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Interaction of Aluminum Ions with Fused Silica/Water Interfaces in the Presence of Oxalic Acid Tracked by Second Harmonic Generation David S. Jordan and Franz M. Geiger* Department of Chemistry, Northwestern University, 2145 Sheridan Road, Evanston, Illinois 60208, United States ABSTRACT: Second harmonic generation (SHG) and the χ(3) technique are used to investigate the effect of oxalic acid on the adsorption of aluminum [Al(III)] to the fused silica/water interface. Al(III) adsorption isotherms are measured in the presence and absence of oxalic acid and fit with the Diffuse Layer model to quantify thermodynamic binding parameters. The evolution of charge density throughout the adsorption process is analyzed and used to derive binding mechanisms at the molecular level. It is found that 0.5 mM oxalic acid irreversibly binds to and increases the negative surface charge on the fused silica surface −0.003 C/m2, corresponding to 1.9 × 1012 bound oxalate ions per cm2 under those conditions if oxalate were singly charged at the interface. Oxalic acid decreases the strength of the Al(III) adsorption interaction: the binding constant, Kads, decreases from 60 000 ± 10 000 to 1100 (200) M−1, corresponding to a reduction in the apparent adsorption free energy, ΔGads, from −37.2(4) to −27.3(5) kJ/mol, while surface charge densities at maximum metal coverage, σm, increase from 0.004(2) to 0.014(3) C/m2. Evidence is presented for a change in the dominant charge determining adsorption mechanism that corresponds with a change in Al(III) speciation. These results reveal important information about the surface activity and speciation of Al(III) in the presence of oxalic acid that have direct implications for the mobility, toxicity, and ultimate environmental fate of this metal pollutant.

I. INTRODUCTION Low-molecular-weight organic acids, such as malonic acid, citric acid, and oxalic acid, are important in soils and surface waters1,2 because they can form coordination complexes with metal ions and thus play an important role in biogeochemical cycling of various nutrients or pollutants.3−5 Because the mineral/water interface is difficult to probe directly, surface-specific investigations into how oxalate impacts the interaction of metal ions with a mineral surface have yet to be carried out. Here, we apply nonlinear optics to access the mineral/water interface directly and study how the presence of oxalic acid impacts the number of aluminum ions bound, their speciation, and their free energy of interaction and observe binding constants at fused silica surfaces held under an aqueous solution of pH 4 and 10 mM NaCl. We find that 0.5 mM oxalic acid binds irreversibly to the fused silica surface, leading to an increase in the negative surface charge, but yet decreases the strength of the Al(III) adsorption interaction when compared with bare fused silica/water interfaces at the same pH and ionic strength. Moreover, we find evidence for a change in the dominant charge-determining adsorption mechanism that corresponds to a change in Al(III) speciation. The implications of this result are discussed in the context of the mobility, toxicity, and ultimate environmental fate of Al(III) in heterogeneous geochemical environments. Among low-molecular-weight organic acids in the environment, oxalic acid is one of the most common;2,6,7 it occurs naturally in many plants and has been found to be a major metabolic product of fungi.6,7 Oxalic acid itself contains two carboxylate groups and has been shown to have a high affinity for positively charged ions.8,9 This property has an important impact on processes that involve or are mediated by metal ions, © 2014 American Chemical Society

such as nutrient cycling and the environmental fate of metal pollutants, making oxalic acid an important species to investigate in an environmental context. Oxalic acid has been shown to be an effective chelator of several metal ions, including aluminum.10,11 Aluminum is toxic to plants and animals,12,13 and the extent of aluminum toxicity depends strongly on its speciation.14 Oxalic acid can form complexes with aluminum ions in solution, thus altering the speciation of aluminum as well as its potential mobility through the environment.10 The identity and the properties of these complexes are quite difficult to probe at solid/water interfaces, where environmental interactions often occur, and the thermodynamic adsorption parameters have not been quantified. Here we take a first step toward bridging this knowledge gap. Several studies have investigated the interaction of oxalic acid with various soil and mineral samples in the absence of metal ions.15−19 Vibrational spectroscopy is particularly effective in this area due to the carbonyl stretches that may be probed in situ, yet these studies have not yet been performed to provide information as to the effect of oxalate on the environmental fate and geochemical cycling of metal species. There have been comparatively fewer published studies investigating the binary oxalic acid/metal ion system. Of these, batch chemical extraction is a common method used to elucidate the overall effect of oxalic acid on the adsorption of metal ions by various Special Issue: John C. Hemminger Festschrift Received: October 4, 2013 Revised: January 7, 2014 Published: January 8, 2014 28970

