Adsorption Mechanisms of EDTA at the WaterâIron Oxide Interface

7762

J. Phys. Chem. C 2009, 113, 7762–7771

Adsorption Mechanisms of EDTA at the Water-Iron Oxide Interface: Implications for Dissolution Katarina Noreń,† John S. Loring,† John R. Bargar,‡ and Per Persson*,† Department of Chemistry, Umeå UniVersity, SE-901 87 Umeå, Sweden, and Stanford Synchrotron Radiation Laboratory, P.O. Box 4349, Stanford, California 94309 ReceiVed: October 17, 2008; ReVised Manuscript ReceiVed: January 22, 2009

The interactions between chelating agents and metal oxide particles play important roles for the distribution and availability of metal ions in aquatic environments. In this work, the adsorption of ethylenediaminetetraacetate (EDTA) onto goethite (R-FeOOH) was studied as a function of pH, time, and background electrolyte concentration at 25.0 °C, and the molecular structures of the surface complexes formed were analyzed by means of infrared spectroscopy using the attenuated total reflectance sampling technique. The collective infrared spectroscopic results of this study show that two surface complexes consisting of HEDTA3- and H2EDTA2predominate at the water-goethite interface within the pH range of 3-9. No direct interactions of these complexes with surface Fe(III) ions were detected; hence, most likely the surface complexes are stabilized at the interface by electrostatic and hydrogen-bonding forces. The formation of the EDTA surface complexes is fast (time scale of minutes), but a slower (time scale of hours to days) dissolution reaction also occurs. The dissolved iron in solution is in the form of the highly stable FeEDTA- solution complex, and the experimental evidence presented indicates that this complex can readsorb to the mineral surface. As dissolution proceeds, the concentration of FeEDTA- in the solution phase increases, and this in turn leads to a buildup of readsorbed FeEDTA- onto goethite. In the pH range of 4-7, this dissolution and readsorption process increases the total EDTA concentration at the surface. Under the experimental conditions in the present study, it is primarily the presence of uncomplexed EDTA in solution that drives the dissolution of goethite resulting in the subsequent readsorption of FeEDTA-, while the HEDTA3- and H2EDTA2- surface complexes are stable during this process. 1. Introduction Ethylenediaminetetraacetate, EDTA, belongs to a group of anthropogenic aminocarboxylate chelating agents widely used in industrial processes. Considerable amounts of EDTA have been found in aquatic environments as a result of incomplete biodegradation,1,2 as well as insufficient photodegradation of Fe(III)EDTA.3,4 Uncomplexed EDTA exhibits relatively low toxicity to aquatic organisms and is not likely to accumulate in the food chain.5 However, the strong complex formation of EDTA with metal ions is prone to alter metal speciation, and EDTA has been found to predominately exist as a metalcomplexed species in aquifers.6 Furthermore, EDTA has been shown to readily adsorb onto mineral particles and may also cause mineral dissolution.7-15 Thus, EDTA likely affects the mobility and bioavailability of harmful and benign metal ions in aquatic environments. Adsorption of uncomplexed EDTA to mineral surfaces has been studied extensively. A quantitative model for the adsorption of EDTA onto hydrous γ-Al2O3 was proposed by Bowers and Huang,7 and EDTA was assumed to bind to the surface only through strong hydrogen-bonding interactions. A model for the adsorption of EDTA to hematite was reported by Chang et al. where both electrostatic and specific interactions were assumed between EDTA and the surface.8 However, these authors were vague about the type of specific surface interaction involved, * Corresponding author. Tel.: +46-90-786 55 73. E-mail: Per.Persson@ chem.umu.se. † Umeå University. ‡ Stanford Synchrotron Radiation Laboratory.

i.e., whether the complexes were inner-sphere (directly coordinated to surface metal ions) or hydrogen-bonded outer-sphere. Only inner-sphere complexes were used to model the adsorption of EDTA to other iron (hydr)oxides such as β-FeOOH (akaganeite), Fe3O4 (magnetite), and R-FeOOH (goethite).9-12 Recently, EDTA was suggested to form predominantly Hbonded surface complexes on amorphous Cr(III) hydroxide nanoparticles.13 In several articles EDTA has also been shown to affect dissolution of iron (hydr)oxides. However, different mechanisms have been proposed in order to explain the dissolution processes.9-15 The objectives of the present work were to investigate the molecular speciation and coordination chemistry of EDTA at the water-goethite interface and to use this information to further the understanding of ligand-assisted dissolution reactions. This was accomplished by studying the EDTA adsorption as a function of time, pH, and ionic strength in combination with infrared spectroscopic measurements of samples prepared in H2O and D2O. 2. Experimental Section 2.1. Chemicals, Solutions, and Suspensions. Experiments were performed at 25 °C in both H2O (deionized Milli-Q Plus, boiled to remove dissolved CO2) and D2O (Aldrich, 99.9 atom %). NaCl (Merck, p.a.) dried at 180 °C was used to provide a constant ionic medium of either 0.01 or 0.1 M (Na)Cl. The parentheses around Na indicates that the sodium ion concentration varied while the chloride ion concentration was held constant. pH adjustments were made with standardized NaOH,

10.1021/jp809190m CCC: $40.75  2009 American Chemical Society Published on Web 04/09/2009

