Kinetics and mechanism of surface reaction of salicylate on alumina in colloidal aqueous suspension

Geochimica et Cosmochimica Acta, Vol. 64, No. 7, pp. 1159 –1172, 2000 Copyright © 2000 Elsevier Science Ltd Printed in the USA. All rights reserved 0016-7037/00 $20.00 ⫹ .00

Pergamon

PII S00167037(99)00360-9

Kinetics and mechanism of surface reaction of salicylate on alumina in colloidal aqueous suspension Z. WANG,1,* C. C. AINSWORTH,2 D. M. FRIEDRICH,1,† P. L. GASSMAN,1 and A. G. JOLY1 1

Environmental and Molecular Science Laboratory Pacific Northwest National Laboratory, Richland, WA 99352 2 Interfacial Geochemistry Group, Pacific Northwest National Laboratory, Richland, Washington 99352, USA (Received January 13, 1999; accepted in revised form September 13, 1999)

Abstract—The reaction kinetics of salicylate with Al(III) in aqueous solution and at the colloidal alumina– water interface was studied by stopped-flow laser fluorescence spectroscopy. Temporal evolution of the fluorescence spectra suggests that formation of a carboxylate monodentate complex was the reaction intermediate that occurs transiently at the beginning of the reaction in aqueous salicylate–Al(III) solution. However, by lowering the pH to 2.0, the formation of such an intermediate can be directly observed as it is the only species formed. The reaction of salicylate with aqueous Al3⫹ is completed within 10 min at pH 3.3 but is significantly slower at pH 2.0. At both pH the aqueous reaction follows a single pseudo-first order rate law. In alumina suspension the reaction was initially fast but slowed down after ⬃30 s. Completion of the reaction took up to 12 h, depending on pH and ionic strength. The formation of a carboxylate monodentate surface complex as a transient species is clearly observed in alumina suspensions at near neutral pH. The initial rapid reaction (⬍30 s), accounting for ⬃70% of the total reaction, can be best described by the Elovich rate equation and the slower reaction, accounting for ⬃30% of the total reaction, obeys pseudo-first order kinetics. These results are consistent with a sorption reaction mechanism that is controlled by the leaving group lability at the surface sites (Al–OH⫹ 2 and Al–OH). The pseudo-first order rate constant varies little with initial salicylate concentration, ionic strength, or pH ⬎ 4, suggesting that the slow reaction pathway involves ligand substitution reactions between salicylate and the hydroxyl groups for which the Al–O binding and activation energy are affected by site heterogeneity or site density to a lesser degree than Al–OH⫹ 2 sites. Copyright © 2000 Elsevier Science Ltd However, through the use of relaxation methods (Sparks and Zhang, 1991) reaction mechanisms for several inorganic oxyanions (Zhang and Sparks, 1989, 1990) and an organic acid anion (Ikeda et al., 1982) have been deduced. The adsorption mechanism for these anions is conceptually analogous to the aqueous phase Eigen-Wilkens-Werner mechanism that is the fundamental pathway for strong metal complex formation in aqueous solution (Sposito, 1994). This mechanism, for monodentate ligands, involves a two-step process with rapid formation of an outer sphere complex followed by the slower formation of an inner sphere complex with the elimination of a H2O molecule. Many adsorption reactions of ligands at a metal oxide–water interface have been observed to share common characteristics of structure, reactivity, and thermodynamic stability with their homogeneous aqueous phase counterparts (Kummert and Stumm, 1980; Ainsworth et al., 1998; Yost et al., 1990; Tunesi and Anderson, 1992). Cylindrical internal reflection–FTIR studies indicate that the spectra of ring-substituted benzoic acids at the TiO2 surface are identical to those obtained from solution phase Ti(IV) complexes (Tunesi and Anderson, 1992), which reflects the similarity in structure of complexes formed in solution and at solid–water interfaces. Similar spectroscopic results have been reported for salicylate–Fe(III) complexes in solution and at the goethite–water interface (Yost et al., 1990), as well as the salicylate–Al(III) complex in solution and at the alumina–water interface (Ainsworth et al., 1998). Using dynamic 17O nuclear magnetic resonance spectroscopy, the water exchange rate in aluminum complexes with hydroxide and fluoride (Phillips et al., 1997b, 1998), oxalic