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soils and minerals.11,20,21 Additional techniques, such as the radiotracer method,22 ATR-FTIR,19 and potentiometry and 27 Al NMR10 have been used to gain a more detailed understanding of the speciation and electrostatics involved in the system. While there is some debate as to the mechanism involved, these studies show that, in general, the presence of surface-bound oxalic acid hinders the adsorption of aqueous metal ions. Here we build on these studies with a label-free interfacespecific nonlinear optical technique, namely, second harmonic generation (SHG), that yields concrete data for absolute metal ion surface coverages, the absorption free energy, and the observed binding constants for Al3+ interaction with fused silica/water interfaces at pH 4, 10 mM salt concentration, and various oxalic acid surface coverages. The mobility of aluminum is assessed in the presence of oxalic acid directly at the silica interface and under flowing conditions used to simulate that of groundwater. The electrostatic properties of the system are also evaluated to elucidate possible adsorption mechanisms. This information is important for understanding the toxicity of aluminum as well as its ultimate environmental fate, which is discussed.

be expressed as a function of surface charge density and bulk electrolyte concentration as follows: Φ0 =

⎛ K ads[M] ⎞ σ = σ0 + σm⎜ ⎟ ⎝ 1 + K ads[M] ⎠

(4)

Here Kads is the equilibrium-binding constant observed at a given ionic strength and for the metal ion at bulk concentration [M]. By determining the observed binding constant from eq 4 for a series of ionic strengths, the Coulombic contribution to the free energy of binding can be determined separately from the non-Coulombic, sometimes called “chemical”, free binding energy.34 By combining eqs 2−4, we obtain a model for the SHG signal intensity as a function of metal ion concentration as follows: ⎡⎛ ⎛ K ads[M] ⎞⎞ ESHG = A + B′ arcsinh⎢⎜⎜σ0 + σm⎜ ⎟⎟⎟ ⎢⎣⎝ ⎝ 1 + K ads[M] ⎠⎠

(1)

Here E2ω is the electric field of the second harmonic, P2ω is the polarization of the second harmonic, Eω is the incident electric field at frequency ω, Φ0 is the interfacial potential, and χ(2) and χ(3) are the second- and third-order nonlinear susceptibility tensors, respectively. It has been shown that χ(2) and χ(3) can be treated as constants over the ionic strengths employed in this work.26 Additionally, the incident electric field is held constant. Thus, eq 1 can be simplified as follows ISHG = E2ω ∝ P2ω = A + BΦ0

(3)

In this equation, σ is the net surface charge density, Celec is the concentration of the electrolyte in bulk solution, z is the charge of the screening electrolyte, ε is the permittivity of bulk water, and kb, T, and e are the Boltzmann constant, the temperature, and the elemental charge, respectively.32 The net surface charge density, σ, can be modeled as the sum of the native surface charge of the oxide interface, σ0, and the interfacial charge density due to the adsorbed metal ions of interest at maximum surface coverage, σm, which is scaled by surface coverage assuming Langmuirian adsorption. This approach was first put forth by Salafsky and Eisenthal33 and is shown in the equation below.

