Environ. Sci. Technol. 2010, 44, 5862–5867
Interactions of Al(III), La(III), Gd(III), and Lu(III) with the Fused Silica/Water Interface Studied by Second Harmonic Generation DAVID S. JORDAN, JESSICA N. MALIN, AND FRANZ M. GEIGER* Department of Chemistry, Northwestern University, 2145 Sheridan Road, Evanston, Illinois 60208
Received March 1, 2010. Revised manuscript received June 9, 2010. Accepted June 29, 2010.
The interactions of the trivalent metal cations Al(III), La(III), Gd(III), and Lu(III) with the silica/water interface were studied using the nonlinear optical technique of second harmonic generation (SHG). Specifically, the Eisenthal χ(3) technique was used to quantify the thermodynamics of trivalent ion adsorption to the bare fused silica surface. SHG adsorption isotherms were measured and fit with the triple layer surface complexation model to obtain adsorption free energies, binding constants, and interfacial charge densities. The adsorption free energy for Al(III) was found to be -37.2(5) kJ/ mol, while the adsorption free energies for the three trivalent lanthanide cations ranged from -29.9(9) to -32.2(7) kJ/ mol. Despite identical ionic charges, the metals under investigation displayed different affinities for the fused silica/ water interface, and this finding is analyzed and interpreted in the context of size-dependent metal cation properties and metal ion speciation. The thermodynamic results from this work are valuable benchmarks for computer simulations of trivalent metal transport in the environment.
I. Introduction Solid/water interfaces are ubiquitous in the environment. Investigating the interactions of metal pollutants with interfaces, specifically the mineral oxide/water interface, is crucial for understanding how these pollutants are transported throughout the environment, what their speciation is, and what their ultimate fate is in groundwater (1-3). In the work presented here, the fused silica/water interface is used to model the binding of trivalent metal pollutants to a naturally occurring mineral/water interface. Silicate minerals are major components of the Earth’s crust and are a common constituent in soils (4), making silica an ideal substrate for studying environmentally relevant heterogeneous geochemical processes. Lanthanide compounds have emerged as important materials in research and industry (5). Examples include the use of lanthanide complexes as emitters in electroluminescent devices (6, 7), as reagents in organic synthesis and asymmetric catalysis (8, 9), and as MRI contrast agents (10). The lanthanides are also utilized as natural analogues for the actinide elements (11, 12). This increased demand for lanthanide materials results in increased mining of lanthanide ores and increased public exposure to these lanthanide * Corresponding author e-mail:
[email protected]. 5862
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elements. Specifically, it has been reported that mining of lanthanide-containing ores and industrial processes, such as fertilizer production (13), releases lanthanides into nearby soils and groundwater (14, 15). Lanthanide elements can then accumulate in humans (16) and other organisms (17), and their toxicity has been reviewed previously (18). Lanthanide elements occur almost exclusively in the +3 oxidation state (19), and the adsorption edge of various lanthanide elements has been previously quantified (20). In the present laboratory model study, we chose lanthanum(III), gadolinium(III), and lutetium(III) to investigate how trivalent lanthanide ions interact with common oxide/aqueous interfaces. Aluminum is another trivalent metal pollutant of concern. Aluminum is a common element in the Earth’s crust (19); however, elevated soil solution concentrations of aluminum are detrimental to plants and threaten drinking water quality (21). Aluminum can leach into groundwater via soil weathering as well as mining activities (22, 23), lake and stream acidification (24, 25), and from building materials (19). Excess aluminum is known to inhibit root growth and function, resulting in decreased plant productivity (1, 26). In humans, aluminum can act as a neurotoxin that alters the function of the blood-brain barrier (27). It has been classified as a metalloestrogen (28), a class of compounds