EDTA Adsorption at the Water-Iron Oxide Interface HCl, or DCl solutions. Stock ligand solutions were prepared by dissolving weighed amounts of dried (80 °C) Na2H2EDTA (Merck) and Na4EDTA (Sigma) in H2O and D2O, respectively. Goethite (R-FeOOH) was synthesized in polyethylene bottles by adding 2.5 M KOH (EKA, p.a.) to 10 L of 0.15 M Fe(NO3)3 (Merck, p.a.) at a rate of 10 mL/min. The precipitates were aged for 96 h at 60 °C and dialyzed for 3 weeks.16 The resulting particles were identified to be goethite by X-ray powder diffraction, and the surface area was determined to be 94 m2/g using N2 BET analysis. Goethite suspensions were prepared at three different sodium chloride concentrations (0, 0.01, and 0.1 M (Na)Cl). Suspensions in D2O were obtained by resuspending goethite dried at 90 °C in D2O and equilibrating for 24 h. Proton adsorption/desorption equilibria for goethite are reached within 24 h, as shown with potentiometric titrations by Boily et al.16 Hence, a 24 h pre-equilibration time in D2O is sufficient for the surface protons to exchange with deuterium. 2.2. Adsorption Experiments. Adsorption experiments were carried out in batch mode in the absence of light to avoid reductive dissolution of goethite and photodegradation of EDTA. Batch samples covered the pH range of 3-10 and were prepared at background electrolyte concentrations of 0, 0.01, or 0.1 M (Na)Cl. Both quantitative adsorption data and infrared spectra were collected for experiments in goethite suspensions in H2O. For experiments conducted in D2O only infrared spectra were obtained, at an ionic strength of 0.1 M (Na)Cl. Stock suspensions of goethite that were used for batch sample preparation were acidified to pH ∼ 5 and purged overnight with N2(g). Each batch sample was prepared by transferring an aliquot of a stock goethite suspension to a 15 mL polypropylene centrifuge tube wrapped in aluminum foil, adding a volume of freshly prepared stock ligand solution, and adjusting the pH to a value between 3 and 10 using standardized acid or base. All samples were diluted to total ligand concentrations of 1.0 µmol/m2 (0.94 mM), 2.0 µmol/m2 (1.88 mM), or 3.0 µmol/m2 (2.82 mM), and the goethite concentration was 10 g/L. During batch sample preparation, the centrifuge tubes were continuously purged with N2(g) to avoid carbonate contamination. After an equilibration time of 10 min, 24 h, 7 days, 12 days, or 21 days at 25 °C on an end-over-end rotator, the pH of each batch sample was measured with a combination electrode (Orion) that was calibrated with commercial buffers (Merck). The outer reference cell of this electrode was filled with 0.01 M NaCl for experiments in 0 and 0.01 M ionic medium, and with 0.1 M NaCl for experiments in 0.1 M ionic medium. For measurements conducted in D2O, pD was calculated by adding 0.4 to the reading from the pH meter.17 After the pH measurement, the samples were centrifuged for 20 min at a relative centrifugal force (rcf) of 3240g, and the supernatant was filtered through a 0.22 µm Millipore filter. The amount of adsorbed EDTA was determined by measuring the concentration of the ligand remaining in the supernatant and subtracting this value from the total ligand concentration. EDTA in the supernatant was analyzed as the Fe(III)EDTA complex using liquid chromatography (Metrohm Ltd.). The stationary phase was a DuPont Zorbax Phenyl (250 mm × 4.6 mm) column with 5 µm particles (Agilent). The mobile phase was an aqueous solution of 0.8 g/L Fe(NO3)3 · 9H2O, 1.5 mL/L HNO3 (65% w/w), and 0.04% (w/w) tetrabutylammonium nitrate. The Fe(III)EDTA complex was detected by UV-vis at λmax ) 320 nm. All adsorption experiments were conducted twice to check for reproducibility. The concentration of EDTA in the supernatant solutions was obtained from the calibration curves created from standard solutions with ligand concentration varying

J. Phys. Chem. C, Vol. 113, No. 18, 2009 7763

Figure 1. Concentration of EDTA in aqueous solution as a function of pH, at [EDTA]tot ) 25 mM and I ) 0.1 M. FEDTA denotes the fraction of the total EDTA species in aqueous solution.

Figure 2. Infrared spectra of aqueous EDTA at pH (a) 2.92, (b) 3.47, (c) 4.46, (d) 5.93, (e) 6.88, (f) 7.75, (g) 8.87, (h) 9.50, (i) 10.04, (j) 12.15, a total EDTA concentration of 0.025 M, and I ) 0.1 M (Na)Cl.

between 0.2 and 1.9 mM. We estimate from calibration curves that the errors in the unknown EDTA concentrations are better than (0.008 mM. The supernatants from the batch samples were also analyzed for dissolved iron. Prior to analysis, the samples were acidified to pH below 2 with concentrated HCl (analytical grade). The total iron content was measured in triplicate using flame atomic absorption spectrometry (Perkin-Elmer AAS 3110). In samples equilibrated for 10 min, the iron concentration was at or below the detection limit of 2 µM, indicating that no significant mineral dissolution had occurred. The amount of iron dissolved in the samples with longer equilibration times is reported below. The infrared (IR) spectra of the wet mineral pastes from the batch samples were recorded using attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy. Spectra

7764

J. Phys. Chem. C, Vol. 113, No. 18, 2009

Noreń et al.

TABLE 1: Tentative Assignments of the Main Experimental IR Frequencies of EDTA in H2O and D2O (within Parentheses) in the Frequency Region of 1200-1800 cm-1 H2EDTA2assignmenta νCdO νC-Oas + δNH νC-Oas + δCH2 νC-Os + δCH2 + δNH νC-Os + νC-C + δCH2 δCH2 + δNH νC-Os + νC-C + δCH2 δCH2 + δNH νC-N + δCH2 νC-OH + δC-O-H

H3EDTA-

exptl

theor

EDTA4HEDTA3-

1695 1618

1618 (1626) 1727-1701 (five peaks) 1618 (1626) 1572 (1583)

1386

1402 (1404) 1403-1372 (five peaks) 1402 (1404) 1357

1317

exptl

theor

1572 (1583)

1634-1605 (four peaks)

1407 (1406)

1418-1397 (four peaks)

1333 (1333)

1342-1317 (three peaks)

1356

1323 (1328) 1336

1320 (1305) 1285, 1258 (1286, 1260) 1280-1259 (three peaks)

1254, 1233, 1212

a

Assignments of the experimental peaks from the ab initio calculations performed herein and from previous works on EDTA and by comparison with similar molecules (refs 22-28). Note that detailed assignments of the bending modes are not made; thus scissoring, wagging, rocking are collectively denoted δ modes.

