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Uranyl Adsorption at the Muscovite (Mica)/Water Interface Studied by Second Harmonic Generation Sarah A. Saslow Gomez, David S. Jordan, Julianne M. Troiano, and Franz M. Geiger* Department of Chemistry, Northwestern University, 2145 Sheridan Road, Evanston, Illinois 60208, United States S Supporting Information *

ABSTRACT: Uranyl adsorption at the muscovite (mica)/water interface was studied by second harmonic generation (SHG). Using the nonresonant χ3 technique and the Gouy−Chapman model, the initial surface charge density of the mica surface was determined to be −0.022(1) C/m2 at pH 6 and in the presence of dissolved carbonate. Under these same conditions, uranyl adsorption isotherms collected using nonresonant χ3 experiments and resonantly enhanced SHG experiments that probe the ligand-to-metal charge transfer bands of the uranyl cation yielded a uranyl binding constant of 3(1) × 107 M−1, corresponding to a Gibbs free energy of adsorption of −52.6(8) kJ/ mol, and a maximum surface charge density at monolayer uranyl coverage of 0.028(3) C/m2. These results suggest favorable adsorption of uranyl ions to the mica interface through strong ion-dipole or hydrogen interactions, with a 1:1 uranyl ion to surface site ratio that is indicative of monovalent cations ((UO2)3(OH)5+, (UO2)4(OH)7+, UO2OH+, UO2Cl+, UO2(CH3COO−)+) binding at the interface, in addition to neutral uranyl species (UO2(OH)2 and UO2CO3). This work provides benchmark measurements to be used in the improvement of contaminant transport modeling.

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the “nuclear renaissance”,6,7 and legacy contamination sites such as those located at Hanford, WA, or Savannah, GA, are large contributors.8,9 Under aerobic conditions, U(VI) is easily transported as hydroxide and carbonate complexes of the uranyl ion (UO22+),10,11 whose complex bulk speciation (Figure 1) makes predicting its interaction with mineral surfaces challenging. Phyllosilicates are thought to be particularly important for uranyl transport in the environment as they are common constituents in the confining layers of mined uranium ores12 and exhibit highly reactive surface sites and high cation exchange capacities.13 Here, we used muscovite (mica), KAl2(AlSi3O10)(OH,F)2, a common soil constituent14 and a tetrahedral-octahedral-tetrahedral (T-O-T) structured phyllosilicate known for its near perfect cleavage (001) basal plane.15 Al substitution for Si atoms results in an inherent negative charge within the crystal lattice, which is balanced by K+ counterions located between the T-O-T silicate layers. When immersed in water, exposed surface counterions are removed, resulting in a net negative structural surface charge density of approximately 0.021 e−/Å2, or −0.34 C/m2.16 Due to this strong negative charge cation adsorption at the interface is believed to be electrostatically driven. Common methods to study uranium-mica interactions include batch studies,17 X-ray techniques (XPS, EXAFS, XANES, and XRF),18 and theoretical modeling.19,20 Samples studied are usually bulk powders in the presence of stagnant

INTRODUCTION Understanding, controlling, and predicting the adsorption of contaminant ions at mineral/water interfaces is important for assessing the mobility, transport, and environmental fate of contaminants in subsurface geochemical environments.1 To this end, surface complexation models have been successfully applied by explicitly taking charge−charge interactions into account via the following fundamental relationship:1 n+ n+ Xads = Xbulk (e−ΨF/ RT )n

Xn+ ads

(1)

Xn+ bulk

Here, and are the interfacial activity and bulk activity of an ion with charge n, respectively,1 Ψ is the interfacial potential, F is Faraday’s constant, and R and T are the ideal gas constant and temperature, respectively. Given the exponential dependence of eq 1 on the charge state of the adsorbing ion for determining the number of adsorbates at the interface, the use of incorrect charge states for interfacial species changes the outcome of eq 1and the corresponding environmental transport rateby orders of magnitude.2 This predicament can be confounded when interfacial equilibria are shifted with respect to their bulk counterparts,3 leading to situations in which the relative abundances of variously charged bulk vs surface-bound species differ substantially,4 or when working with contaminants with complex bulk speciation, such as hexavalent uranium. These two issues motivate the need for surface-specific speciation measurements. The U.S. Environmental Protection Agency (USEPA) limits human exposure to uranium to 0.13 μM, beyond which an individual’s risk of various cancers is thought to increase.5 The primary point of contact for humans with uranium is through anthropogenic contamination of drinking water, to which uranium mining activities, increasing due to the emergence of © 2012 American Chemical Society