1. INTRODUCTION

The adsorption of aqueous inorganic oxyanions and organic acid anions to hydrous metal oxides has been described in terms of surface complexation reactions involving the formation of mono- and bidentate or binuclear surface complexes. Although the surface speciation resulting from adsorption has during the past 20 years been intensely studied, adsorption kinetics studies are limited and less successful because of the inherent difficulties in data collection and analysis. Kinetic adsorption data have often been treated as a pseudo-first order reaction, a series of pseudo-first order reactions, or a diffusioncontrolled reaction (Hingston and Raupach, 1967; Chen et al., 1973; Huang, 1975; Hingston, 1981; Ainsworth et al., 1985; Raven et al., 1998; Westall, 1987). The physical meaning of simple rate laws applied to such complex systems is often open to question. A simple rate law may be observed if the parameters relevant to the kinetic rate constants are not too sensitive to site or sorbent heterogeneity. However, under the conditions that the binding or reaction activation energy varies significantly with site heterogeneity, the observed reaction rate may evolve continuously with increasing surface coverage (Lasaga, 1981). Because of their inherently heterogeneous nature, adsorption reaction pathways or mechanisms are difficult to elucidate. *Author to whom correspondence should be addressed (zheming.wang @pnl.gov). † Present address: Optical Coating Laboratory, Inc., 2789 Northpoint Parkway, Santa Rosa, California 95407-7397, USA. E-mail: dmfriedrich @ocli.com. 1159

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Fig. 1. Structures of salicylate complexes at alumina surface in aqueous alumina suspension: (a) bidentate; (b) carboxylate-bonded monodentate; (c) phenolate-bonded monodentate; (d) outer sphere ionic complex.

acid (Phillips et al., 1997a), and methylmalonate (Casey et al., 1997) was determined. Complexation of Al(III) with these anions increases the exchange rate of inner hydration sphere water molecules as much as two orders of magnitude or more. Such results are consistent with the interpretation of Secco and Venturini (1975), Perlmutter-Hayman and Tapuhi (1977, 1979), and Rakotonarivo et al. (1989) for the faster reaction rate of hydrolysed Al(III) with organic acids than the nonhydrolysed Al(III) in aqueous solution. In such a reaction removal of the first water molecule from the inner hydration sphere is the rate determining step. Binding of an anion weakens the Al–OH2 bond of other water molecules and therefore, accelerates the rate of ligand complexation. It has also been suggested that the increased water exchange rate can be correlated with the ligand-enhanced dissolution rate of minerals (Pulfer et al., 1984; Zutic and Stumm, 1984; Phillips et al., 1997a,b, 1998; Casey et al., 1998). In a previous fluorescence spectroscopy investigation of salicylate adsorption at the ␦-Al2O3–aqueous interface we identified four surface complexes (Ainsworth et al., 1998). These consisted of one outer sphere and three inner sphere species with a bidentate inner sphere complex being the dominant species under the conditions of the study (Fig. 1a). At the low surface coverages of this study the bidentate surface species accounted for ⱖ90% of the total adsorbed salicylate. At equilibrium a monodentate phenolate surface complex (Fig. 1c) was present and could be distinguished from the bidentate species. The monodentate carboxylate surface complex (Fig. 1b) was not as unambiguous. At low salicylate surface coverage between pH 4 and 6, the equilibrium concentration of the monodentate carboxylate surface complex was beyond the limit of

spectral resolution, although it was observed in pH 2.0 aqueous Al(III) solutions. Although the above study addressed a number of questions regarding salicylate surface speciation, issues concerning salicylate adsorption kinetics and mechanism arose. First, preliminary kinetic data (unpublished) suggested that, similar to oxyanions and other organic acids, salicylate time-dependent adsorption is composed of a very rapid adsorption followed by a slower process. Second, investigation of the monodentate carboxylate surface complex (Ainsworth et al., 1998) suggested that the bidentate surface species is formed through a monodentate intermediate. Together with the observed presence of an outer sphere complex (Fig. 1d), this suggested that the path of formation of the predominate salicylate bidentate surface species is similar to the aqueous phase Eigen-Wilkens-Werner mechanism. This study was undertaken to determine the rate and possible mechanism of formation for the bidentate surface species that were previously shown to dominate the surface speciation of sorbed salicylate. Adsorption kinetics of salicylate on ␦-Al2O3 was studied by stopped-flow with fluorescence detection. The temporal evolution of the salicylate fluorescence spectra and the growth and decay kinetics of the fluorescence of salicylate complexes, over a range of pH, ionic strength, and initial salicylate concentrations, were used to elucidate the overall adsorption process leading to the observed time-dependent adsorption curve. 2. MATERIALS AND METHODS

2.1. Materials Aqueous suspensions of aluminum oxide (Aluminum Oxide C, ␦-Al2O3, Degussa AG.) and solutions of sodium salicylate were prepared as described previously (Ainsworth et al., 1998). Briefly, the commercial ␦-Al2O3 was washed, suspended, and pH adjusted for several days to achieve a stable pH of 6.0. After centrifugation, the gel-like top layer of the sediment was carefully removed and suspended again in the supernatant solution. This produced a stable suspension with 70 nm average colloidal particle size. Sodium salicylate (2hydroxybenzoate; 100.1% assay, Mallinckrodt Chemical Co.; sal or HA⫺) was used as received and made up into pH-adjusted 10⫺7 mol/L and 10⫺5 mol/L salicylate solutions in background electrolyte ionic strength (IS) of 0.01 mol/L NaClO4. AlCl3 solution (2 ⫻ 10⫺3 mol/L) was prepared by dissolving reagent grade AlCl3 (Aldrich) and used immediately after preparation. The pH of the AlCl3 solutions were adjusted to either pH 3.3 or 2.0 at the time of dissolution using dilute perchloric acid. Deionized water (18 M⍀) from a Millipore reverse osmosis and ion exchange system with ultraviolet sterilization of the finished water was used throughout the study. Care was taken to avoid long periods of water contact with plastic tubing and vessels to eliminate contamination by residual fluorescing impurities (e.g., phthalates).