II. BACKGROUND AND THEORY SHG is a nonlinear optical phenomenon in which two photons of a fundamental frequency, ω, are combined to create one photon of twice the fundamental frequency, 2ω.23,24 This process occurs where the centrosymmetry of a system is broken and thus is ideal for probing interfaces.25 The incoming laser light induces a nonlinear polarization from the surface and can be described as follows: E2ω ∝ P2ω = χ (2) EωEω + χ (3) EωEω Φ0

⎞ ⎛ 2k bT π ⎟⎟ arcsinh⎜⎜σ ze ⎝ 2εk bTCelec ⎠

⎤ π ⎥ 2εk bTCelec ⎥⎦

(5)

A and B are parameters composed of constants from eqs 2 and 3. We thus arrive at a Diffuse Layer model-derived expression that we apply to the experimentally obtained SHG data to obtain binding constants, interfacial charge densities, and free energies of adsorption at a given ionic strength and pH. Other models describing the electrical double layer may be applicable but are likely to yield results that are indiscernible from those derived from eq 5, at least within thermal energy (i.e., kT). For example, the Triple Layer model, which describes the electrical double layer in terms of three distinct planes, has been used in our previous work. This model yielded identical results to those obtained with the Double Layer expression employed here; therefore, given the increased complexity of the Triple Layer expression, we opted to include the results obtained using the Diffuse Layer model in this work.

(2)

where ISHG is the intensity of the SHG signal and is the measurable. Equations 1 and 2 represent the mathematical foundation of the χ(3) technique and show that the SHG signal intensity is modeled to be directly related to the interfacial potential. Given that the signal detection itself is optical, the χ(3) technique can be thought of as an optical voltmeter.27 The χ(3) technique is appropriate for investigating the interactions of charged analytes with oxide interfaces without the need for electrochemical labels or probes. By applying the appropriate surface complexation model for the interfacial potential, this technique can yield a great deal of concrete information regarding adsorption processes. The Diffuse Layer model was chosen as an analytedependent expression for the interfacial potential. This model has been described in detail elsewhere.28,29 In brief, the surface potential decays exponentially into the bulk according to Gouy−Chapman theory.30,31 The interfacial potential, that is, the electrostatic potential at zero distance from the surface, can

III. EXPERIMENTAL METHODS A. Lens and Solution Preparation. A fused silica hemisphere (ISP Optics) was used for each experiment. Prior to each experiment, the lens was treated for 1 h with NoChromix solution (Godax Laboratories), a commercial glass cleaner. The lens was then submerged in methanol and placed in a sonicator for 6 min, blown dry with a stream of N2, and placed in a 100 °C oven for 30 min. The lens was then 28971

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claiming to have observed oxalic-acid-catalyzed quartz dissolution have shown that the quartz dissolution process depends strongly on solution pH, with no quartz surface dissolution occurring at low pH values.38 It is therefore unlikely that surface dissolution is a significant problem in the experimental system employed here. Oxalic acid has been observed to alter the electrostatic properties of soil and mineral surfaces due to its propensity to form strong surface complexes. Specifically, the presence of oxalic acid has been shown to increase the negative surface charge density along with a decrease in the number of positively charged surface sites of various soil and mineral samples.21,42 This result has been attributed to the formation of stable oxalic acid surface chelates having five-membered ring structures.42 Such a strong surface interaction is not likely to be reversible under isothermal (room temperature) flow conditions, such as those used in this study. Figure 1A shows an adsorption/desorption trace for oxalate adsorbing to fused silica at pH 4 and in 10 mM NaCl. We find

