O). The solid line (right axis) is the bulk UV-vis spectrum of the same uranyl solution. The uranyl acetate solution concentration for both spectra was 5 × 10-4 M. Inset: Bulk UV-vis spectra of the same uranyl solution for a range of pH.

proximation, the SHG process is symmetry forbidden in centrosymmetric media,61-65 therefore the SHG response from the silica/water interface studied here only results from the interfacial region between the bulk phases where centrosymmetry is broken. To study uranyl adsorption, resonantly enhanced SHG was used. The SHG signal response from an interface (ISHG) is proportional to the induced second order polarization (P2ω) as follows:61-63,65

√ISHG ) ESHG ∝ P2ω ) χ

(2)

EωEω

(1)

In this expression, ESHG is the SHG electric field, Eω is the incident electric field and χ(2) is the nonlinear susceptibility of the interface. When the input frequency (ω) is tuned such that the second harmonic response (2ω) is in resonance with an electronic transitions of the adsorbed uranyl species, the nonlinear susceptibility is described by a resonant (χR(2)) and (2) ) as follows.61,65-71 nonresonant component (χNR (2) i∆φ 2 ESHG ∝ √ |χ(2) | 2 ) √ |χR(2) + χNR e |

(2)

In eq 2, the SHG E-field is proportional to the square modulus of the nonlinear susceptibility composed of the resonant and nonresonant terms that are linked through a phase factor ∆φ. For resonantly enhanced SHG studies, the phase difference between the resonant and nonresonant contributions is assumed to be 90°, thus eliminating cross terms from the square modulus.72,73 For the case of resonantly enhanced SHG of uranyl species at the silica/water interface, the SHG signal is dominated by the resonant susceptibility. The nonresonant contribution from the underlying solid/water interface is weak and assumed to undergo negligible changes throughout the experiments. The resonant contribution to the nonlinear susceptibility (χR(2)) is described as the product of the adsorbate number density (Nads)

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and their second order molecular hyperpolarizability (R(2)) averaged over all molecular orientations.61,62,65

χR(2) ) Nads

〈 〉 T (2) R

(3)

Furthermore, the molecular hyperpolarizability is expressed as a summation over states according to the following:61,62,65

〈 〉

T (2) -4π2e3 ijk ) R h2

∑ b,c

f |b > SiO-, >SiOH, and >SiOH2+ exist on quartz under pH 7 water.85 Singly charged anionic uranyl species would likely adsorb at positively charged, >SiOH2+, surface sites. To be consistent with a ∆z of zero, a singly charged adsorbing uranyl anion would have to displace a hydrated Cl- ion (from the screening electrolyte) coordinated to a positively charged surface site for a net change in charge of zero. For a uranyl adsorbate carrying a single positive charge, a ∆z of zero results from displacing a bond, hydrated Na+ ion coordinated to a negatively charged, >SiO-, surface site. The results of our free energy analysis are consistent with Gabriel and co-workers who also identified univalent surface-active uranyl species adsorbed on amorphous silica in their work using time-resolved laser induced fluorescence spectroscopy.21 Figure 5 shows the bulk uranyl species, and their abundance, present under the experimental conditions over the bulk uranyl concentration range covered in the adsorption isotherms. The speciation calculations were performed using ChemEql (EAWAG, v. 3.0)86 for a bulk uranyl acetate solution equilibrated with dissolved CO2 and containing 10 mM NaCl as the background

Malin and Geiger electrolyte. All of the species formation constants used in the calculations were taken from the OECD Nuclear Energy Agency (NEA).10,19 In Figure 5, the possible surface-active species, based on the ∆z analysis, are presented as solid lines, and those species that do not have the proper valency to be considered are presented as dotted lines. It can be seen in Figure 5 that the most abundant species are neutral or univalent, providing good agreement with our valency deductions from our free energy versus potential data. On the basis of the speciation calculations and the free energy-potential relationship, the possible surface-active uranyl species at pH 7 are the following, in order of decreasing abundance, UO2(OH)2, UO2(OH)3-, (UO2)2CO3(OH)3-, (UO2)3(OH)5+, UO2OH+, and UO2CO3. To further narrow down the speciation possibilities for the uranyl adsorbate at the fused silica/water interface at pH 7, the adsorbed uranyl surface charge density was determined with a separate SHG technique. These results are discussed in the next section. E. Adsorbed Uranyl Charge Density and Surface Coverage. The Eisenthal χ(3) technique is a nonresonant variant of SHG that directly detects changes in interfacial potential.83,87,88 Details of the theory behind this method, and its application to studying metal ion adsorption at solid/water interfaces can be found in our previous publications.74,76,84,89 Briefly, the χ(3) technique can be applied as an “optical voltmeter”90 which is sensitive to the interfacial potential resulting from static charge at an interface. In the case of a charged interface, such as the fused silica/water interface at pH 7, the SHG response from the interface can be written as follows:

√ISHG ) ESHG ∝ P2ω ) χ

EωEω + χ(3)EωEωΦ0

(2)

(8)

From eq 8, it is clear that changes in the interfacial potential (Φ0) produce a direct change in the measured SHG signal intensity (ISHG). In eq 8, Eω is the incident electric field and χ(2) and χ(3) are the second- and third-order susceptibilities of the interface. The Eisenthal χ(3) technique was used to measure the surface charge density of adsorbed uranyl species at the silica/water interface, at pH 7 and under carbonate rich conditions. These χ(3) charge screening experiments were performed in the following way. The input laser frequency was tuned to 690 nm to ensure that the second harmonic signal wavelength at 345 nm was off the interfacial uranyl resonance at 306 nm. By carrying out the experiments off electronic resonance, the second order contributions in eq 8 (χ(2)EωEω) are minimized, and the changes in the SHG response can be attributed to changes in the interfacial electrostatics. The experiments were carried out using a bulk solution of Millipore water equilibrated with atmospheric CO2 containing a constant uranyl acetate concentration. This uranyl solution was first flowed through the system and a baseline SHG signal level was determined. The interfacial charge setup by the adsorbed uranyl species was then sequentially screened out by the addition of increasing amounts of NaCl to the same constant uranyl solution. The inset of Figure 4 shows the salt screening isotherms for constant uranyl concentrations of 1.8 × 10-4 M (100% coverage) and 3 × 10-6 M (50% coverage). As expected from the χ(3) SHG equation, as the concentration of NaCl added to the uranyl solution increases, more and more of the charge carried by the adsorbed uranyl species is screened, thereby decreasing the interfacial potential and thus decreasing the SHG signal. To fit the salt screening data, the Gouy-Chapman eq 7 was inserted into the χ(3) expression (eq 8) as follows:

Uranyl at the Fused Silica/Water Interface

(

ESHG ) A + B × sinh-1 (σ0 + σU) ×

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30.2

√Celec

)

(9)

In eq 9, the overall surface charge density for the system was modeled as the sum of the initial surface charge density carried by fused silica at pH 7 (σ0) and the surface charge density carried by the adsorbed uranyl species (σU), Celec is the concentration of NaCl, and A and B are constants containing the second- and third-order susceptibilities, χ(2) and χ(3), and the incident electric field strengths (Eω). The surface charge density (σ0) of the silica/water interface at pH 7 was previously determined to be - 0.013 C/m2.74 By fitting the salt screening isotherms in the Figure 4 Inset with eq 9 the adsorbed uranyl surface charge density (σU) was determined to be 0.0069(7) C/m2 at 100% uranyl coverage and 0.001(2) C/m2 at 50% uranyl coverage. These surface charge density results reveals three points about uranyl adsorption at the silica/water interface at neutral pH. First, the uranyl contribution to the surface charge density, σU, is positive for both 50% and 100% coverage indicating that there was a cationic uranyl species adsorbing. From the bulk solution speciation presented in Figure 5, (UO2)3(OH)5+ is the most abundant positively charged species under the experimental conditions, with significant amounts of UO2OH+ also present, thus making these two species the most plausible candidates for the charged surface-active uranyl species at pH 7. A second key observation was drawn from the surface charge density at 100% uranyl surface coverage. Initially, the silica/water interface, at pH 7, carries a surface charge density of -0.013 C/m2. The adsorbed uranyl surface charge density at saturation coverage was found to be 0.0069(7) C/m2, which is, within error, equal to half the initial surface charge density. On the basis of this analysis, we conclude that each uranyl cation interacts with two negatively charged surface sites. This finding is consistent with the observation of bidentate uranyl binding to both silica and iron oxides.12,21,91 The third significance of the uranyl surface charge density results is that the speciation of the adsorbed uranyl does not change upon surface loading. When moving from 50% to 100% coverage, the uranyl surface charge density should double if the speciation of the adsorbed uranyl remains the same. Within the experimental error of the σU values, the uranyl surface charge density was observed to double when the uranyl concentration was increased from 50% to 100% coverage (0.003 to 0.0062 C/m2). The large error associated with the surface charge density value for 50% uranyl coverage was attributed to the fact that the amount of uranyl adsorbed at the low bulk concentration (3 × 10-6 M) was approaching the sensitivity limit of the χ(3) technique for the silica/water interface at pH 7. Also, it should be noted that the value of 3 × 10-6 M for 50% coverage was determined from the Langmuir fit to the uranyl adsorption isotherm and therefore introduced a second source of error stemming from the precision in the bulk uranyl concentration used to represent 50% coverage. Though the surface charge density data points to a positively charged surface-active uranyl species, neutral species may still also be adsorbing. Since the most abundant uranyl species based on the bulk speciation calculations, UO2(OH)2, carries no charge, it would not be detected by the χ(3) charge density experiments, but is certainly detected by the resonantly enhanced SHG measurements. Due to its abundance, it is likely to be adsorbing as well as the positive species. At pH 7, 75% of the surface sites are in the neutral, >SiOH, state and thus provide adequate adsorption sites for UO2(OH)2 via hydrogen bonding. Overall, we believe that a mixture of neutral and singly charged cationic