2.2. Experimental Methods A schematic diagram of the apparatus used for the kinetics measurements is shown in Figure 2. The experiments involved mixing the reactants rapidly, followed by UV excitation, and detection of visible fluorescence. Mixing of aqueous sodium salicylate solution, typically between 10⫺7 mol/L and 10⫺5 mol/L, with either 1 g/L alumina suspension at pH 6.0 and IS 0.01 mol/L (sodium perchlorate) or 0.002 mol/L fresh AlCl3 solution at pH 2.0 or 3.3 was performed in a Hi-Tech Scientific SFA-211 stopped-flow kinetics accessory with a deadtime of 50 ms. The temperature-controlled sample cuvette has three polished quartz windows allowing fluorescence detection at 90° relative to the excitation beam. To record the temporal evolution of fluorescence spectra, the solu-

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Fig. 2. A schematic drawing of the experimental setup for the study of salicylate adsorption kinetics in aqueous alumina suspension. (a) Stopped-flow apparatus; (b) sample cell; (c) collection and focusing optics; (d) filters; (e) mirror; (f) monochromators; (g) intensified diode array detector; (h) computer; (i) photomultiplier tube; (j) photon counter and computer (see text for detail).

tion mixture was excited with a continuous-wave argon laser (Coherent Innova 400) at either 300 or 334 nm. Fluorescence collected by a 2 inch diameter f/2 lens was focused by an f/4 lens into a 0.22 m focal length spectrograph (Spex 270). Wavelength-dispersed emission was detected by an intensified diode array detector (1024 elements Princeton Instruments IPDA with CSMA data acquisition software). To record accurate growth and decay kinetics of the fluorescence from salicylate complexes, a SPEX Flourolog II fluorimeter was used to record the fluorescence intensity as a function of time at fixed excitation and emission wavelength. The fluorimeter was equipped with a 450 W xenon arc lamp, double monochromators (SPEX 1680) for excitation and emission, and a cooled photomultiplier in photon counting mode. Because of the higher spatial stability of the xenon light focused at the sample, fluorescence intensity data recorded with xenon lamp excitation have higher signal-to-noise ratios than with argon laser excitation. Therefore, data used in the analysis of the kinetic parameters were all recorded with xenon lamp excitation using narrow bandpass (1–2 nm) excitation and emission wavelengths of the fluorimeter. To correct for minor variation of the lamp intensity, light from the excitation monochromator was reflected by a thin quartz plate into a Rhodamine dye-cell. Red emission from this “quantum counter” was detected by a second photomultiplier tube and recorded by the fluorimeter in an analog reference channel. Fluorescence intensity from the sample was normalized at each time to the corresponding excitation intensity in the reference channel. As described previously (Ainsworth et al., 1998) the bidentate Al– salicylate (Al:A⫹) complex (Fig. 1a) is the predominant species at low salicylate concentration (10⫺7 to 10⫺5 mol/L) at pH 6.0 and IS 0.01 mol/L. Both free salicylate and Al:sal have distinct excitation and emission maxima. Because both free and complexed salicylates are excited at the absorption maximum near 300 nm, both species contribute to the emission intensity at 415 nm (␭max of salicylate). However, at 334 nm the complexed salicylate is predominantly excited, therefore

at 380 nm only the complexed salicylate is detected (Ainsworth et al., 1998). The monodentate complex Al 䡠 HA2⫹ shows an excitation maximum at 311 nm and emission maximum at 412 nm. Therefore, by performing the kinetics experiments at characteristic excitation and emission wavelengths the complexes Al:A⫹ and Al 䡠 HA2⫹ can be studied with high selectivity. Salicylate samples in both homogeneous aqueous solution and heterogeneous alumina suspension were examined for the effect of possible photobleaching. The results indicated that within 10 min the decrease in fluorescence intensity at the excitation and emission wavelength for the samples was ⬍1%. Because the time ranges used for kinetics analysis were within even shorter periods, the effect of photobleaching was negligible.