plasma-cleaned (Harrick Plasma) for 30 s before it was submerged under Millipore water until further use. Solutions of Al(III) and oxalic acid were prepared using AlCl3·6H2O (Alfa Aesar, 99.995%) and oxalic acid dihydrate (Sigma Aldrich, ACS Reagent ≥99%), respectively. Each solution was prepared using Millipore water (18.2 MΩ/cm). 10 mM of NaCl (Alfa Aesar, 99+%) was added to each solution to control for ionic strength, unless otherwise noted. The pH of each solution was adjusted using dilute solutions of HCl (E.M.D., ACS grade) and NaOH (Sigma Aldrich, 99.99%). B. Laser System. A detailed description of the laser system used in this study can be found elsewhere.35−37 In brief, SHG studies were performed using a Ti:sapphire laser system (Hurricane, Spectra-Physics, 1 kHz repetition rate, 120 fs pulse) pumping an optical parametric amplifier (OPA-CF, Spectra-Physics) tuned to a fundamental frequency of 600 ± 5 nm. The beam was directed through a variable density filter and the power was attenuated to 0.4 ± 0.05 μJ/pulse, far below the level at which optical breakdown of the surface occurs. The beam was focused onto the interface between the oxide of interest and a flowing aqueous phase at an angle of 60° from surface normal. The second-harmonic signal generated from this interface was isolated from the reflected fundamental beam using a UV-grade Schott filter and a monochromator set to 2ω. The second-harmonic signal was sent through a photomultiplier tube, amplified, and collected using a gated singlephoton detection system. All experiments were performed under flow conditions (∼1 mL/s) using our previously published flow system. The flow system and flow cell were flushed with 1 L of pure Millipore water before and after each experiment. Correct power dependencies and spectral responses of the SHG signal were verified regularly in between runs. C. X-ray Photoelectron Spectroscopy (XPS). XPS experiments were performed using a Thermo Scientific ESCALAB 250Xi spectrometer equipped with a monochromatic Al Kα X-ray source and flood gun for charge compensation. Binding energies were corrected by reference to the C1s peak at 284.5 eV. A fused silica window (ISP Optics) was cleaned using the procedure previously described in Section III.A. All solutions used in the following procedure were held at pH 4 and contain 10 mM NaCl. The window was clamped atop a custom built Teflon flow cell and exposed to a flowing salt solution (∼1 mL/s) for several minutes. The sample was blown dry with N2, and XPS spectra were collected. Next, the sample was again clamped atop a custom-built Teflon flow cell. A salt solution was flowed across the surface for several minutes. The sample was then exposed to a 0.1 mM oxalic acid solution for 10 min (1 mL/s flow rate), followed by a salt solution for several minutes. The sample was blown dry with N2, and XPS data were collected.

Figure 1. (A) SHG signal intensity (raw, not normalized) versus time trace for 0.1 mM oxalic acid binding to the fused silica/water interface at pH 4 and in 10 mM NaCl. (B) XPS spectra of the C1s region taken of a fused silica window after plasma cleaning (black), after exposure to a 10 mM NaCl solution at pH 4 (red), and after exposure to a pH 4 solution of 0.5 mM oxalic acid and 10 mM NaCl, which was followed by a salt solution rinse (blue).

IV. RESULTS AND DISCUSSION A. Interaction Between Oxalic Acid and the Fused Silica Surface. To assess the effect of oxalic acid on the adsorption of Al(III) to fused silica, we measured Al(III) adsorption isotherms in the presence of a constant concentration of oxalic acid. Oxalic acid is a strong chelator of positively charged ions and as such has been observed to form strong, inner sphere complexes with mineral surfaces.39−43 Several studies have observed enhanced quartz dissolution in the presence of oxalic acid;38,39 however, these claims have been contested by subsequent studies.40,41 Additionally, studies

that the observed SHG signal intensity decreases upon oxalate adsorption. In general, inorganic anions adsorbing to negatively charged fused silica substrates show SHG signal increases, which are attributable to a more negative interfacial potential upon anion adsorption (viz. eq 1) as well as the notion that the SHG active water molecules do not change their net overall orientation upon anion adsorption. The observation of an SHG signal intensity decrease upon oxalate adsorption to the 28972