Figure 5. Total amount of EDTA adsorbed on goethite as a function of pH at background electrolyte concentrations of (b) 0, (2) 0.01, and (f) 0.1 M (Na)Cl, and at a total ligand concentration of 2.0 µmol/m2.

Figure 3. Infrared spectra and the corresponding Fourier selfdeconvoluted spectra of EDTA species in D2O for (a) D2EDTA2-, (b) DEDTA-, and (c) EDTA4- at a total EDTA concentration of 0.025 M, and I ) 0.1 M (Na)Cl.

Figure 4. Total amount of EDTA adsorbed on goethite as a function of pH at total ligand concentrations of (2) 1.0, (f) 2.0, and (O) 3.0 µmol/m2, in 0.1 M (Na)Cl.

were collected with a Bruker IFS 66v/S spectrometer equipped with a deuterated triglycine sulfate (DTGS) detector and a watercooled globar source. All measurements were performed under vacuum conditions, thereby diminishing contributions from H2O(g) and CO2(g). A horizontal ATR accessory was used with a diamond/KRS5 internal reflection element with nine reflections (SensIR Technologies). The angle of incidence for this ATR

Figure 6. Infrared spectra of EDTA adsorbed onto goethite at pH (a) 3.09, (b) 3.57, (c) 3.99, (d) 4.43, (e) 4.98, (f) 5.98, (g) 6.42, (h) 7.53, and (i) 8.43. The samples were prepared as H2O suspensions in batch mode at a total EDTA concentration of 2.0 µmol/m2 and I ) 0.1 M (Na)Cl.

cell is ∼45°. A plastic lid with a rubber gasket was pressed tightly against the ATR cell to protect the sample from the vacuum conditions of the spectrometer. This lid was not in direct contact with and exerted no extra pressure on the sample. For each batch sample, a background spectrum of the empty ATR cell was collected, and then absorbance spectra were measured of both the filtered supernatant solution and the wet mineral paste. Spectra were an average of 500 scans that were

EDTA Adsorption at the Water-Iron Oxide Interface

Figure 7. IR spectra of EDTA adsorbed on goethite at pH (a) 2.93, (b) 3.12, (c) 3.47, (d) 3.98, (e) 4.48, (f) 5.10, (g) 5.55, (h) 5.91, (i) 6.22, (j) 6 54, (k) 6.88, (l) 7.80, (m) 8.86, and (n) 9.77 at a total EDTA concentration of 2.0 µmol/m2, and I ) 0.1 M (Na)Cl. These spectra were collected during a simultaneous infrared and potentiometric titration.

collected at a resolution of 4 cm-1. The absorbance spectrum of a wet mineral paste of a batch sample includes contributions from adsorbed ligands, ligands in solution, bulk water (ionic medium), and goethite. We used the following procedure to isolate the spectrum of the adsorbed ligands. First, the absorbance spectrum of the batch sample’s supernatant was subtracted from the corresponding wet mineral paste spectrum in order to remove the contributions from bulk water and ligands in solution. The critical step was to correctly remove the water peak at 1638 cm-1, originating from bending mode of bulk water. Second, a spectrum of goethite was used to subtract away the infrared peaks of the mineral. The spectrum used for this subtraction was that of a wet mineral paste of goethite that already had the contributions from bulk water removed. This goethite was at the same pH and ionic strength as for the batch sample. Both of these subtractions were achieved by using subtraction factor that varied between 0.96 and 1.0. IR spectra were obtained of suspensions prepared in D2O as well as H2O. As will be shown, the spectra in D2O have superior signal-to-noise ratios in comparison with the spectra of the aqueous suspensions. This effect is primarily ascribed to the shift of the water bending mode to lower frequency, at around 1200 cm-1, which causes more light to reach the detector in the carboxylate stretching frequency region. Fourier self-deconvolution was applied to some IR spectra in order to enhance their apparent peak resolution. This procedure assumes that the spectra consist of peaks that have been broadened by the same type of peak-broadening function. These peaks are resolved by multiplying the measured interferogram with a deconvolution function for a specific peak shape. In this study, a Lorentzian peak shape was assumed.