Received: Revised: Accepted: Published: 11154

July 17, 2012 September 7, 2012 September 11, 2012 September 11, 2012 dx.doi.org/10.1021/es302879y | Environ. Sci. Technol. 2012, 46, 11154−11161

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Φ=

ESHG ∝

(3)

(2) |χNR + χR(2) e iΔϕ|2

(4)

(2) Here, χ(2) R and χNR represent the resonant and nonresonant components, respectively, and ϕ is the phase factor linking the two terms. Aqueous uranyl ions exhibit a pH and liganddependent absorption band between 300−310 nm that has been used for SHG resonance enhancement.27,31 We are therefore able to track uranyl adsorption at the interface, as this absorption band serves as an intrinsic label for the adsorbed uranyl species. As the number of adsorbates, Nads, at the interface increases the resonant second order susceptibility also increases according to3,21,30

uranyl solutions with concentrations as low as several μM. Here, we use a complementary method, the nonlinear optical technique second harmonic generation (SHG) to provide interface-specific, molecular-level measurements for elucidating the thermodynamics, electrostatics, and speciation, including charge states, of uranyl at the muscovite (mica)/water interface under environmentally relevant electrolyte concentrations and pH conditions. In this work, results are obtained in real time, under flow conditions, and utilize the high molecular sensitivity of SHG to study uranyl adsorption at concentrations ranging from as low as about 3 nM to about 80 μM, to which the batch, synchrotron, and computational studies can be readily compared.

χR(2) = Nads⟨α⃡ (2)⟩

(5)

Here, ⟨α⃡ (2)⟩ is the second order molecular hyperpolarizability averaged over all molecular orientations. According to eqs 4 and 5, the SHG response increases with the number of uranyl adsorbates at the interface, allowing for the determination of surface coverages, binding constants, and subsequent free energies of adsorption for uranyl binding to the mica surface via adsorption isotherm measurements.

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MATERIALS AND METHODS In this work, two forms of SHG3,21−23 were used: we apply the Eisenthal χ3 technique to study the electrostatic properties of the mica/water interface as a function electrolyte concentration at pH 6, and resonantly enhanced SHG to quantify uranyl adsorption at the mica/water interface. Both methods are briefly described below. A. The Eisenthal χ3 Technique. With this technique, the SHG response is modeled to depend linearly on the static interfacial potential, Φ, provided it is present at a charged mineral/water interface, according to3,22,24−26

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EXPERIMENTAL SECTION A. Sample Preparation. For all experiments V1 quality muscovite mica disks (Ted Pella, Inc.), 0.21 mm thick and 20 mm in diameter, were used. Each disk was cleaned by sonicating in methanol for 6 min, followed by 10 min in a drying oven set to 110 °C, and plasma cleaned for 30 s at the highest setting. After cleaning, the disk was soaked overnight in Millipore water (18.2 MΩ). This same cleaning procedure was used to clean the surface of a fused silica hemisphere (ISP Optics, one inch diameter) after being treated with NoChromix (Godax Laboratories) for one hour. Additionally, a 20 L carboy was filled with Millipore (18.2 MΩ) water and was equilibrated with atmospheric CO2 overnight before each experiment. NaCl (VWR, 99.0%) was added to adjust for the desired electrolyte concentration. This electrolyte solution was then used to prepare NaOH (Sigma-Aldrich, 99.99%) and HCl (EMD ACS

(2)

Here χ and χ are the second and third order susceptibility tensors, respectively, Eω is the incident electric field, and ISHG is the observed SHG signal intensity. The interfacial potential cannot be directly measured with this technique, but it can be related to the surface charge density (σ) and background electrolyte concentration (Celec) using common electrical double layer theories, such as the Gouy−Chapman equation:2,27−29 2