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negatively charged fused silica surface can be attributable to the strength of the oxalate−silica interaction, which is likely to lead to a disruption of the aligned water molecules that contribute to the SHG signal. In fact, the oxalate−silica interaction is so strong that it leads to irreversible adsorption, as seen in Figure 1A, which shows no SHG signal intensity recovery after flushing the system with plain electrolyte solution. The dipolar array of SHG-active water molecules is probably severely disrupted upon oxalate binding to silica, decreasing the magnitude of the χ(3) term in eq 1 and causing a decrease in the SHG signal intensity upon oxalate adsorption, contrary to what is observed when an anion adsorbs reversibly to the surface. Moreover, given that the χ (2) term contains fundamental physical constants related to the outermost fused silica surface sites and given that the fused silica surface is altered upon irreversible oxalic acid adsorption, a change in the χ(2) term of eq 2 can be expected. Such a change could then contribute the observed decrease in SHG signal intensity upon oxalic acid adsorption. Nevertheless, given that all experiments here are carried out with a constant background concentration of oxalic acid (0.5 mM), we treat the χ(2) term of the fused silica−oxalate system, if it is indeed different from that of the plain fused silica system, as a constant in the subsequent salt screening or Al(III) isotherm experiments. XPS was used to provide further evidence of irreversible adsorption of oxalic acid onto fused silica (Figure 1B). This Figure shows the C1s region collected from a fused silica sample before and after exposure to a 0.1 mM solution of oxalic acid at pH 4 and in 10 mM NaCl. A large peak is present at 285 eV after the sample was exposed to a solution of oxalic acid and rinsed with the background electrolyte solution, indicating that a carbon-containing molecule, most likely oxalic acid, adsorbed irreversibly to the silica surface. Furthermore, the prominent peak centered around 285 eV is consistent with the presence of a C−C moiety, while the large shoulder toward higher binding energies is consistent with the presence of C−O groups on the surface, both of which are present in the structure of oxalic acid.43 A charge screening experiment was conducted with a constant oxalic acid concentration of 0.5 mM to determine if the surface charge density was altered due to the presence of oxalic acid. This experiment was carried out at pH 4 using the established salt screening protocol outlined in previous work.35,44 In this experiment, increasing amounts of NaCl were added to screen out the charge present at the fused silica interface, which, according to eq 1, should result in an SHG signal intensity decrease. Figure 2 shows that this observation is indeed made. The data were fit by combining eqs 2 and 3 to yield the fused silica surface charge density in the presence of 0.5 mM oxalic acid. Assuming the surface charge density of fused silica substrate remains constant, the resulting surface charge was found to be −0.007(2) C/m2, which is slightly more negative than the neat fused silica surface charge density of −0.004(1) C/m2 at pH 4.44 Regarding our notation, the number in parentheses is the standard error of the point estimate. This result demonstrates that oxalic acid alters the electrostatic properties of that surface. If oxalate were singly negatively charged, then the excess negative charge density of −0.003 C/m2 would correspond to 1.9 × 1012 bound oxalate ions per cm2 and half that if oxalate were doubly deprotonated. The surface charge density value of −0.007(2) C/m2 will be used to fit the Al(III)/oxalic acid adsorption data presented in the following section.

Figure 2. Charge screening isotherm performed at the fused silica/ water interface at pH 4 and with a constant oxalic acid concentration of 0.5 mM. The blue line represents the fit of the Gouy−Chapman model to the data.

B. Co-Adsorption of Al(III) and Oxalic Acid to the Fused Silica Interface. To investigate the effect of oxalic acid on the adsorption of Al(III) ions to the fused silica interface, SHG and the χ(3) technique were used to measure an Al(III) adsorption isotherm in the presence of 0.5 mM of oxalic acid and 10 mM NaCl at pH 4. Oxalic acid and NaCl were present in both the background solution and in the Al(III) solution. Only the concentration of Al(III) was varied, and the SHG signal intensity was collected. The presence of a constant concentration of 10 mM NaCl ensures that any changes in surface potential, and thus SHG signal intensity, are due only to the presence of adsorbed Al(III) ions and not due to screening of the surface charge from a change in overall ionic strength of the solution. Detailed experimental procedures used to collect SHG adsorption isotherms can be found in previous publications.35,44,45 An example adsorption/desorption trace is shown in the inset of Figure 3. This Figure shows that the SHG signal intensity decreases with the introduction of the Al(III) species. This result can be understood as follows: the oxalicacid-saturated fused silica surface possesses a slight negative charge under the employed experimental conditions. The presence of positively charged species decreases the magnitude of the surface potential, thus decreasing the SHG signal intensity, consistent with the χ(3) effect (eq 1). The SHG signal intensity then increases back to the baseline level after the metal species is flushed from the interface, indicating that the adsorption event was fully reversible. Full reversibility regarding aluminum interaction was observed at every Al(III) concentration. An adsorption isotherm was recorded by collecting adsorption/desorption traces for a range of Al(III) concentrations. Figure 3 shows the adsorption isotherm for Al(III) binding to the fused silica surface in 0.5 mM oxalic acid and 10 mM NaCl at pH 4. The data were fit using the Diffuse Layer model, which is shown in eq 5. From this fit, an apparent binding constant, Kads, was found to be 1100 (200) M−1, which corresponds to an apparent adsorption free energy, ΔGads of −27.3(5) kJ/mol, and a surface charge density at maximum metal coverage, σm, was found to be 0.014(3) C/m2. Comparatively, Kads, ΔGads, and σm for Al(III) adsorbing to 28973