J. Phys. Chem. C, Vol. 113, No. 18, 2009 7765 2.3. Simultaneous Infrared Spectroscopy and Potentiometric Titration. The experiment was performed in the absence of light to avoid reductive dissolution of goethite and photoreduction of EDTA. A goethite suspension (10 g/L) was blanketed with a nitrogen atmosphere in a titration vessel thermostatted at 25.00 ( 0.05 °C. It was pumped peristaltically in a closed loop through fluoroelastomer (Chemsure, Gore Industries) and PTFE tubing from the titration vessel to a flow-through ATR cell inside an evacuated infrared spectrometer (Bruker IFS-66v/s with a DTGS detector in a thermostatted (0.15 °C room). The flow-through attachment was custom built of inert materials (e.g., Pyrex glass, PEEK, PTFE) and was mounted on a singlereflection 45° ATR accessory (FastIR, Harrick Scientific) equipped with a ZnSe internal reflection element (IRE). The volume of the flow-through cell was approximately 1.5 cm3, and mixing was facilitated within the cell by a propeller stirrer that was fixed above the IRE. Prior to the titration, a mineral overlayer was deposited onto the IRE by evaporating 0.7 mL of an approximately 2 g/L goethite suspension onto the crystal at 75 °C for 2.5 h under nitrogen. During the experiment, the goethite suspension was flowed above this overlayer. First, a background single-beam spectrum (4096 scans, 4 cm-1 resolution) was collected of the overlayer and goethite suspension at pH ) 9.4, which is at the point of zero charge of the mineral. Second, the suspension was titrated up to about pH ) 10.5, and a volume of stock EDTA solution was added so that the total concentration of EDTA was 2.0 µmol/m2. Finally, the EDTA-goethite suspension was titrated in the acidic direction by an automated and computer-controlled system. After each addition of titrant, the solution was equilibrated for 10 min. Sample single-beam spectra were the average of 256 scans, and the absorbance spectra presented here are negative decadent logarithm of the ratio of the sample single-beam spectrum to the background single-beam spectrum. 2.4. Two-Dimensional (2D) Infrared Correlation Spectroscopy. The IR spectra were analyzed by means of the general 2D correlation spectroscopy formalism as implemented in OPUS v.5.5 (Bruker). The spectra were truncated at 1200 and 1800 cm-1 and baseline-corrected by fitting a straight line through these end points. The obtained spectral data set displays variation as a function of pH, which is caused by changes in the total amount of ligand adsorbed and in the relative distribution of different surface species. The objective of the 2D correlation analysis was primarily to study the latter effect; hence, the variation in total surface concentration was significantly reduced by normalizing the spectra to the same total peak area over the interval of 1200-1800 cm-1. Subsequently, the truncated, baseline-corrected, and normalized spectral data set was used to calculate synchronous and asynchronous 2D correlation plots. The synchronous plot was analyzed by identifying the diagonal auto peaks and the off-diagonal cross peaks. The former provide information on peaks responsible for the major spectral variation as a function of pH, whereas the latter are a measure of the correlated response to the pH perturbation at two different wavenumbers. The asynchronous plot does not contain autopeaks, but the off-diagonal cross peaks in this plot show the uncorrelated peak responses, which are partly or completely outof-phase, as a function of the pH perturbation.18 The collective information provided by the 2D correlation spectroscopy analysis indicates which peaks belong to the same surface species and also the number of dominating species.19 2.5. Molecular Orbital Calculations. Gaussian 0320 was used to perform a geometry optimization and calculate the infrared spectrum of EDTA4- and H2EDTA2- species. This

7766


Noreń et al.

Figure 8. Synchronous (left) and asynchronous (right) 2D correlation contour plots obtained from the 2D correlation spectroscopic analysis of the infrared spectra in Figure 7.

Figure 9. Infrared spectra and the corresponding Fourier selfdeconvoluted spectra of EDTA adsorbed on goethite at (a) pH 3.47, (b) pH 4.48, (c) pH 5.91, (d) pH 6.54, (e) pH 7.80, (f) pH 8.86, (g) pD 3.45, (h) pD 4.52, (i) pD 5.77, (j) pD 6.64, (k) pD 7.20, and (l) pD 8.10, at a total EDTA concentration of 2.0 µmol/m2, and I ) 0.1 M (Na)Cl.

TABLE 2: IR Frequencies (cm-1) of H2EDTA2- and HEDTA3- in Aqueous Solution and Adsorbed at the Goethite Surface in H2O and D2O (within Parentheses) H2EDTA2-

HEDTA3-

aqueous

surface

aqueous

surface

νC-O

1611 (1625)

1613 (1628)

νC-Os

1403 (1402)

1410 (1407)

1618 (1626) 1572 (1581) 1402 (1404)

1613 (1624) 1570 (1578) 1405 (1406)

as

calculation was based on the B3LYP/6-31+G(d,p) model chemistry, and each EDTA species was solvated by eight explicit water molecules. Only positive frequencies were observed, indicating that the optimized structure represents a minimum in the potential energy surface. Visualization of the atomic displacements corresponding to each calculated frequency was performed with GaussView.20

3. Results and Discussion 3.1. Speciation and IR Spectra of EDTA in Aqueous Solution. Five EDTA species predominate within the pH range of 0-12.5, H4EDTA, H3EDTA-, H2EDTA2-, HEDTA3-, and EDTA4- (Figure 1),21 and the most basic four of these different protonation states were detected by IR spectroscopy (Figure 2, Table 1); attempts to collect a spectrum of the neutral H4EDTA species were unsuccessful due to its low solubility in water. The IR spectral assignments of the aqueous EDTA species are based on results from the ab initio calculations on the microsolvated EDTA4- · 8H2O H2EDTA2- · 8H2O and on assignments from previous works on EDTA and similar molecules (Table 1).22-28 At pH 2.92 ∼35% of the total ligand concentration is in the form of the H3EDTA- species, and in the corresponding spectrum (Figure 2a), a peak at 1695 cm-1 and weaker peaks at ∼1200 cm-1 can be distinguished, which are unique compared to spectra collected at higher pH values. Hence, the peaks most likely originate from H3EDTA-, are indicative of a protonated carboxylic acid group, and are assigned to νCdO (1695 cm-1) and νC-OH and δC-O-H (∼1200 cm-1) modes. At pH 3.47, the carboxylate group is deprotonated, and the peak at 1618 cm-1 is attributed to the asymmetric carboxylate stretching mode (νasC-O) of H2EDTA2- that originates from carboxylate groups involved in strong intramolecular hydrogen bonding to neighboring protonated amine groups.22 As pH increases, this intramolecular hydrogen bonding is lost on one side of the molecule due to the deprotonation of an amine group, which results in the formation of the HEDTA3- species. Accordingly, the HEDTA3- species is associated with two νasC-O peaks, one at 1618 cm-1 from the carboxylate groups that are strongly intramoleculary hydrogen bonded to the remaining protonated amine group, and another at 1572 cm-1 from the carboxylate groups that are not strongly hydrogen bonded. The peak at 1618 cm-1 is relatively broad and weak due to coupling with the bending modes of solvation waters and the protonated amine group (δNH+). Hence, it is difficult to distinguish from the one at 1572 cm-1 when the latter is comparatively intense (Figure 2, spectra e and f). With further increase in pH, only one νasC-O frequency is observed at 1572 cm-1, in accordance with the equivalent carboxylate groups of the unprotonated EDTA4- species. In contrast to νasC-O, there are only minor differences in the frequencies of the symmetric carboxylate stretching peaks (νsC-O) of H2EDTA2-, HEDTA3-, and EDTA4- species. For similar molecules, the νsC-O frequency is known to couple with other molecular vibrations such as νC-C, thus making it less



Figure 10. (a) Concentration of Fe(III) dissolved (primarily in the form of the Fe(III)EDTA solution complex) and (b) the concentration of uncomplexed EDTA remaining in the supernatant as a function of pH after reaction times of (9) 24 h, (b) 7 days, (1) 12 days, and (f) 21 days, at a total EDTA concentration of 2.0 µmol/m2 (1882 µM), and I ) 0.1 M (Na)Cl.