which can potentially promote breast cancer development. Aluminum has also been linked to the development of Alzheimer’s disease (29, 30), where a strong correlation exists between aluminum found in dinking water and development of the disease (21). Due to the potential threat posed to human health by aluminum and the lanthanides, adsorption studies of trivalent metals at mineral/water interfaces are crucial to pollutant risk assessment and remediation efforts. The development of molecular-level insight into the adsorption interactions between trivalent ions and the silica/ water interface is crucial for assessing their environmental and biogeochemical fate. Previous work on the adsorption of Al(III), La(III), Gd(III), and Lu(III) at interfaces has focused on using batch techniques to study their interaction with biological membranes (31-33) and mineral oxides, including manganese oxide (34), gibbsite (35), alumina (35, 36), silica (20), and goethite (37). Additionally, solid state NMR was used to investigate the association of Al(III) with silica (38), and EXAFS was employed to model the binding of Lu(III) to hectorite (39). Overall, these studies complement the work presented here in that the lanthanides are shown to coordinate weakly with mineral interfaces, whereas aluminum has been shown to associate to a greater extent. Despite this previous work, there is a need for a molecular-level description of trivalent metal association with mineral-water interfaces to serve as a basis for interpreting macroscopic adsorption data. To the best of our knowledge, the data obtained from the experiments presented here represents the first measurement of the binding constant, adsorption free energies, and interfacial charge densities of these four ions binding to the fused silica/water interface. Our second harmonic generation studies fill this gap by quantifying the thermodynamics of trivalent metal adsorption in situ, under flow conditions, and without the use of labels, as well as provide adsorbate number densities derived from measured surface charge densities. In this work, the adsorption of Al(III), La(III), Gd(III), and Lu(III) to the silica/water interface is evaluated under environmentally relevant conditions.
II. Background and Theory A. The Eisenthal χ(3) Technique. Second harmonic generation (SHG) (40, 41) is a coherent nonlinear optical spec10.1021/es100665c
2010 American Chemical Society
Published on Web 07/13/2010
troscopy which, in the case of the Eisenthal χ(3) technique (42), is highly sensitive to the electrostatic potential set up by interfacial charged species. The χ(3) technique allows us to quantify the interaction of trivalent metal cations with mineral oxide interfaces directly and without the need for electrochemical or spectrophotochemical labels. Briefly, the SHG E-field (ESHG) is proportional to the induced second order polarization (P2ω) and is expressed as follows
for water at 25 °C, and Celec is the bulk concentration of the screening electrolyte in solution. By substituting eqs 3 and 4 into eq 2, we arrive at a triple layer expression for the SHG response as a function of the bulk metal concentration
ESHG ∝ P2ω ) χ(2)EωEω + χ(3)EωEωΦ0
In the equation above, the charge density at the β-plane, σβ, is modeled as the maximum charge density established by metal ion adsorption, σm, scaled by surface coverage (48) where θ is given by the Langmuir model (49)
(1)
where Eω is the applied electric field at frequency ω, Φ0 is the interfacial potential obtained by integrating the third-order polarization from 0 to ∞ in the direction perpendicular from the surface, and χ(2) and χ(3) are the second- and third-order nonlinear susceptibilities, respectively. The χ(2) contribution is a result of the populations and nonlinear polarizabilites of the interfacial species, while the χ(3) contribution is due to the induced alignment of water dipoles resulting from the interfacial potential established by a static charge located at zero distance from the surface (42). For a constant incident electric field and nonlinear susceptibilities, eq 1 simplifies to
√ISHG ) ESHG ∝ P2ω ) A + BΦ0
(2)