⎞ ⎟⎟ ⎠

Here, kB is the Boltzmann constant, T is temperature, z is the charge on the electrolyte ion, e is the charge of an electron, ε is the dielectric constant of water at 25 °C, and ε0 is the permittivity in a vacuum. Equation 3 shows that an increase in the bulk electrolyte concentration leads to a decrease in the interfacial potential. Furthermore, a change in background electrolyte concentration will result in a proportional increase or decrease in SHG signal intensity, according to eq 2. By varying the electrolyte concentration at constant pH while monitoring the SHG signal intensity, the surface charge density of the mica surface is readily determined. B. Resonantly Enhanced SHG. To study uranyl adsorption, resonantly enhanced SHG was used. Similar to the χ3 technique, resonantly enhanced SHG occurs when two photons of the same incident frequency combine to form one photon at twice the frequency. However, for adsorbates with a resonance at the second harmonic frequency, the electromagnetic radiation oscillating at the second harmonic depends on the second order susceptibility, which consists of a resonant and a nonresonant term,21,30 according to

Figure 1. Speciation plot for a 10 μM uranyl solution at pH 6, in the presence of 10 mM NaCl and equilibrated with atmospheric CO2 (400 ppm). Speciation concentrations were calculated using ChemEql (EAWAG, v. 3.0)),48 and formation constants taken from the Nuclear Energy Agency (NEA) database. Uranyl species with concentrations less than 10−15 M are not shown and considered negligible.

ISHG = ESHG ∝ χ 2 EωEω + χ 3 EωEω Φ

⎛ 2kBT π sinh−1⎜⎜σ ze ⎝ 2εε0TCelec

3

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grade) solutions of ∼1 M, used to regulate pH conditions throughout the duration of the experiment, and to prepare a 0.5 mM uranyl solution using uranyl acetate, UO2(CH3COO)2·2H2O (SPI Supplies, 99.6%). B. Laser and Flow System. A detailed description of our SHG laser set up can be found in our previous publications.24,26,32−34 Briefly, experiments were conducted using a regeneratively amplified Ti:Sapphire laser (Hurricane, Spectra Physics, 120 fs pulse) pumping an optical parametric amplifier (OPA-CF, Spectra Physics, 1 kHz repetition rate). The output beam was attenuated to 0.30 ± 0.02 μJ using a variable density filter, passed through a half-wave plate selecting for ppolarization, and focused at the mica/water interface at an angle just below total internal reflection. The exigent beam was then directed through a Schott filter blocking the fundamental (ω) and allowing the SHG signal to pass through a monochromator set to the SHG wavelength into a photomultiplier tube. Following signal preamplification, a gated single photon counter provides the number of SHG photons collected each second. All experiments were conducted in triplicate under environmentally relevant flow conditions using a dual pump flow system for switching between background electrolyte and uranyl reservoirs that fed a custom-made Teflon flow cell at ∼1 mL/sec. Details regarding the flow cell design are described in our previously published work.26,32−34 The mica disk was clamped atop the flow cell between a Viton O-ring and the fused-silica hemisphere, forming a leak tight seal. A ring of background electrolyte solution was placed around the hemisphere and disk to ensure full saturation of the mica interlayers throughout the duration of the experiment. Before conducting the experiment, two liters of background electrolyte were flown through both reservoirs and flow tubes to rinse the flow system. While conducting the experiment, a sample of each uranyl solution was collected and used to verify sample concentrations with ICP-MS. An internal standard of 5 ppb 115 In was added to each sample before analysis. Concentrations were quantitatively determined for select samples ranging from 1 to 100 nM U(VI) verifying the accuracy of our experimental results. NaCl concentrations were verified using a conductivity meter (Fisher Traceable Conductivity and TDS meter, Fisher Scientific). Finally, X-ray photoelectron spectroscopy (XPS) analysis of the mica surface was conducted using an Omicron Electron Spectroscopy for Chemical Analysis (ESCA) probe equipped with an E125 energy analyzer and a monochromated Al Kα X-ray source. Binding energies were calibrated to the C1s peak at 284.5 eV.

to the stock Millipore water while maintaining constant pH. The SHG signal intensity was then normalized to the water baseline and plotted as a function of NaCl concentration, as shown in Figure 2. To fit the salt screening data, the Gouy−

Figure 2. Salt screening plot of normalized ESHG as a function of NaCl concentration. The initial surface charge density (σ0) for the muscovite basal plane under pH 6 conditions was determined to be −0.022(1) C/m2. The black line represents the fit of the Gouy−Chapman model. Fit equation values for constants A and B were derived to be 0.74 and 0.06, respectively.