Figure 11. Total amount of EDTA adsorbed on goethite as a function of pH for a reaction time of (b) 10 min, (4) 24 h, (f) 7 days, (O) 12 days, and (2) 21 days, at a total ligand concentration of 2.0 µmol/m2 (1882 µM), and I ) 0.1 M (Na)Cl.

sensitive to environmental changes around the carboxylate group than the νasC-O frequency.22 Significant changes in the infrared spectra of the aqueous EDTA species occur when D2O is used as a solvent instead of H2O (Table 1, Figure 3). The bending mode of water and the NH modes of the protonated amines are shifted downward several hundred wavenumbers in D2O due to the atomic-mass effect.23 These shifts lead to decoupling of the νasC-O modes, and this causes upward frequency shifts of the νasC-O peaks, as well as an increase in the intensities of the νasC-O peaks relative to the peaks of νsC-O (Figure 3). As a consequence of the loss of coupling, the νasC-O peaks in D2O are more narrow, and this effect is most evident for HEDTA3-, where the two νasC-O peaks are clearly resolved in both the regular and Fourier deconvoluted spectra (Figure 3). Although a small shoulder exists in the νasC-O peak in the Fourier self-deconvoluted spectrum of EDTA4(Figure 3c), the νasC-O peaks of both EDTA4- and H2EDTA2are arguably quite narrow and symmetric (Figure 3 spectra a and c), which indicates a comparatively equivalent nature of the carboxylate groups in these species. 3.2. Adsorption and IR Spectra of EDTA on Goethite under Negligible Dissolution Conditions. 3.2.1. Adsorption Results. The adsorption of EDTA on goethite was initially studied under conditions where dissolution did not consume a significant amount of goethite (reaction time of 10 min), thus leaving the morphology of the mineral surface relatively unaltered by the dissolution process. The criterion for these conditions was that the amount of dissolved iron(III) should be not more than the detection limit of the AAS technique, which was 2 µM. Considering the surface area and solid concentration

of our goethite suspensions (94 m2/g and 10 g/L), [Fe]aq ) 2 µM corresponds to a surface iron concentration 0.002 µmol/ m2. This is a negligible surface concentration compared to the concentration of proton-active surface sites of 11.0 µmol/m2 estimated from crystallographic data and site densities reported by Boily,29 as well as to the adsorption densities of EDTA (see below). The trend under negligible dissolution conditions for all total ligand concentrations studied is that the concentration of EDTA adsorbed decreases with increasing pH in accordance with anion adsorption in general (Figure 4). A significant increase in adsorption at pH values below pH 6 is observed when the total concentration of EDTA increases from 1.0 to 2.0 µmol/m2. This increase is most obvious at pH 3, where the surface coverage at the adsorption maximum changes from 0.9 to 1.16 µmol/m2. A further increase in total ligand concentration, from 2.0 to 3.0 µmol/m2, yields no increase in adsorption, indicating that the surface is saturated at a total ligand concentration of 2.0 µmol/ m2 (Figure 4). The adsorption of EDTA at the water-goethite interface was also studied as a function of ionic strength (Figure 5). The maximum amount of EDTA adsorbed within the studied pH range decreases by 31% as the background electrolyte is increased from 0 to 0.1 M (Na)Cl. Although no definitive conclusions may be drawn from these results, this type of ionic strength dependence suggests that electrostatic interactions are important for the adsorption of EDTA onto goethite. 3.2.2. Infrared Spectroscopic Results. The IR spectra of samples prepared in batch mode after a 10 min of reaction time display pH-dependent features indicating that the surface speciation of EDTA changes with pH (Figure 6). Most obvious are the shift of νasC-O to lower frequency and the disappearance of the peak at 1357 cm-1 with increasing pH. Comparison with the spectra of the EDTA solution species (Figure 2) suggests that both observations are in agreement with gradual deprotonation of the adsorbed EDTA molecules. IR spectra of EDTA adsorbed at the water-goethite interface under similar conditions as in the batch adsorption experiments were also obtained by means of the simultaneous infrared spectroscopy and potentiometric titration technique (Figure 7). A 10 min reaction time was allowed between additions of titrant, and the titration was complete within 3 h. Thus, the longer reaction time may lead to slightly increased goethite dissolution (see below), but from the practically identical spectra in Figures 6 and 7, we can conclude that the EDTA surface speciation is the same in both experiments. Since the signal in the IR spectra from the titration originates almost exclusively from the goethite overlayer, the identical spectra in Figures 6 and 7 show that