Here, A and B are constants that include Eω and the nonlinear susceptibilities, and the square root of the second harmonic signal intensity, ISHG, is directly proportional to the interfacial potential. Given that any change in the interfacial potential changes SHG signal intensity, the Eisenthal χ(3) method can be viewed as an “optical voltmeter” (43). B. The Triple Layer Model. At the pH and ionic strengths employed here, the silica surface possesses a net negative charge (3). The addition of positive cations should decrease the magnitude of the interfacial potential and thus result in a decrease of the second harmonic signal proportional to the concentration of metal analyte. In order to quantify metal ion adsorption from the χ(3) measurements, the triple layer model was employed as an analyte-dependent expression for the interfacial potential (3). This framework is widely used to model adsorption to mineral oxides due to its applicability over a wide range of ionic strengths and analytes (2, 44-46). Briefly, the interfacial region is taken to consist of the zero plane, the β-plane, and the diffuse layer plane (3). The zero plane represents the surface of the solid substrate. The acid/ base chemistry of this plane determines the surface charge density, σ0, and the interfacial potential, Φ0. The intermediate β-plane represents the location of specifically adsorbed ions and carries the charge density σβ and the interfacial potential Φβ. Finally, the outermost diffuse plane represents a layer of charge-balancing ions and carries the potential Φd. The innermost layer, bounded on either side by the zero and β-planes, is treated with a constant capacitance approach, as is the layer bound by the β and diffuse planes (45, 47). An expression for the interfacial potential (Φ0) can then be derived as follows Φ0 )
σ0 σ β - σ0 + + Φd C1 C2
(3)
The potential of the diffuse layer (Φd) is assumed to decay according to Gouy-Chapman theory (45, 47) Φd )
(
2kBT sinh-1 (σ0 + σβ) ze
π 2εTkBCelec
)
(4)
Here, kB, T, and e have their usual meaning, z is the valency of the screening ions in solution, ε is the dielectric constant
ESHG ) A′+B
{
(
2kBT σmθ + sinh-1 (σ0 + σmθ) C2 ze
θ)
π 2εTkBCelec
)}
Kads[Cmetal] 1 + Kads[Cmetal]
(5)
(6)
In eq 5, the β-layer capacitance, C2, has been assumed to be the constant value of 0.2 F/m2, which is standard in many applications of the triple layer model (44). In order to test the robustness of this assumption, we performed a sensitivity analysis by taking the partial derivative of eq 5 with respect to C2 (Figure S1 of the Supporting Information), which shows that the value of C2 can be varied by as much as (50% around 0.2 F/m2 without having an appreciable effect on model results.
III. Experimental Methods A. Lens and Solution Preparation. A hemispherical fused silica lens (ISP Optics) was used as the adsorption substrate. Prior to each experiment, the lens was treated for one hour with NoChromix solution (Godax Laboratories), a commercial glass cleaner. Following this treatment, the lens was rinsed thoroughly with Millipore water (18.2 MΩ cm) before it was sonicated in methanol for six minutes. The lens was then rinsed in methanol and placed in a 110 °C oven for 30 min. Finally, the lens was oxygen plasma cleaned (Harrick Plasma) for 30 s and stored under Millipore water until further use. Stock solutions of Al(III), La(III), Gd(III), and Lu(III) were prepared using AlCl3 · 6H2O (Alfa Aesar, 99.9995%), LaCl3 · 6H2O (Aldrich, 99.999%), GdCl3 · 6H2O (Alfa Aesar, 99.9%), and LuCl3 · 6H2O (Aldrich, g99.99%) salts, respectively. Each solution was prepared in Millipore water with a background NaCl (Alfa Aesar, 99+%) electrolyte concentration of 0.01 M. Each solution used in this study was maintained at a pH of 4 using dilute solutions of HCl (E.M.D., ACS grade) and NaOH (Sigma Aldrich, 99.99%). The speciation of each dissolved salt under experimental conditions was assessed using the ChemEQL program (50), which indicates that the metals under investigation are present in solution as trivalent cations at pH 4 with an overall charge of +3. B. Laser and Flow System. A detailed description of the laser system used in this study can be found in previous publications (51-54). Briefly, our SHG studies were performed using a regeneratively amplified Ti:Sapphire laser (Hurricane, Spectra-Physics) pumping an optical parametric amplifier (OPA-CF, Spectra-Physics, 1 kHz repetition rate) which