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adsorption free energy between Al(III) and the fused silica surface at pH 4 and 10 mM NaCl. This reduction occurs despite the increased negative surface charge due to the presence of surface-bound oxalic acid molecules. To gain further insight into the adsorption mechanism(s) operative in the Al(III)/oxalic acid system, several salt-screening experiments were performed to assess the evolution of surface charge throughout the Al(III) adsorption process. Salt-screening experiments were performed at 20 and 50% of Al(III) saturation coverage, corresponding to a bulk Al(III) concentration of 0.4 and 0.9 mM, respectively. Each experiment was performed with a constant oxalic acid concentration of 0.5 mM, and the resulting adsorption isotherms are shown in Figure 4. These salt-screening isotherms were fit as previously described to yield an interfacial charge density of −0.005(1) C/ m2 for the system with 20% Al(III) coverage, while a value of +0.006(2) C/m2 was derived for the system with 50% Al(III) surface coverage. Note that the surface charge density at 100% surface saturation was found to be 0.014(3) C/m2 from the fit of the data shown in Figure 3. The two plots in Figure 4 show that the SHG signal intensity either decreases or increases in the presence of constant oxalic acid concentration and increasing NaCl concentration, depending on Al(III) surface coverage and speciation. This observed inversion of the change in the SHG signal intensity with increasing salt concentration is attributable to the direction of the alignment of the dipolar arrays of water molecules at the solid/liquid interface, which flips when the overall charge state changes from negative (−0.005(1) C/m2 for the system with 20% Al(III) coverage) to positive (+0.006(2) C/m2 for the system with 50% Al(III) surface coverage). These various scenarios can be described conceptually in terms of constructive or destructive interference between the χ(2) and χ(3) terms (see eq 2), following Higgins et al.46 When the surface is negatively charged, the χ(2) and χ(3) terms constructively interfere to give the total SHG signal. When an anion adsorbs to the negative surface, more water dipoles are aligned and χ(3) increases, increasing the SHG signal intensity as a result. When a cation adsorbs to the negatively charged surface, the water layer is disrupted by the presence of the opposing charge. The χ(3) term decreases in magnitude and the

Figure 3. Adsorption isotherm for Al(III) adsorbing to the fused silica/water interface at pH 4 and in the presence of 0.5 mM oxalic acid and 10 mM NaCl. The blue line represents the fit of the Diffuse Layer model to the data (eq 5). Inset: Adsorption/desorption trace for Al(III) adsorbing to the fused silica/water interface at pH 4 and in the presence of 0.5 mM oxalic acid and 10 mM NaCl (note: count values are not normalized).

the fused silica/water interface at pH 4 and in 10 mM NaCl with no oxalic acid present were found to be 60 000 ± 10 000 M−1, −37.2(4) kJ/mol, and 0.004(2) C/m2 (with σ0 = −0.004(1) C/m2 in this system), respectively.44 It should be noted that there are several assumptions involved in using the Langmuir equation to derive the binding parameters for the Al(III)/oxalic acid system (see Section II). However, given the low concentration regime used in this work, these assumptions are valid in our system with the exception of a uniform adsorption mechanism. On the basis of the discussion in the following sections, we note that the derived quantified binding parameters represent an average of multiple Al(III) binding modes. C. Possible Al(III)/Oxalic Acid Binding Modes. From the previous discussion, it can be seen that the presence of 0.5 mM oxalic acid in solution greatly reduces the magnitude of the