2.3. Data Analysis Procedure The fluorescence intensity of the aluminum-salicylate complex increases rapidly initially and then asymptotically reaches a maximum limit for both the homogeneous (aqueous) and heterogeneous (colloidal) reactions. The asymptotic intensity limit I⬁ can be estimated accurately by fitting the longer time portion of the fluorescence increase curve to smooth functions such as exponentials (omitting data for the first 30 s). For aqueous phase experiments I⬁ represents the fluorescence intensity of the complexes with equilibrium concentrations that can be accurately calculated from the stability constants of the complexes and the protonation constants of the acid. Thus, the concentration of the complex, [AlA], representing either Al:A⫹ or Al:HA2⫹, depending on the excitation and emission wavelengths and solution pH, at time t can be calculated using Eqn. 1: [AlA] ⫽

It [AlA]e I⬁

(1)

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Fig. 3. Emission spectra of Al3⫹–salicylate(aq) as a function of reaction time at pH 3.3. [Al3⫹] ⫽ 0.002 mol/L, [Sal] ⫽ 2 ⫻ 10⫺5 mol/L. ␭ex ⫽ 300 nm. Time range: 0 – 4 min.

where [AlA]e is the concentration of the complex of interest at equilibrium. For clarity, charge was omitted in all concentration expressions. For the kinetics measurements involving alumina suspensions the total salicylate concentration is much less than the alumina reaction sites. Under similar conditions previous work by Ainsworth et al. (1998) indicated that during 24 h of reaction time free salicylate concentration was about 10% as determined by 14C activity of 14Clabeled salicylate left in the supernatant after centrifugation. However, with careful centrifugation of the suspensions at extended adsorption times (⬎24 h), under the experimental conditions of the present study, the concentration of free salicylate was ⬍2% as determined from the fluorescence intensity of the supernatant, and therefore the equilibrium adsorption of salicylate-to-alumina is essentially complete. Thus, the long time asymptotic limit concentration of the surface complex [Al: Al]infin; is assumed to be equal to the initial salicylate concentration [A]tot. Interference from the fluorescence of free salicylate is minimized at the excitation and emission wavelengths (␭ex ⫽ 334 nm, ␭em ⫽ 380 nm). At these wavelengths the recorded fluorescence intensity I(t) is proportional to the concentration of the emitting complex. The concentration of the complex at time t, [A]t can be calculated by [AlA]t ⫽ I(t) [A]tot/I⬁

k Al3⫹ ⫹ HA⫺ | 0 AlA⫹ ⫹ H⫹ k⬘ The reaction rate d[AlA] d[A] ⫽⫺ dt dt ⫽ k[Al] 䡠 [A] ⫺k⬘[AlA][H]

[A]t ⫽ [A]tot ⫺ [Al:A]t ⫽ [A]tot (1 ⫺I(t)/I⬁) 3⫹ (aq)

[Al]0 ⫺ [Al] ⫽ [AlA]

(6)

[A]0 ⫺ [A] ⫽ [AlA]

(7)

where subscript 0 denotes reaction at time zero. Therefore, d[A] ⫽ k[Al] 䡠 [A] ⫺ k⬘([A]0 ⫺ [A])[H] dt

⫺

(8)

when [Al]0 ⬎⬎ [AlA], [Al] ⫽ [Al]0

(9)

d[A] ⫽ k[Al]0[A] ⫺ k⬘[A]0 ⫹ k⬘[A][H] dt

⫺

(3)

The reaction kinetics data of salicylate with aqueous Al and colloidal alumina at time ⬎30 s were found to be best fit by pseudo-first order reaction and therefore, analyzed based on the following treatment. For the reaction

(5)

Because only Al and A are present at the start,

(2)

and the unreacted salicylate concentration at time t can be calculated as

(4)

⫽ ⫺k⬘[A]0 ⫹ (k[Al]0 ⫹ k⬘[H])[A]

(10)

Integration yields ln

k[A]0[Al]0 ⫽ (k[Al]0 ⫹ k⬘[H])t (k[Al]0 ⫹ k⬘[H])[A] ⫺ k⬘[H][A]0 (11)

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Fig. 4. Emission spectra of Al3⫹–salicylate(aq) as a function of reaction time at pH 3.3. [Al3⫹] ⫽ 0.002 mol/L, [Sal] ⫽ 2 ⫻ 10⫺5 M. ␭ex ⫽ 334 nm. Time range: 0 – 4 min.

3⫹ ⫹ Al(aq) ⫹ HA⫺ 3 Al:A(aq) ⫹ H⫹

d[A] ⫽0 dt

At equilibrium, ⫺

k[Al]0 䡠 [A]e ⫽ k⬘[H][AlA]e ⫽ k⬘[H]([A]0 ⫺ [A]e)

(12)

where the subscript e refers to equilibrium concentrations then, [A]e ⫽

k⬘[H] [A]0 k[Al]0 ⫹ k⬘[H]

(13)

and Eqn. 11 can be rewritten as ln

冉

冊 冉

[A]0 ⫺ [A]e [AlA]e ⫽ ln [A] ⫺ [A]e [AlA]e ⫺ [AlA]