7768


Figure 12. ATR-FTIR spectra of EDTA adsorbed on goethite as a function of pH and time: (a) pH 3.09, 10 min; (b) pH 3.18, 24 h; (c) pH 3.25, 7 days; (d) pH 3.22, 12 days; (e) pH 3.06, 21 days; (f) pH 4.43, 10 min; (g) pH 4.28, 24 h; (h) 4.52, 7 days; (i) pH 4.24, 12 days; (j) pH 4.40, 21 days; (k) pH 4.97, 10 min; (l) pH 5.29, 24 h; (m) pH 5.27, 7 days; (n) pH 5.29, 12 days; (o) pH 5.37, 21 days; (p) pH 5.98, 10 min; (q) pH 6.10, 24 h; (r) pH 6.49, 7 days; (s) pH 6.42, 12 days; (t) pH 6.16, 21 days; (u) pH 7.53, 10 min; (v) pH 7.28, 24 h; (w) pH 7.49, 7 days; (y) 7.59, 21 days; (x) pH 8.46, 10 min; (z) pH 8.17, 24 h; (a′) pH 8.30, 7 days; (b′) pH 8.35, 12 days; (c′) pH 8.50, 21 days, at a total EDTA concentration of 2.0 µmol/m2 and I ) 0.1 M (Na)Cl.

the reactions occurring at the interface between the solution and the particles in the overlayer are representative of the reactions at the water-particle interface in the bulk suspension. One advantage with the IR titration technique is the good contact between the overlayer and the IRE of the ATR accessory resulting in spectra with superior signal-to-noise ratio; this is evident from comparing the noise level between 1700 and 1800 cm-1 to the νasC-O peak intensities in Figures 6 and 7.

Noreń et al. Consequently, whenever possible the IR spectra obtained using the titration technique will be used in the following discussion and analyses. The pH-dependent surface speciation of EDTA was further investigated by subjecting the spectra in Figure 7 to a 2D correlation spectroscopic analysis. Since, this analysis relies on good signal-to-noise, we limited the frequency region to that of νasC-O vibrations (1450-1750 cm-1), which contains the strongest peaks and also shows interesting pH-dependent features. The synchronous contour plot shows two well-resolved auto peaks, along the diagonal, at 1613 and 1570 cm-1 (Figure 8), and together with the negative off-diagonal peak (cross peak) we can conclude that the intensity increase of the 1613 cm-1 peak is accompanied with a decrease at 1570 cm-1, and vice versa. Thus, the carboxylate groups are present in two states at the interface and the distribution among these states varies as a function of pH. It follows that the existence of two predominating surface complexes each with a different ratio of carboxylate groups in these states explains the synchronous 2D results. The asynchronous contour plot shows one cross peak at (1570, 1613) implying that an intensity increase at 1613 cm-1 is not perfectly correlated to a decease at 1570 cm-1; i.e., the magnitudes of the rates of the change in intensity as a function of pH are different at the two wavenumbers. The system can therefore not be viewed as the conversion of one surface complex into the other. In summary, the 2D correlation spectroscopic analysis has indicated the existence of at least two predominating EDTA surface complexes on goethite and has demonstrated that the concentrations of these complexes increase and decrease at different pH-dependent rates. A key observation in the analysis of the coordination modes of the predominating surface complexes is that the IR spectra of EDTA adsorbed onto goethite show a strong resemblance to the spectra of HEDTA3-(aq) and H2EDTA2-(aq) at high and low pH, respectively (Figures 2, 3, and 8). These similarities are also evident from the changes in peak positions, intensities and widths when D2O is substituted for H2O as the solvent (Figure 9 and Table 2). At low pH, the EDTA surface complex in both H2O and D2O is characterized by a νasC-O peak with a frequency that is identical, within the resolution of the measurements (4 cm-1), to the corresponding frequencies of the solution species, whereas the νsC-O frequencies show a slightly larger divergence (5-7 cm-1). Also the gain in νasC-O intensity relative to νsC-O and the disappearance of the peak at 1357 cm-1 observed in the solution spectra (Figures 2b and 3a) are reproduced in the spectra of the surface complexes when the solvent is changed from H2O to D2O (Figure 9, spectra a and g). Hence, at low pH the transfer of H2EDTA2- from solution to the water-goethite interface results in only small perturbations in its spectrum, and this implies the formation of either solvent-surface hydration-separated or surface hydration-shared ion pairs.19 Although the very close similarity to the solution species tentatively suggests a predominance of solvent-surface hydration-separated ion pairs, most likely a distribution of both types of surface complexes exists at the interface. However, at present we cannot quantify this distribution from the IR spectroscopic data. One difference between the spectra of EDTA adsorbed at low pH and of aqueous H2EDTA2- is the appearance of a weak peak at 1520 cm-1 in the surface spectra (Figures 2b, 3a, and 9, spectra a and g). This peak is not affected by a change of solvent, and the frequency is low for being associated with νasC-O. Instead a possible explanation for this peak is that it originates from a vibration of the molecular backbone (e.g., νC-C or δCH2), and its variant IR activity might be associated



Figure 13. Asynchronous correlation contour plots obtained from the 2D correlation spectroscopic analysis of the IR spectra of EDTA adsorbed onto goethite at pH 5 as a function of time (left) and a blow-up of the region 1350 to 1450 cm-1 (right).

Figure 14. Infrared spectra of (a) Fe(III)EDTA adsorbed on goethite at pH 3.84, (b) Ga(III)EDTA adsorbed on goethite at pH 3.84, (c) EDTA adsorbed on goethite at pH 3.65 and a reaction time of 10 min. Total concentrations of Fe(III)EDTA and EDTA were 2.0 µmol/m2; the Ga(III)EDTA sample was prepared at 2.3 µmol/m2.

with a change in the conformation of intramolecularly hydrogenbonded H2EDTA2- when it adsorbs at the interface. This interpretation is supported by the fact that νsC-O, which is coupled with δCH, shows the largest shift upon adsorption, and thus this shift might primarily be caused by a perturbation of the backbone and not of the carboxylate group. Accordingly, in the spectra of the EDTA surface complexes, the 1520 cm-1 peak could be indicative of strong hydrogen bonding to the surface and to surface hydration-shared ion pairs that favor certain conformations of H2EDTA2-. At pH between 8 and 9, the IR spectra of EDTA adsorbed onto goethite in both H2O and D2O display peak positions and intensities in close agreement with the spectra of aqueous HEDTA3- (Figures 2g, 3b, and 9, spectra f and l). In accordance with the discussion above this surface complex is assigned to either solvent-surface hydration-separated or surface hydrationshared ion pairs, and most likely both coexist at the interface.