Figure 4. Charge screening isotherms measured in the presence of (A) 0.4 mM Al(III) and 0.5 mM oxalic acid and (B) 0.9 mM Al(III) and 0.5 mM oxalic acid. The blue line represents the fit of the Gouy−Chapman model to the data. 28974

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At a 0.9 mM bulk Al(III) concentration, or 50% surface coverage, the free Al3+ ion and the AlOxalate+ ion are equal in concentration. The surface charge density changes from −0.005(1) C/m2 for 20% Al(III) surface coverage to +0.006(2) C/m2 for 50% Al(III) surface coverage, that is, by more than the factor by which the surface coverage changes. This result is not surprising given the changes in bulk speciation over this concentration range with the increasing amount of free Al3+ ions in solution discussed in the previous paragraph. Conversely, the surface charge doubles from 50% surface coverage to 100% coverage (the value at 100% coverage is 0.014(3), derived from the fit shown in Figure 3), which indicates that the dominant charge-determining surface mechanism does not change from 50 to 100% surface coverage. Given that the bulk concentration of the free Al3+ ion is the dominant species in solution over this concentration range and that the bulk concentration of the AlOxalate+ ion remains constant over this same range, the adsorption mechanism dominant in the 50 to 100% surface coverage regime most likely involves the hydrated Al3+ ion. The surface charge density at 100% surface coverage, 0.014(3) C/m2, is greater in magnitude than the initial surface charge density, −0.007(2) C/m2. This result indicates that adsorption is not purely electrostatic in nature and that a specific, chemical interaction is taking place for surface coverages ranging from at least 50% surface coverage until surface saturation.49 Given the abundance of free Al3+ ions in solution and the available oxalic acid molecules on the surface of the fused silica, this chemical adsorption mechanism most likely involves the reversible formation of an AlOxalate+ surface complex. The reaction free energy for the formation of an Al(H2O)4Oxalate+ complex from the fully hydrated Al(H2O)63+ ion in water was calculated to be −28.9 kJ/mol by Aquino and coworkers.50 This free-energy value corresponds well with adsorption free-energy value obtained from the measured adsorption isotherm shown in Figure 3, indicating that this is a plausible adsorption mechanism when the Al3+ ion is the dominant species in solution. This non-Coulombic mechanism is only operative in the presence of oxalic acid; the adsorption of Al(III) to bare fused silica was determined to be purely electrostatic,44 further underscoring the dramatic effect of oxalic acid on Al(III) adsorption to fused silica.

overall SHG signal decreases (Figure 4A). Conversely, when the surface is positively charged, the water molecules rotate by 180°47,48 and the χ(2) and χ(3) terms destructively interfere with each other. When an anion adsorbs to this positively charged surface, the interfacial water layer once again becomes more disordered, decreasing the magnitude of the χ(3) term. This results in less destructive interference and the SHG signal increases. (See Figure 4B.) When a cation adsorbs to this net positively charged surface, the interfacial water layers become more ordered, increasing the magnitude of the χ(3) term and thus increasing the deconstructive interference between the χ(2) and χ(3) terms, and the SHG signal decreases. To elucidate the possible adsorption mechanisms operative in this system, the bulk speciation of the Al(III)/oxalic acid system was calculated and is shown Figure 5. Vertical lines are

Figure 5. Bulk Al(III) speciation calculated in the presence of 0.5 mM oxalic acid and 10 mM NaCl using ChemEQL.53 Formation constants were adapted from Sjoberg and Ohman.10 Negatively charged species and species in with low concentrations have been omitted for clarity. Vertical lines are included to indicate the speciation at varying degrees of Al(III) saturation surface coverage.