uranyl species are adsorbing at the silica/water interface at pH 7, in the presence of aqueous carbonate. On the basis of our SHG studies, UO2(OH)2, (UO2)3(OH)5+, and UO2OH+ are expected to be the viable candidates for the surface-active uranyl species under our experimental conditions. A mixture of uranyl adsorbates is likely responsible for the scatter in the free energy versus potential plot. Since resonantly enhanced SHG does not differentiate between varying UO22+ species, a mixture of adsorbing species would result in some convolution in the free energy-potential analysis. Conclusions We have reported the first resonantly enhanced second harmonic generation (SHG) spectra of uranyl adsorbed at the fused silica/water interface, which allowed us to identify strong SHG resonance enhancement peaked at 306 nm. Utilizing this spectroscopic signature, we then studied uranyl adsorption in real-time, under flow conditions, and with solution electrolyte concentrations within the range of those found in natural groundwater. Adsorption isotherms collected at pH 7 as a function of electrolyte concentration and in the presence of aqueous carbonate yielded adsorption free energies determined via the Langmuir isotherm that are consistent with physisorption via hydrogen bonding. The speciation of the surface-active uranyl species at pH 7 was elucidated via a free energy versus interfacial potential analysis combined with surface charge density data, obtained using the Eisenthal χ(3) technique, which reveals that the uranyl adsorbates are either neutral or univalent cationic species. It is proposed that a mixture of neutral and univalent cationic uranyl species is surface active at pH 7 and in the presence of carbonate ions at environmentally relevant electrolyte concentrations. Our thermodynamic results are expected to serve as important benchmarks for electronic structure calculations of U species,92,93 and of computer simulations of U(VI) transport in heterogeneous geochemical environments.13 Furthermore, insofar as the experimental conditions model those found in natural groundwater systems, the results of this work are valuable for the prediction and assessment of uranium pollution transport in groundwater and soils via the use of the Kd model.37 Specifically, on the basis of our previously published analysis of ion adsorption isotherms in terms of geochemical transport,8,94-97 the thermodynamic data reported here indicate that U(VI) will be highly mobile in silica-rich soil environments at circumneutral pH and environmentally relevant ionic strengths. Future work will focus on an a priori prediction of U(VI) mobility for various pH, electrolyte, and soil conditions using our thermodynamic data, which are obtained directly from the mineral/water interface. Acknowledgment. This work was supported through an Alfred P. Sloan Foundation Fellowship to FMG and by the Director, Chemical Sciences, Geosciences and Biosciences Division, of the U.S. Department of Energy under Grant No. DE-FG02-06ER15787, and by the National Science Foundation Experimental Physical Chemistry program under Grant No. CHE-0348873. References and Notes (1) Baird, C. EnVironmental Chemistry, 2nd ed.; W. H. Freeman: New York, 1999. (2) Langmuir, D. Aqueous EnVironmental Geochemistry; Prentice-Hall, Inc: NJ, 1997. (3) Morel, F. M. M.; Hering, J. G. Principles and Applications of Aquatic Chemistry; John Wiley & Sons, Inc.: New York, 1993.

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