冊

(14)

or ln([AlA]e ⫺ [AlA]) ⫽ ⫺(k[Al]0 ⫹ k⬘[H])t ⫹ ln[AlA]e

(15)

A plot of ln([AlA]e ⫺ [AlA]) or ln([sal] ⫺ [sal]e) vs. time t will yield a straight line with slope of ⫺(k[Al]0 ⫹ k⬘[H]), the pseudo-first order rate constant kobsd and intercept of ln[AlA]e. 3. RESULTS AND DISCUSSION 3ⴙ 3.1. Salicylate Reaction Wth Al(aq)

The product of the reaction between Al3⫹ and the salicylate anion (HA⫺) in slightly acidic solutions (pH ⱖ 3) has been shown to be a 1:1 bidentate complex (Secco and Venturini, 1975; Rakotonarivo et al., 1989; Thomas et al., 1993; Ainsworth et al., 1998). At the ratio of Al to salicylate used (100:1) in the present study and the one by Ainsworth et al. (1998), the reaction is essentially complete and the only equilibrium salicylate species observed is the monosalicylate–Al complex.

(16)

The temporal evolution of the fluorescence spectra from the aqueous reaction at pH 3.3 is shown in Figure 3. Because the pKa1 of salicylate is 2.95, approximately 31% of salicylate is in the protonated salicylic acid form. The fluorescence of salicylic acid is very weak and contributes ⬍1% to the observed intensity (Ainsworth et al., 1998). With 300 nm excitation both free salicylate anion (␭max ⫽ 296 nm) and the Al:A⫹ complex (␭max ⫽ 312 nm) are excited. If present, however, the Al 䡠 HA2⫹ monodentate carboxylate complex (␭max ⫽ 306 nm) would also be excited. As shown by Ainsworth et al. (1998), the bidentate Al:A complex (binding aluminum at both its carboxy and phenoxy oxygens) is characterized by its strong fluorescence maximum at ⬃383 nm. Thus, Figure 3 shows the fluorescence spectrum of the reaction mixture evolving from initially free salicylate (410 nm) to predominantly bidentate complexed salicylate (⬃383 nm) at longer times. Because of the 15-nm red shift in the excitation band maximum of Al:A⫹ compared to HA⫺ (Ainsworth et al., 1998), excitation at 334 nm only excites the Al:A⫹ complex, which produces timeevolving fluorescence spectra dominated by the growth of the bidentate Al:A⫹ complex (Fig. 4). By performing the same experiments at pH 2.0, time-evolving fluorescence spectra of the monodentate species can be isolated (Fig. 5). The ability to isolate this species arises from the observations that (1) in the absence of Al3⫹ 90% of salicylate is in the weakly emitting protonated salicylic acid form at pH 2.0 and (2) that the fluorescence intensity of Al–salicylate

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Fig. 5. Emission spectra of Al3⫹–salicylate(aq) as a function of reaction time at pH 2.0. [Al3⫹] ⫽ 0.002 mol/L, [Sal] ⫽ 2 ⫻ 10⫺5 M. ␭ex ⫽ 300 nm. Time range: 0 – 4 min.

complexes in such low pH Al3⫹ solutions are many times more intense than in the absence of Al3⫹ (Ainsworth et al., 1998). Therefore, the growth of fluorescence intensity in Figure 5 is assigned to the formation of an aluminum complex that binds the salicylate ion through the carboxylate functional group (nominally monodentate), but still allows an intramolecular H-bond from the phenol group to the carbonyl group of the carboxylate moiety. As explained in Ainsworth et al. (1998), the evidence for this intramolecular H-bond is the large energy loss between the excitation maximum (⬃300 nm) and the fluorescence maximum (⬃410 – 415 nm). This energy difference is due to proton transfer from the phenol group to the carbonyl group in the excited electronic state (excited state intramolecular proton transfer, ESIPT) (Gormin and Kasha, 1988; Nagaoka et al., 1988; Barbara et al., 1989). An intramolecular H-bond between the phenol (H-donor) and the carbonyl of the ortho-carboxylate group (H-acceptor) is required for ESIPT to occur. Monitoring the kinetics at fixed excitation and emission wavelengths produced Al:A⫹ and Al 䡠 HA2⫹ growth kinetic curves I(t) of high signal-to-noise ratio. For Al:A⫹ excitation at 334 nm and monitoring the emission at 383 nm produces a direct observation of the Al:A⫹ complex growth (Fig. 4). Similarly, at low pH the formation of the Al 䡠 HA2⫹ monodentate complex may be followed by excitation at 300 nm and monitoring of the emission at 412 nm. A plot of log ([sal]t ⫺ [sal]e) at a function of time t yields straight lines with the pseudo-first order rate constants of 0.040 s⫺1 (Al:A⫹ at pH 3.3) and 0.009 s⫺1 (Al 䡠 HA2⫹ at pH 2) (Fig. 6). An important