was tuned to a fundamental frequency of ω ) 600 ( 5 nm, a wavelength that is far from any electronic resonances present within the system. This beam was directed through a variable density filter, where the power was attenuated to 0.55 ( 0.05 µJ per pulse, far below the power level at which optical breakdown of the surface under examination occurs. The input beam was focused onto the silica/water interface at an angle just below that of total internal reflection. The generated second harmonic (2ω) beam was then isolated from the reflected fundamental beam by using a UV-grade Schott filter and a monochromator set to 2ω, directed into a photomultiplier tube, amplified, and recorded using a gated VOL. 44, NO. 15, 2010 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 1. The adsorption isotherm for Al3+ at the fused silica/ water interface taken in a 0.01 M NaCl background at pH 4. The solid line represents the triple layer model fit. Inset: the adsorption/desorption trace observed for 0.2 mM Al3+ binding to the fused silica/water interface in a 0.01 M NaCl background at pH 4. single-photon detection system. The quadratic power dependence and appropriate bandwidth of the SHG signal were verified before each experiment. All SHG measurements were performed under flow conditions using our previously published flow setup (51-54). The fused silica hemisphere was clamped atop a custombuilt Teflon flow cell using a Viton O-ring. Peristaltic pumps draw from one reservoir containing Millipore water and a 0.01 M background NaCl concentration adjusted to pH 4.0 ( 0.1, and another reservoir containing the metal analyte solution of interest adjusted to an identical pH in the same background electrolyte solution. A flow rate of ∼1 mL/s was monitored and maintained throughout all experiments using a flow meter. Aliquots of the output flow were collected and analyzed using inductively coupled plasma atomic emission spectroscopy (ICP-AES, Varian) to determine the bulk analyte concentration.
IV. Results and Discussion Adsorption/desorption studies were performed at pH 4 under dynamic flow conditions in order to determine the reversibility of Al(III), La(III), Gd(III), and Lu(III) binding to the fused silica/water interface. Initially, water containing the 10 mM background electrolyte was flowed past the fused silica substrate while the SHG signal intensity was recorded. After a stable signal level was obtained, the flow from the water reservoir was stopped, while the flow from the reservoir containing the metal analyte was initiated at the same time. The SHG signal intensity was again monitored until a stable signal level was reached, at which point the adsorption/ desorption processes reached steady state. After collecting a stable SHG signal for several minutes, the analyte flow was stopped, and water was once again flowed across the interface until a stable signal level was reached. The magnitude of the SHG E-field was obtained from these adsorption/desorption studies by taking the square root of the raw SHG signal intensity. This value was normalized to the baseline water SHG E-field calculated from the SHG signal intensity in the absence of any metal analyte. The inset of Figure 1 shows that the SHG E-field decreases from the baseline water level with the introduction of a trivalent metal cation, an observation that is consistent with the χ(3) effect. In all cases, the SHG signal increased back to baseline level once the metal ion solution was flushed from the interface with the background electrolyte solution, indicating that the binding of each cation is fully reversible. 5864
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FIGURE 2. Charge screening experiment performed on a bare silica surface at pH 4. The line represents the fit of the Gouy-Chapman model to the experimental data. SHG χ(3) adsorption isotherms were recorded by collecting adsorption/desorption traces for a range of metal ion concentrations. Figure 1 shows the adsorption isotherm for Al(III) at the fused silica/water interface at pH 4 and in the presence of 10 mM NaCl. The ESHG values on the y-axis were calculated by normalizing the square root of the SHG signal intensity in the presence of Al(III) to the square root of the average background water signal level before and after