Chapman equation was used to express the interfacial potential as a function of the initial surface charge density (σ0) and the electrolyte concentration (Celec). In this work, σ0 is not just the structural charge density as defined by Sposito35 but in fact the sum of all charged species within the interfacial region. This sum includes contributions from the structural charges of mica, adsorbed protons and hydroxide ions, protonated and/or deprotonated oxygen atoms at the basal plane in contact with the aqueous phase, and adsorbed ions that are part of the aqueous solution at nominally zero salt concentration. We point out that a full sensitivity analysis and an application of other electrical double layer models are available in our prior work.25,36 Substituting eq 3 into eq 2 yields the following fit function: ISHG = ESHG ⎡ (30.19M1/2m 2C −2) ⎤ ⎥ = A + B × arcsinh⎢σ0 × ⎢⎣ ⎥⎦ Celec

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RESULTS AND DISCUSSION A. Initial Surface Charge Density. Prior to studying uranyl adsorption, the initial surface charge density of the mica basal surface was determined under pH 6 aqueous conditions, which is above the point of zero charge of the mica basal plane.28 pH 6 was chosen because of its relevance in natural groundwater. The surface charge density of the mica surface was determined by tuning the laser input wavelength to 650 nm, generating a second harmonic response at a wavelength of 325 nm, which is located to the red of the mica bulk absorption around 280 nm (vide infra). A bulk solution of Millipore water, adjusted to pH 6 and equilibrated with atmospheric CO2, was then flowed through the system in order to establish a baseline SHG signal intensity level. The initial interfacial surface charge density was then sequentially screened with additions of NaCl

(6) (2)

Here, A and B are constants composed of the second (χ ) and third order (χ(3)) susceptibility terms and the incident electric field (Eω). The determined values for the constants presented in eq 6 and later in eq 7 may be found in the Supporting Information (SI), Table S1. This fit equation yields an initial surface charge density of the mica basal plane of −0.022(1) C/ m2 at pH 6, which is consistent with literature values reported for similar aqueous conditions.37,38 We point out here that the surface charge density calculated from the SHG data for the mica/liquid interface is approximately ten times smaller than the calculated density of structural charges of −0.34 C/m2, in air.16,39−41 The difference of +0.318 C/m2 is then the sum of charges due to adsorbed protons, hydroxide ions, protonated 11156

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Figure 3. A. XPS spectrum of the mica substrate used in experiments. Inset: Adapted with permission from Xu, L.; Salmeron, M., An XPS and Scanning Polarization Force Microscopy Study of the Exchange and Mobility of Surface Ions on Mica. Langmuir 1998, 14 (20), 5841−5844. Copyright 1998, Anerican Chemical Society. Literature reference of the K mica spectrum. Black and gray lines represent K mica and Ca substituted mica, respectively.49 B. XPS spectrum after running a 10 μM uranyl solution in the presence of 10 mM NaCl at pH 6 that has dried on the surface. Inset: Enlarged spectrum of the UO2 4f7/2 (green dashed line) and K 2s1/2 (blue dashed line) peak region illustrating two distinguished, separate peaks. C. XPS spectrum after running a 10 μM uranyl solution in the presence of 10 mM NaCl at pH 6 and rinsing with Millipore water. Inset: Enlarged spectrum of the UO2 4f7/2 (green dashed line) and K 2s1/2 (blue dashed line) peak region. The peak present is indicative of K with little to no contributions from UO2, confirming the reversibility of uranyl adsorption.

Figure 4. Adapted with permission from Malin, J. N.; Geiger, F. M. Uranyl adsorption and speciation at the fused silica/water interface studied by resonantly enhanced second harmonic generation and the χ3 method. J. Phys. Chem. A 2010, 114, 1797−1805. Copyright 2010, American Chemical Society. A. On-resonance adsorption isotherm of uranyl on fused-silica at pH 7, 10 mM NaCl and equilibrated with pCO2. The black line represents the Langmuir fit.27,43 Adapted with permission from Malin, J. N.; Holland, J. G.; Saslow, S. A.; Geiger, F. M. U(VI) adsorption and speciation at the acidic silica/water interface studied by resonant and nonresonant second harmonic generation. J. Phys. Chem. C 2011, 115, 13353−13360.Copyright 2011, American Chemical Society. B. χ(3) adsorption isotherm of uranyl binding to fused-silica at pH 4, 1 mM NaCl and equilibrated with atmospheric CO2 (400 ppm). The black line represents the Gouy−Chapman fit C. An on-resonance uranyl adsorption isotherm on mica exhibits an initial decrease due to destructive interference between the resonant mica substrate and uranyl adsorbates.The black line represents the derived fit equation incorporating both resonant and χ(3) contributions. Error bars are indicative of averaging over several uranyl concentrations. The binding constant, 3(1) × 107 M−1, and Gibbs free-energy of adsorption, −52.6(8) kJ/mol were determined.