feature of the aqueous reaction kinetics at both pH 2 and 3.3 is that both are pseudo-first order (i.e., second order) over 1.5 decades of intensity data. In the aqueous system there is no evidence of competing reaction mechanisms over this time range. Although the reaction between aqueous Al and salicylate has been studied previously (Secco and Venturini, 1975; Rakotonarivo et al., 1989), these studies relied on detection of UV absorption at 310 nm and calculations of intermediate step rates rather than direct detection of the monodentate intermediate species. They characterized the reaction path for the Al–salicylate reaction as an Eigen-Wilkens-Werner mechanism followed by the elimination of H3O⫹ and ring closure (Secco and Venturini, 1975; Rakotonarivo et al., 1989): (H2O)6Al3⫹ ⫹ HA⫺ N [(H2O)5 Al(H2O) ⫺ HA]2⫹

(17)

[(H2O)5Al(H2O) ⫺ HA]2⫹ N [H2O)5AlHA]2⫹ ⫹ H2O

(18)

[(H2O)5AlHA]2⫹ N [(H2O)4AlA]⫹1 ⫹ H3O⫹

(19)

The elimination of water from the inner sphere coordination sphere of Al(III) (Eqn. 18) has been suggested to control the reaction rate for the formation of the bidentate product rather than potential steric factors associated with ring closure (Eqn. 19) (Secco and Venturini, 1975). The present results compare favorably with these previously published results on the Al– 3⫹ salicylate reaction (if differences in the Al(aq) concentrations and the solution acidity are taken into account) and directly confirm the formation of a monodentate intermediate. That is,

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Fig. 6. Pseudo-first order kinetics of Al3⫹ complexation with salicylate as a function of reaction time at pH 3.3 (a) and pH 2.0. (b) [Al3⫹] ⫽ 2 ⫻ 10⫺3 mol/L, [Sal] ⫽ 2 ⫻ 10⫺5 mol/L, IS ⫽ 0.01 mol/L. At pH 2.0, ␭ex ⫽ 310 nm, ␭em ⫽ 410 nm. At pH 3.3, ␭ex ⫽ 334 nm, ␭em ⫽ 383 nm.

the results of the current study show that formation of the Al 䡠 AH species is the slow step in the aqueous reaction and hence follows an Eigen-Wilkens-Werner mechanism. It was noticed that although the formation of the monodentate complex is the rate-determining step, at elevated pH (e.g., pH 3.3 in this study) the overall reaction rate for the formation of the bidentate complex is actually greater than the rate of formation of the monodentate complex. Similar results were also obtained by Secco and Venturini (1975) and PerlmutterHayman and Tapuhi (1977, 1979) for Al(III) reaction with substituted salicylates, and Rakotonarivo et al. (1989) for salicylate complexation with hydrolysed Al(III). The increased reaction rate can be explained by reaction between salicylate and hydrolysed Al(III), such as AlOH2⫹:

(Secco and Venturini, 1975; Perlmutter-Hayman and Tapuhi, 1977, 1979; Phillips et al., 1997b, 1998). From calculations based on the available hydrolysis data of Al(III) (Martell and Smith, 1995), at pH 3.3 the equilibrium concentration of hydrolyzed Al(III), such as (H2O)5AlOH2⫹, is about 0.3% of the total Al(III) concentration, which is comparable to the salicylate concentration considering the large [Al(III)]:[sal] ratio in the solutions.

(H2O)5AlOH2⫹ ⫹ HA⫺ N [(H2O)4AlOH(H2O) ⫺ HA]1⫹

2⫺i s ⫺ Al(OH)i(OH2)(2⫺i)⫹ ⫹ HA⫺ 3 s ⫺ Al:A ⫹ (2 ⫺ i)H2O

(20) [(H2O)4 AlOH(H2O) ⫺ HA]1⫹ N [(H2O)4AlOH ⫺ HA]1⫹ ⫹ H2 O [(H2O)4AlOH ⫺ HA]1⫹ N [H2O)4AlA]1⫹ ⫹ H2O

(21) (22)

In these reactions removal of H2O from the inner sphere of Al(III) (Eqn. 21) is the rate-determining step, following the Eigen-Wilkens-Werner mechanism, but the hydroxyl group in the inner sphere of Al(III) reduces the strength of the Al–OH⫹ 2 bond and results in increased reaction rate with salicylate

3.2. Reaction With Colloidal Alumina Figure 7 shows the temporal evolution of the fluorescence spectra (␭ex ⫽ 334 nm) from the heterogeneous salicylate reaction

⫹ iOH⫺ ⫹ H⫹

i ⫽ 0,1,2

(23)

where s- represents the solid and i is the stoichiometric number of nonprotonated hydroxyl sites (0, 1, or 2). The study was conducted at approximately one order of magnitude lower salicylate concentration than in the aqueous solution reaction, and the ratio of the total number of surface sites to salicylate was about 1000:1 (Ainsworth et al., 1998). The spectra obtained from excitation at 334 nm evolve in a manner similar to the aqueous reaction (Fig. 4). As expected only the surface bidentate species (383 nm) is observed. However, excitation of