V. CONCLUSIONS AND ENVIRONMENTAL IMPLICATIONS This study marks the first surface specific investigation into the interaction of oxalic acid and Al(III) to the fused silica interface. SHG was used to assess the impact of oxalic acid on the adsorption and speciation of Al(III) at the fused silica/water interface under slightly acidic conditions. We find evidence of a strong surface interaction between oxalic acid and the fused silica surface under slightly acidic conditions where the oxalate ion irreversibly binds to the interface. This finding has large implications for surface complexation models that rely on fundamental surface charge parameters to accurately predict adsorption behavior.28,29 We show that the presence of 0.5 mM oxalic acid reduces the binding constant by over an order of magnitude compared with Al(III) adsorption without oxalic acid under the same conditions, corresponding to a large increase in the mobility of Al(III) throughout the environment in the presence of oxalic acid. Moreover, a series of chargescreening experiments were performed to assess the surface speciation of Al(III) under varying degrees of Al(III) surface

included to indicate the speciation at 20, 50, and 100% Al(III) saturation coverage studied in our SHG experiments. We interpret the charge flipping when going from 20 to 50% Al(III) surface coverage as follows: according to Figure 5, at a 0.4 mM bulk Al(III) concentration, or 20% surface coverage, AlOxalate+ is the dominant species in solution. Given that the surface charge becomes slightly more positive in the presence of this species (−0.005(1) C/m2 at 20% Al(III) saturation coverage compared with −0.007(2) C/m2 with no Al(III) present), we may conclude that the AlOxalate+ ion adsorbs to the surface in such a way that the positive charge is retained. Therefore, it is unlikely that the AlOxalate+ ion forms a complex with surface-immobilized oxalic acid groups, which would form a negatively charged surface species, such as Al(Oxalate)2−. The AlOxalate+ species most likely adsorbs to the surface through a Coloumbic adsorption mechanism or through a hydrogen-bonding mechanism in which the charge of the adsorbing ion remains positive. 28975

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loading. Our data are consistent with the notion that, initially, Al(III) adsorbs to the fused silica surface as the AlOxalate+ cation through an electrostatic mechanism. AlOxalate+ is the relevant species for adsorption until the concentration of the free Al3+ ion becomes equal to that of the AlOxalate+ ion, at which point the Al3+ adsorbs through a chemically specific mechanism with surface-immobilized oxalate molecules. These findings have broad environmental implications in that the presence of oxalic acid has be shown to increase the mobility of Al(III) through groundwater and soils. The Kd model, a model used by regulatory agencies to quantify changes in pollutant transport,51 was employed to assess the differences in the environmental mobility of the two pollutant systems. Specifically, Kd values were calculated and found to be 1.32(5) × 10−4 mL/g for the Al(III) system and 2.78(4) × 10−5 mL/g for the Al(III)/oxalic acid system, a reduction in Al(III) mobility of an order of magnitude imparted by the presence of oxalic acid. In addition, we find that oxalic acid changes the interfacial speciation of Al(III), thus altering the toxicity of Al(III) under the given experimental conditions. This finding complements previous work that has shown a decrease in Al(III) toxicity in plants in the presence of oxalic acids.4 This change in interfacial speciation also has important implications for surface complexation models that explicitly rely on the charge state of the ion at the interface to accurately predict pollutant transport and assess the risk posed by a given pollutant.29,31 Given the prevalence of oxalic acid and other low-molecular-weight organic acids in the environment and as components in more complex humic acids,52 the results of this SHG study provide important surface-specific information as to how these chemical species can influence the mobility, toxicity, and overall environmental fate of various toxic metal pollutants.

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AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Sarah A. Saslow Gomez for the collection of XPS data, which was completed at the Keck Interdisciplinary Surface Science Center(Keck II) of Northwestern University. This work was supported by the National Science Foundation Environmental Chemical Sciences program under grant no. CHE-0950433. We also acknowledge the International Institute for Nanotechnology (IIN) at Northwestern University for capital equipment support. We acknowledge Spectra-Physics Lasers, a division of Newport Corporation, for equipment support. The ICP-AES analysis was completed at the Northwestern University Integrated Molecular Structure Education and Research Center (IMSERC).

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