B. Adsorption/Desorption of Uranium. The interaction of uranyl species with the mica basal surface was studied by carrying out adsorption/desorption experiments with the dual pump capabilities of our experimental setup while probing the ligand-to-metal charge transfer band of uranyl with resonantly enhanced SHG at an SHG wavelength of 310 nm. To start, a signal baseline was established using a 10 mM NaCl

and/or deprotonated oxygen atoms, adsorbed ions that are part of the aqueous solution at nominally zero salt concentration, and any other charged species at the interface. The low surface charge density of −0.022(1) C/m2 at pH 6 is consistent with work showing that the majority of the mica surface consists of neutral adsorption sites.42 11157

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background electrolyte stock solution, adjusted to pH 6. Once the SHG signal intensity had been at steady state for 400 s, the solution was switched to the uranyl reservoir, which was adjusted to the same pH and electrolyte concentration, and an increase in the SHG signal, consistent with SHG resonant enhancement (eqs 4 and 5), was observed upon uranyl adsorption. Finally, 10 mM NaCl was flowed again through the cell and the SHG signal decreased to the established baseline, demonstrating the reversibility of uranyl adsorption at the mica surface. Further investigations using XPS confirmed SHG adsorption/desorption results. Three mica disks were cleaned and analyzed using XPS; one prior to uranium exposure (Figure 3A), one with a layer of air-dried 10 μM uranyl/10 mM NaCl solution on the surface (Figure 3B), and finally one rinsed with Millipore water after exposure to the same uranyl solution (Figure 3C). XPS scans were performed in the UO2 4f7/2 binding energy (375−395 eV) region with constant pass energy. As anticipated, the UO2 4f7/2 peak at ∼380 eV was only observed for the mica disk with a dried layer of uranyl solution on the surface (Figure 3B), confirming XPS sensitivity for the desired uranyl peak. After rinsing the mica disk exposed to a 10 μM uranyl/10 mM NaCl solution under flow conditions (Figure 3C) the UO2 peak was not observed and the XPS spectrum is very similar to the spectrum taken prior to uranium exposure (Figure 3A). These XPS results confirm desorption of uranyl adsorbates as observed in the SHG on−off traces. C. Adsorption Isotherms. The binding constant, Gibbs free energy of adsorption, and maximum surface charge density at monolayer coverage for uranyl adsorption to the mica surface was quantified by recording adsorption isotherms using resonantly enhanced SHG. In our previously published work, resonantly enhanced adsorption isotherms for uranyl on fused silica resulted in SHG signal intensity increases up until surface saturation occurred (Figure 4A) and nonresonant χ (3) adsorption isotherms resulted in SHG signal intensity decreases until surface saturation occurred (Figure 4B).27,43 The adsorption isotherm for uranyl interaction with the mica/ water interface, Figure 4C, exhibits an SHG signal intensity decrease for uranyl concentrations up to about ∼40 nM. Higher uranyl concentrations result in SHG signal intensity increases until saturation occurs. We find that the SHG responses of the mica/aqueous interface in the presence and absence of adsorbed uranium are comparable (Figure 5). Likewise, the linear absorption spectrum of mica reveals an absorption that coincides with the uranyl electronic transition in the 300−310 nm region studied here. The small number of 310 nm-photons produced by resonantly enhanced SHG from submonolayer amounts of uranyl adsorbates may therefore be absorbed by the mica substrate, and under those conditions the nonresonant χ(3) response may dominate. Therefore, the uranyl/mica SHG adsorption isotherm needs be described by a combination of eqs 2 and 4 for low and high uranyl concentrations and surface coverages, respectively. In other words, the decrease in SHG signal intensity is due to the decrease in interfacial potential upon uranyl adsorption, and when the uranyl surface coverage reaches sufficiently high values at concentrations exceeding ∼40 nM, SHG resonant enhancement exceeds the nonresonant χ(3) response. This effect is similar to one reported by Petersen and Saykally, who studied the adsorption of ferrocyanide at the water−air interface using resonant and nonresonant SHG.44 The initial SHG signal decrease reported in their work was attributed to destructive phase interference between the