Thus, the collective spectroscopic data indicate that under negligible dissolution conditions, EDTA forms two predominating surface complexes consisting of HEDTA3- and H2EDTA2-. These are bonded to the surface via electrostatic interactions of EDTA ions retaining their solvation shells and/or via hydrogen bonding of partially desolvated EDTA to surface hydroxyls or surface-bound water molecules. The weak peak at 1570 cm-1 in the Fourier self-deconvoluted spectra (Figure 9, spectra a and g) indicates the presence of interfacial HEDTA3- at pH values as low as 3. Hence, in agreement with several previous studies on adsorption of carboxylic acids, a particular protonation state of the adsorbed molecules occurs at lower pH values as compared to the corresponding protonation state in solution (cf. Figure 1),30,31 or in other words more negatively charged species are stabilized at the interface. Finally, no peaks indicative of inner-sphere coordination between Fe(III) and EDTA (see discussion below) were detected, although inner-sphere surface complexes might exist at low steady-state surface concentrations as dissolution proceeds. 3.3. Adsorption and IR Spectra of EDTA on Goethite under Dissolution Conditions. Extending the reaction time of EDTA with goethite leads to increased mineral dissolution (Figure 10), and after 24 h the iron concentration in solution reaches levels between 100 and 200 µM (Figure 10a). Furthermore, the iron concentrations show an increasing trend with decreasing pH. At longer reaction times there is no net increase in the dissolved iron concentration below pH 8 as indicated by the similar results obtained after 12 and 21 days (Figure 10a). The goethite dissolution has a pronounced effect on the solution speciation of EDTA. For example, at a 10 min of reaction time and a total EDTA concentration of 2.0 µmol/m2, between 50% and 60% remains in solution at pH 4 (Figure 4), and under these conditions H2EDTA2- predominates in solution. As iron dissolves from goethite, it complexes with EDTA and primarily forms the highly stable FeEDTA- solution complex (log K(FeEDTA-) ) 25.0 at I ) 0.1 M).21 Below pH 5, practically all the EDTA is in the form of the FeEDTA-. This strong solution complexation might be primarily what drives the dissolution, and it is clear that the dissolved iron concentration

7770


stops increasing once most of the free EDTA below pH 5 has been consumed (Figure 10b). The adsorption of EDTA at the water-goethite interface is influenced by the increasing reaction time and the changing EDTA speciation in solution. Between pH 4 and 7, EDTA adsorption increases, whereas at both higher and lower pH values the adsorption remains unaltered (Figure 11). Interestingly, the adsorption of Ga(III)EDTA complexes on goethite displays an adsorption maximum around pH 5, and these complexes have been shown to adsorb as ternary surface complexes.32 Thus, we hypothesize that dissolved Fe(III)EDTA solution complexes can also adsorb as ternary Fe(III)EDTA surface complexes, and it is this dissolution-readsorption process that leads to an increase in the total surface concentration of EDTA. The IR spectra of EDTA adsorbed on goethite collected as a function of time reveal that regardless of pH there is very little spectral variation and accordingly very little change in EDTA surface speciation (Figure 12). Hence, despite the goethite dissolution, the varying speciation in solution, and the increased adsorption within the pH range of 4-7, the outer-sphere and hydrogen-bonded HEDTA3- and H2EDTA2- surface complexes always predominate under the experimental conditions studied. In order to investigate possible reasons for the increased adsorption with time within the pH range of 4-7, the spectra collected at pH 5, i.e., at the adsorption maximum, were subjected to a 2D correlation spectroscopic analysis. The low signal-to-noise ratio of the asynchronous correlation contour plot is a consequence of the very similar spectra analyzed (Figures 12 and 13), but the fact that some peaks may be distinguished indicates that spectral changes, although small, do occur as a function of time. We can define a noise level using the region between 1700 and 1800 cm-1 where no peaks appear in the regular IR spectra, and then only consider peaks with intensities of at least 3 times the noise. On the basis of this criterion for significant peaks, asynchronic cross peaks clearly exist both in the νasC-O and νsC-O regions. Of particular interest are the peaks in the νsC-O region shown in the blow-up in Figure 13, indicating that the peak at 1406 cm-1, representing a mixture of H2EDTA2- and HEDTA3- surface complexes, is asynchronically correlated to a peak at 1384 cm-1, which is not obvious from just visual inspection of the regular IR spectra in Figure 12. The peak at 1384 cm-1 coincides with a peak in the νsC-O region recently shown to be indicative of EDTA coordinated to Ga(III) both in solution and at the surface.32 In Figure 14 the spectrum of the Ga(III)EDTA surface complex on goethite is compared to the spectrum of a surface species formed when goethite particles were reacted with a solution of FeEDTA-(aq). Clearly there is good agreement between these spectra, and this supports our hypotheses that the peak at 1384 cm-1 detected in the asynchronous 2D contour plot is indicative of formation of a ternary Fe(III)EDTA surface complex on goethite and that readsorption of Fe(III)EDTA is occurring as the goethite dissolution progresses. 4. Conclusions The collective infrared spectroscopic results of this study have shown that two surface complexes consisting of HEDTA3- and H2EDTA2- predominate at the water-goethite interface. No direct interactions of these complexes with surface Fe(III) ions were detected; hence, they are probably stabilized at the interface by electrostatic and hydrogen-bonding forces. The formation of these EDTA surface complexes is fast (time scale of minutes), but a slower (time scale of hours to days) dissolution reaction