Al(III) flow. It should be noted that adsorption isotherms were measured in triplicate for each metal. These data were then fit using eq 5. The fitting parameters are A, B, σm, the maximum charge density established by adsorbed metal cations at saturation coverage, and Kads, the binding constant. The value for σ0, the surface charge density of the bare silica surface under water at pH 4, was determined experimentally through a salt-screening χ(3) experiment. Following our previously published procedure for SHG determination of silica surface charge density (53), the value of σ0 was determined to be -0.004(1) C/m2 at pH 4 (see Figure 2, uncertainties within one standard deviation are given in the parentheses). From the triple layer fits, binding constants of 6(1) × 104 -1 M , 6(3) × 103 M-1, 3.1(9) × 103 M-1, and 8(2) × 103 M-1 were found for Al(III), La(III), Gd(III), and Lu(III), respectively. Clearly, the binding constant for Al(III) is about ten times larger than that of the lanthanide ions. The corresponding adsorption free energies were calculated with reference to 55.5 M water, the standard state for adsorption from aqueous solution (55), and found to be -37.2(4) kJ/mol, -31(1) kJ/ mol, -29.9(9) kJ/mol, and -32.2(7) kJ/mol for Al(III), La(III), Gd(III), and Lu(III), respectively. To the best of our knowledge, these findings mark the first thermodynamic measurement of these specific trivalent cations binding to silica at pH 4. These adsorption free energies are on the order of one or two hydrogen bonds. This finding, along with the observation that binding is fully reversible, suggests that water plays a critical role in the metal ion-surface site interactions. Please note that our experimental methods do not provide the spatial information required to determine whether or not the binding interaction occurs via an outer or an inner sphere mechanism. The free energy values for the lanthanide elements are comparable to those obtained by Malin (56) and co-workers for Ca(II), Zn(II), and Cd(II) ions binding to a silica surface, which were found to be -29.1(3) kJ/mol, -30.8(3) kJ/mol, and -28.3(2) kJ/mol, respectively. Our thermodynamic adsorption results reveal a substantial difference between the binding energies of Al(III) and the three lanthanide elements. Figure 3A presents the measured binding constants of the four metals as a bar graph for ease in comparison. Insight into the cause of the varying
4 shows that this is not the case. We conclude that other ion properties are important. Figure 4 shows that the smaller Al3+ ion binds to the fused silica/water interface with a larger binding constant than the larger lanthanide ions. This is attributed to the fact that the valence-to-radius ratio increases with decreasing radius. A large valence-to-radius ratio leads to a more compact hydrated species and lower coordination number (Al(H20)6 compared to Ln(H20)8-9), allowing the central cation to approach close to the surface sites, which should lead to an increased binding constant. The maximal surface charge density established by the adsorbed metal cations, σm, was also obtained from the triple layer fits of the χ(3) adsorption isotherms. The surface charge densities due to the presence of Al(III), La(III), Gd(III), and Lu(III) were found to be +0.004(2) C/m2, +0.009(4) C/m2, +0.004(3) C/m2, and +0.009(3) C/m2, respectively. These values are all within error of one another (see Figure 3B) and are of comparable magnitude to the initial surface charge density of -0.004(1) C/m2 for bare silica at pH 4. From this finding we conclude that overcharging of the surface due to metal cation binding is minimal. Not surprisingly, this result indicates that electrostatic attraction is the driving force for the adsorption process. From the metal adsorbate surface charge densities, the adsorbate number densities were calculated and can be found in Table S2.
V. Environmental Implications and Conclusions FIGURE 3. Bar graph representing the range of binding constants (A) and interfacial charge densities (B) measured experimentally. The red bar represents the lowest possible value, and the blue bar represents the highest possible value dictated by the error range obtained from the one σ standard deviation of three adsorption isotherm measurements per cation.