Figure 5. UV-vis (black line) and SHG (black circles) spectra of adsorbed uranyl at the muscovite (mica)/water interface. A 5 × 10−4 M uranyl acetate solution, at pH 6, in the presence of 10 mM NaCl, and equilibrated overnight with atmospheric CO2 (400 ppm) was used. A UV-vis spectrum of the muscovite disk (gray line) and SHG spectrum of the mica/water interface in the absence of uranium were also collected.

resonant response from the adsorbed ions and the nonresonant contribution from water molecules present at the interface and was modeled assuming Langmuir adsorption behavior. The sharp increase in SHG signal intensity at high ferrocyanide concentrations was modeled separately and determined to be linearly dependent on the bulk ferrocyanide concentration. Unlike in the work published by Petersen and Saykally, which involved one resonant adsorbate (ferrocyanide) and one nonresonant adsorbent (the air/water interface), the uranyl/ mica system involves two resonant SHG contributions, namely the mica substrate and the adsorbed uranyl species. In Figure 6, a vector model is used to illustrate the interplay between the resonant χ2 susceptibilities from the mica surface, the adsorbed uranyl ions, and the nonresonant χ3 susceptibility from the charged interface. The resonant susceptibilities are complex numbers and thus are plotted in the real and imaginary axes, whereas nonresonant susceptibilities contain only real components and are plot solely in the real axis. Given that we are not exactly on electronic resonance in our SHG experiments, the phase angles32 between the nonresonant χ3 susceptibility from the charged interface and the resonant χ2 susceptibilities from the mica surface and the adsorbed uranyl ions are not exactly 90 degrees. We therefore plot the resonant χ2 susceptibilities nonorthogonally to the nonresonant χ3 susceptibility. We also assume that the resonant χ2 susceptibility from the mica surface remains constant and is not aligned with the resonant χ2 susceptibility of uranyl, again because we are not exactly on electronic resonance with either species in our SHG experiments. Our vector representation then works as follows and yields the following physical insight: as the number of adsorbates at the interface increases, the magnitude of the uranyl vector increases while the χ(3) vector decreases along the real axis with decreasing interfacial potential. The sum of the three vectors, which describes the observed SHG signal in our vector representation, is initially large, then goes through a minimum, and then increases again once the resonant χ2 susceptibility of uranyl is long enough, until surface saturation occurs. 11158

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ion-dipole interactions. Compared to uranyl adsorption to fused silica/water interfaces, the ΔGads for uranyl adsorption to mica is significantly more favorable (SI Table S2).27,31 Immediate future work will focus on the uranyl adsorption dependence on interfacial potential, or background electrolyte concentration. As published previously, uranyl adsorption showed little to no dependence on interfacial potential at the fused-silica/water interface,27,31 but this situation may be very different for the case of the mica/water interface. Determining the thermodynamic properties of uranyl adsorption at the mica/water interface allows us to propose an adsorption binding mechanism. A comparison of the initial surface charge density, −0.022(1) C/m2, and the maximum surface charge density at monolayer, 0.028(3) C/m2, coverage suggests a 1:1 uranyl ion-to-surface site ratio. This ratio and the positive surface charge density at monolayer coverage suggest monovalent cations are adsorbing at the interface. Therefore, according to the speciation plot presented in Figure 1, the monovalent uranyl cations adsorbing at the interface are most likely the (UO2)3(OH)5+, (UO2)4(OH)7+, UO2Cl+, and UO2OH+ ions and possibly the monovalent uranyl acetate species, UO2(CH3COO−)+. However, resonantly enhanced SHG does not distinguish between charged and neutral species adsorbates (both produce resonantly enhanced SHG); therefore, UO2(OH)2 and UO2CO3 are possible active adsorbing species as well.

Figure 6. Total SHG signal intensity is proportional to the square length of the total nonlinear suceptability vector. The total nonlinear suceptibility is shown as the vector sum of the complex, resonant contributions from uranyl adsorbates and the mica substrate, and the real, nonresonant χ3 contributions. The complex mica vector (green) is assumed to remain constant, whereas an increase in uranyl concentration results in an increase in the uranyl nonlinear susceptibility (blue) and a decrease in χ3 (red). Destructive interference between the mica and uranyl adsorbates results in an initial decrease in SHG signal intensity at low concentrations, but begins to increase with increasing concentration as the uranyl vector excedes both the mica and χ3 vectors.