Noreń et al. also occurs. The dissolved iron in solution is in the form of the highly stable FeEDTA- solution complex, and evidence has been presented here that this complex can readsorb to the mineral surface. As dissolution proceeds, the concentration of FeEDTA- in the solution phase increases, and this in turn leads to a buildup of readsorbed FeEDTA- onto goethite. In the pH range of 4-7, this dissolution and readsorption process increases the total EDTA concentration at the surface. Under the experimental conditions in the present study, it is primarily the presence of uncomplexed EDTA in solution that drives the dissolution of goethite resulting in the subsequent readsorption of FeEDTA-, while the HEDTA3- and H2EDTA2- surface complexes are stable during this process. Acknowledgment. The Kempe foundation is acknowledged for providing funding of the infrared spectrometer. This work was supported by the Swedish Research Council. References and Notes (1) No¨rtemann, B. Appl. Microbiol. Biotechnol. 1999, 51, 751. (2) Brucheli-Witschel, M.; Egli, T. FEMS Microbiol. ReV. 2001, 25, 69. (3) Knepper, T. P. Trends Anal. Chem. 2003, 22, 708. (4) Schmidt, C. K.; Fleig, M.; Sacher, F.; Brauch, H.-J. EnViron. Pollut. 2004, 131, 107. (5) Schmidt, C. K.; Brauch, H.-J. EnViron. Toxicol. 2004, 19, 620. (6) Xue, H.; Sigg, L.; Kari, F. G. EnViron. Sci. Technol. 1995, 29, 59. (7) Bowers, A. R.; Huang, C. P. J. Colloid Interface Sci. 1985, 105, 197. (8) Chang, H.-C.; Healy, T. W.; Matijevic, E. J. Colloid Interface Sci. 1983, 92, 469. (9) Rubio, J.; Matijevic, E. J. Colloid Interface Sci. 1979, 68, 408. (10) Blesa, M. A.; Borghi, E. B.; Maroto, J. G.; Regazzoni, A. E. J. Colloid Interface Sci. 1984, 98, 295. (11) Rueda, H.; Grassi, R. L.; Blesa, M. A. J. Colloid Interface Sci. 1985, 106, 243. (12) Nowack, B.; Sigg, L. J. Colloid Interface Sci. 1996, 177, 106. (13) Carbonaro, R. F.; Gray, B. N.; Whitehead, C. F.; Stone, A. T. Geochim. Cosmochim. Acta 2008, 72, 3241. (14) Chang, H.-C.; Matijevic, E. J. Colloid Interface Sci. 1983, 92, 479. (15) Campbell, J. L.; Eick, M. J. Clays Clay Miner. 2002, 50, 336. (16) Boily, J.-F.; Lu¨tzenkirchen, J.; Balmes, O.; Beattie, J.; Sjo¨berg, S. Colloids Surf., B 2001, 179, 11. (17) Glasoe, P. K.; Long, F. A. J. Phys. Chem. 1960, 1, 188. (18) Noda, I.; Ozaki, Y. Two-Dimensional Correlation Spectroscopy. Applications in Vibrational and Optical Spectroscopy; John Wiley & Sons, Inc.: New York, 2004. (19) Noreń, K.; Persson, P. Geochim. Cosmochim. Acta 2007, 71, 5717. (20) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Montgomery, J. A., Jr.; Vreven, T.; Kudin, K. N.; Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J. J.; Barone, V.; Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson, G. A.; Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Klene, M.; Li, X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Ayala, P. Y.; Morokuma, K.; Voth, A.; Salvador, P.; Dannenberg, J. J.; Zakrzewski, V. G.; Dapprich, S.; Daniels, A. D.; Strain, M. C.; Farkas, O.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Ortiz, J. V.; Cui, Q.; Baboul, A. G.; Clifford, S.; Cioslowski, J.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.; Johnson, B.; Chen, W.; Wong, M. W.; Gonzalez, C.; Pople, J. A. Gaussian 03; Gaussian, Inc.: Wallingford, CT, 2004. (21) Smith, M.; Martell, A. E. Critical Stability Constants, Vol 1: Amino Acids; Plenum Press: New York, 1989. (22) Noreń, K.; Loring, J. S.; Persson, P. J. Colloid Interface Sci. 2008, 319, 416. (23) Diem, M. Modern Vibrational Spectroscopy; John Wiley & Sons, Inc.: New York, 1993; Chapter 7. (24) Chapman, D. J. Chem. Soc. 1955, 2, 1766. (25) Sawyer, D. T.; Paulsen, P. J. J. Am. Chem. Soc. 1959, 81, 816. (26) Nakamoto, K. Infrared and Raman Spectra of Coordination Compounds; John Wiley & Sons, Inc.: New York, 1986. (27) Persson, P.; Axe, K. Geochim. Cosmochim. Acta 2005, 69, 541. (28) Ryczkowski, J. Appl. Surf. Sci. 2005, 252, 813.

EDTA Adsorption at the Water-Iron Oxide Interface (29) Boily, J.-F. The Surface Complexation of Ions at the Goethite (RFeOOH)/Water Interface: A Multisite Complexation Approach. Ph.D. Thesis, Umeå University, Umeå, Sweden, 1999. (30) Johnson, S. B.; Yoon, T. H.; Kocar, B. D.; Brown, G. E. Langmuir 2004, 20, 4996. (31) Johnson, B. J.; Sjo¨berg, S.; Persson, P. Langmuir 2004, 20, 823.

J. Phys. Chem. C, Vol. 113, No. 18, 2009 7771 (32) Noreń, K. Coordination Chemistry of Monocarboxylate and Aminocarboxylate Complexes at the Water/Goethite Interface. Ph.D. Thesis, Umeå University, Umeå, Sweden, 2007; http://www.diva-portal.org/umu/ abstract.xsql?dbid)1337.

JP809190M

Adsorption Mechanisms of EDTA at the WaterâIron Oxide Interface

Recommend Documents

Adsorption Mechanisms of EDTA at the WaterâIron Oxide Interface