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Fig. 7. Emission spectra of salicylate in 1 g/L alumina suspension as a function of reaction time at pH 6.0. [Sal] ⫽ 2 ⫻ 10⫺6 mol/L. IS ⫽ 0.01 mol/L. ␭ex ⫽ 334 nm. Time range: 0 –2 min.

this reacting system at 300 nm reveals a spectral evolution that appears to differ from its aqueous analogue in three important aspects (compare Fig. 8 to Fig. 3): (1) the early phase of this reaction appears more rapid, (2) the asymptotic approach to the product (383 nm) appears slower (note that after about 60 s the 383-nm emission has yet to dominate the spectra as in Fig. 3), and (3) the initial spectrum is significantly red-shifted (⬃423 nm) relative to free salicylate (412 nm). The later point suggests that an intermediate (⬃423 nm) is rapidly formed at this higher pH and evolves toward the bidentate product. The intermediate is likely to be the monodentate surface complex in which the salicylate phenolic hydrogen is held in an intramolecular hydrogen bond to the salicylate carbonyl oxygen, whereas the phenolic oxygen is hydrogen bonded to the proton of a neighboring surface–aluminol or hydronium group (Fig. 1b). This would preserve the intramolecular H bond, which is responsible for the ESIPT emission above 400 nm. The electrostatic field of the phenol-to-surface intermolecular hydrogen bond could stabilize the phenoxide form of the ESIPT excited state, leading to a red shift of the fluorescence relative to a complex with a free or unpolarized phenolic oxygen. Importantly, in our earlier equilibrium study of salicylate surface speciation, identification of the transient monodentate carboxylate surface complex was ambiguous and beyond the limit of spectral resolution (Ainsworth et al., 1998). Although in the present study we can observe the evolution of the monodentate surface species, we cannot adequately isolate

its growth and subsequent disappearance in a manner that would allow the calculation of separate rates for the formation of Al 䡠 HA2⫹ and Al:A⫹. Rakotonarivo et al. (1989) investigated the rate of aqueous salicylate complexation with the Al3⫹ monomer and Al13 polycation and concluded that the reaction of salicylate with the polycation was faster than that with monomers. However, their stopped-flow system used UV detection of absorption at 310 nm. At this wavelength the growth of the monodentate (if present as an intermediate in their system) and the final bidentate product would be detected as a single species. Hence, it is unclear as to what was being monitored in that study. However, the current results suggest that the reaction path is similar to the aqueous reaction, in that a monodentate intermediate complex is formed, and supports the conclusion (at least for the initial stage) that the surface complexation reaction is faster than its aqueous counterpart. In addition, surface formation of the Al 䡠 HA2⫹ monodentate intermediate through the elimination of H2O (Eqn. 25) does not appear to be the rate-limiting step under the conditions of this study. Rather the slow step appears to be ring closure (Eqn. 26). s ⫺ Al(OH)(OH2)⫹ ⫹ HA⫺ N [(s ⫺ Al(OH)(OH2) ⫺ HA] (24) [(s ⫺ Al(OH)(OH2) ⫺ HA] N [(s ⫺ Al(OH) ⫺ HA] ⫹ H2O (25)

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Fig. 8. Emission spectra of salicylate in 1 g/L alumina suspension as a function of reaction time at pH 6.0. [Sal] ⫽ 2 ⫻ 10⫺6 mol/L. IS ⫽ 0.01 mol/L. ␭ex ⫽ 300 nm. Time range: 0 –2 min (reaction is not completed yet).

[(s ⫺ Al(OH) ⫺ HA] N [(s ⫺ Al:A] ⫹ H2O

(26)

To simplify the equations (Eqns. 24 –26), the number of nonprotonated hydroxyl sites (i in Eqn. 23) was assumed to be one. It should also be pointed out that there are significant differences between the reactions of a ligand with Al(III) in homogeneous phase and with the solid–water interface. In light of the fact that in ligand substitution reactions the leaving group lability determines the reaction rate, hydrolysis, or complexation of Al(H2O)3⫹ will enhance the rate of additional com6 plexation because of the weakening of Al(III) binding to the remaining inner hydration sphere water molecules (Secco and Venturini, 1975; Perlmutter-Hatman and Tapuhi, 1977, 1979; Phillips et al., 1997b). However, this effect is less likely for reactions on the surface where there are no more inner sphere water molecules to be replaced except those hydronium or hydroxyl groups, which themselves will be directly involved in the ligand substitution reactions (Eqn. 23). Kinetic growth curves of salicylate surface complexes in colloidal alumina suspensions were obtained by excitation at 334 nm and viewing emission at 380 nm (Fig. 9). Plots of log ([sal]t ⫺ [sal]e) vs. time for salicylate reaction with alumina suspensions were pseudo-first order only after approximately 30 s, accounting for ⬇30% of the total reaction (Fig. 10), depending on pH and ionic strength. As can be seen in Figure 10, the rapid early portion of the reaction accounts for ⬇70% of the total reaction and cannot be fit to a pseudo-first order rate law. Indeed, this data cannot be fit reasonably to any simple