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IMPLICATIONS The results presented here indicate favorable adsorption to the muscovite basal surface; however, the interfacial chemistry observed at the basal plane is expected to differ greatly from that of the positively charged muscovite edges. This positive charge is due to the hydroxyl groups located on the edges of the muscovite octahedral layer with a pHpzc of ∼8.46 The difference in adsorption chemistry between the basal and edge surfaces was exemplified by Strawn and Sparks, who reported that Pb ions adsorbed via electrostatic interactions to the basal plane of montmorillonite, which has a similar mineral structure to muscovite, but formed covalent bonds at the edges face, resulting in Pb polymer complexes.47 Furthermore, while muscovite is known to exhibit little to no swelling, it is hypothesized that should intercalation of uranyl species occur it would occur through the edges of the mineral structure. Given these interesting differences of the basal and edge planes of mica, future work studying uranyl adsorption to muscovite will focus on adsorption at the edges. Finally, these results will be compared to theoretical calculations of uranyl adsorption to muscovite to provide experimentally determined electrostatic, thermodynamic, spectroscopic, and structural benchmarks for the further development of contaminant transport models.

In order to calculate the binding constant, monolayer surface charge density, and free energy of adsorption from the adsorption isotherm in Figure 4C, a fit equation was developed that incorporates both the χ(3) contributions at low uranyl concentrations and the resonant SHG behavior at uranyl concentrations exceeding ∼40 nM. Following our prior work,31 we modified eq 6 to express the surface charge density (σ) as the sum of the initial surface charge density of mica, σ0, and the maximum uranyl surface charge density at monolayer uranyl coverage, σM, modeled using the Langmuir adsorption model, which contains the uranyl binding constant, K, and the bulk uranyl concentration, M (eq 7). Alone, this part of the fit function accurately models the 1−40 nM range. However, in order to combine this nonresonant behavior with the resonant behavior observed above 40 nM, an additional Langmuir term and scaling coefficient (D) is included in eq 7 according to ⎡⎛ ⎛ KM ⎞⎞ ⎟⎟ ESHG = A + B × arcsinh⎢⎜σ0 + σM⎜ ⎝ 1 + KM ⎠⎠ ⎢⎣⎝ ⎛ 30.2M1/2 m 2C−2 ⎞⎤ ⎛ KM ⎞ ⎟ ⎟⎟⎥ + D⎜ × ⎜⎜ ⎝ 1 + KM ⎠ M + Celec ⎠⎥⎦ ⎝

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(7)

Equation 7 contains the resonant χ susceptibilities from the mica surface (the A term), the nonresonant χ3 signal contribution from the charged interface (the B term), and the resonant χ2 contribution from the adsorbed uranyl ions (the D term). The background electrolyte concentration, Celec, was held constant at 0.01 M NaCl. Using this equation, a uranyl binding constant of 3(1) × 107 M−1 was calculated as a function of bulk uranyl concentration, M. The corresponding ΔGads value of −52.6(8) kJ/mol, referenced to water (55.5 M), indicates very favorable adsorption at the interface,45 most likely through strong electrostatic, hydrogen bonding, and/or 2

ASSOCIATED CONTENT

* Supporting Information S

Variables and constants determined by the fit equations presented in this work and a comparison between surface charge densities and free energy of adsorptions determined for fused-silica and mica are provided. This information is available free of charge via the Internet at http://pubs.acs.org/

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. 11159

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Notes

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The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This material is based upon work supported by National Science Foundation Graduate Research Fellowships to S.A.S.G. and J.M.T. This work was also supported by the National Science Foundation Environmental Chemical Sciences program under grant no. CHE-0950433. F.M.G. gratefully acknowledges an Irving M. Klotz professorship. We thank Spectra-Physics Lasers, a division of Newport Corporation, for equipment support. The ICP-MS analysis was completed at the Northwestern University Integrated Molecular Structure Education and Research Center (IMSERC). The XPS work was completed at the Keck Interdisciplinary Surface Science Center (Keck II) of Northwestern University.

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