FIGURE 4. Binding constants as a function of valence-to-ionic radius (bottom axis, closed circles) and the water exchange rate constants for each hydrated ion (top axis, logarithmic scale, open circles). The same vertical error bars shown on the closed circles also apply to the corresponding open circles. binding affinities between Al(III) and the lanthanides is demonstrated in Figure 4, in which the binding constants are plotted as a function of the valence-to-radius ratio (calculated by dividing the cation valence (+III) by the ionic radius (57), bottom x-axis) and their water exchange rate constants (top x-axis) (58). This latter rate is a measure of the rate of exchange of bulk water and of water within the inner hydration sphere of the metal ion. If the metal ion were to directly bind to an interface, the chemical nature of the metal ion could be important as well as partial dehydration of the hydration sphere (59). For the latter case, the binding constants should scale with the water exchange rate. Figure
This study marks the first direct determination of the surface charge densities, binding constants, free energies of adsorption, and an assessment of reversibility for the aqueous trivalent metal cations Al(III), La(III), Gd(III), and Lu(III) interacting with a buried oxide/water interface under dynamic flow and environmentally relevant electrolyte conditions. The thermodynamics and electrostatics of binding were quantified at the silica/water interface using the Eisenthal SHG χ(3) technique. The ability to track the adsorption of trivalent cations directly at metal oxide interfaces and to obtain detailed thermodynamic parameters at the molecular level without the use of labels demonstrates the utility of nonlinear optical techniques in investigating heterogeneous phenomena. The ions studied in this work have the same empirical charge state in solution (plus three), yet the results presented here show a large variation in the binding constants and the adsorption free energies between these ions. Our work not only quantifies this variation in terms of binding constant correlations with size-dependent properties but also suggests differences in metal ion speciation for aluminum versus the lanthanide cations. Specifically, several studies have shown that the lanthanide ions form weak chloride complexes in solution when they are present in 4-14 M chloride solutions (60-63). One EXAFS study by Allen and co-workers reported that chloride complexation decreases with increasing atomic number across the lanthanide series to the point where no chloride complexation is observed in the Yb(III) compound (11). This situation is also applicable to interfaces, albeit at dramatically lower electrolyte concentrations: recently, we reported that Coulomb repulsion at interfaces leads to a situation in which chloride complexation does take place when strontium interacts with fused silica/water interfaces at NaCl concentrations as low as 10 mM (64). In this present work, we find that the binding constants for the lanthanides studied here are considerably lower than that for aluminum, which carries a charge of plus three in solution, but similar in magnitude to the binding constants we recently reported for calcium, strontium, and barium (53, 56), which carry a two plus charge at 10 mM electrolyte concentration. Chloride complexation would serve to decrease the positive charge on the metal adsorbates and therefore lead to a decreased observed binding constant. Similar to our recent report of VOL. 44, NO. 15, 2010 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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salt concentration-dependent speciation changes for strontium (64), lanthanides may thus change their charge state from plus three to plus two via chloride complexation or via acid base chemistry involving surface hydroxyl groups and water molecules. Changes in the charge state n of an adsorbing ion X of bulk concentration Xbulkn+ would have a large impact on electrostatic adsorption and surface complexation models (3) that quantify its absolute surface coverage, Xadsn+, and the corresponding equilibrium adsorption/desorption constant, via the potential at the adsorption plane according to n+ n+ ( -ΨF/RT)n Xads ) Xbulk ·e
(7)
where F is the Faraday constant, and R and T have their usual meanings. For a potential at the adsorption plane of -100 mV, which is typical for fused silica maintained at pH 7 and a 10 mM NaCl solution (64), and room temperature, the Boltzmann term would change by multiple orders of magnitude if n were to change from +3 to +2. Therefore, assuming the charge state of an adsorbed species to be commensurate with that of the bulk has the potential to result in dramatically underestimated absolute surface coverages. This result underscores the necessity to determine absolute surface coverages of metal cations adsorbed at oxide/water interfaces and provides important benchmarks for theory calculations addressing the interactions of metal cations with oxide/aqueous interfaces in geochemistry. Insofar as the experimental conditions model those found in acidic groundwater systems, the quantitative data obtained from this model study and the environmental implications of our findings are valuable for improving the prediction and assessment of trivalent metal cation transport in groundwater and soils.
Acknowledgments This work was supported by the National Science Foundation Environmental Chemical Sciences program under grant no. CHE-0950433, the National Science Foundation Experimental Physical Chemistry program under grant no. CHE-0348873, the Director, Chemical Sciences, Geosciences and Biosciences Division, of the U.S. Department of Energy under grant no. DE-FG02-06ER15787, the International Institute for Nanotechnology at Northwestern University, and an Alfred P. Sloan Fellowship to F.M.G.
Supporting Information Available A sensitivity analysis of the triple layer expression with respect to the C2 parameter, details on the triple layer model, a table of fitting parameters, and a table summarizing the results of this paper. This material is available free of charge via the Internet at http://pubs.acs.org.
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