combination of first or second order or two- or three-constant rate laws. However, this initial rapid reaction is fit easily to the integrated form of the Elovich rate equation (Low, 1960; McLintock, 1967; Hingston, 1981; Ungarish and Aharoni, 1981; Sposito, 1994; Aharoni et al., 1991) over a wide range of pH, ionic strengths, and initial salicylate concentrations. In the Elovich rate law, the rate of adsorption dq/dt, which we take to be the rate of formation of surface complexes, exponentially decreases with increasing coverage q: dq/dt ⫽ a 䡠 exp(⫺␣q)

(27)

where “a” is the reaction rate at zero coverage, or, in practice, at the beginning of the measurement. The coverage scale factor ␣ is the reciprocal of the coverage q1/e at which the adsorption rate has fallen to 1/e of its initial value. (dq/dt)l/e ⫽ a 䡠 exp(⫺␣ql/e) ⫽ a/e

(28)

The integrated form of the Elovich equation is q(t) ⫽ (2.303/␣)log(1 ⫹ t/t0)

(29)

where the fitting constant t0 ⫽ (a␣)⫺1 is the time at which the surface coverage q(t0) has increased to ␣⫺1ln2 ⫽ q1/eln2. An example of fitting the early time growth of the surface complex [Al:A] to the integrated Elovich rate law is shown in Figure 11, which plots the concentration of salicylate adsorbed to the surface [Al:sal]t (equivalent to the surface coverage q(t)) vs.

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Fig. 9. Fluorescence intensity of salicylate in 1 g/L alumina suspension as a function of reaction time at pH 6.0. [Sal] ⫽ 8 ⫻ 10⫺6 mol/L. IS ⫽ 0.01 mol/L. ␭ex ⫽ 334 nm. ␭em ⫽ 383 nm. The dashed line is the asymptotic limit (see text for details).

ln(1 ⫹ t/t0). The fitting parameter t0 is adjusted to achieve the best linear fit to the data and the slope is ␣. The Elovich equation was developed for and is widely used in describing the kinetics of heterogeneous chemisorption of gases on solid surfaces (Low, 1960; Adamson, 1990). The equation assumes a heterogeneous distribution of adsorption or activation energies that vary continuously with surface coverage. It has been used successfully to describe the adsorption or desorption kinetics of phosphate (and other ions) to soils and soil minerals (Chien and Clayton, 1980; Hingston, 1981; also see Sparks, 1989, and references therein). As discussed by Sparks (1989) several investigators have used changes or multilinear segments in the Elovich plot as an indication of multisite binding or shifts for one site type to another. However, concerns have been raised regarding the use of the Elovich equation to conclude mechanistic information concerning multiple sites, changes in adsorption energies with surface coverage, or site heterogeneity (Sposito, 1994; Aharoni and Sparks, 1991; Stumm, 1992). Yet under conditions that the binding energy or reaction activation energy varies significantly with site heterogeneity, the observed reaction rate constant may evolve continuously with increasing surface coverage, hence the usefulness of the Elovich equation. In the present study it is extremely useful as a characterization of rate data and as a way

of bounding the extent of the adsorption reaction described by a pseudo-first order rate law. The variation of the Elovich parameters (a,␣) and the pseudo-first order rate constant kobsd with respect to [sal]tot, pH, and IS were studied to explore the relationship between these experimental conditions and both the fast reaction and slower reactions. In every case the kinetics could be accurately fit at long times to a single exponential (pseudo-first order) rate constant k and at short times to an integrated Elovich rate law. The pseudo-first order rate constant (4 ⫾ 0.3 ⫻ 10⫺3 s⫺1) is practically independent of [sal]tot, and hence the rate of this reaction increases only slightly with concentration (Table 1). However, the initial rate (or instantaneous rate at t ⫽ 0) for the Elovichian part of the reaction (defined by the “a” term) increases with [sal]tot. As [sal]tot increases from 1 to 16 ␮mol/L, the portion or extent of the reaction described as pseudo-first order increases from 16% to 28%. The first-order rate constant is also invariant with pH and IS, yet the extent of its contribution to the overall reaction increases as pH and IS increase (Table 2). The Elovich initial reaction rate, however, is affected substantially by changes in pH and IS. The pH-induced changes in the rates, rate constants, and extent of reaction for the two parts of the overall reaction appear to reflect changes in the surface charge and speciation at

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1169

Fig. 10. Pseudo-first order plot of salicylate adsorption kinetics from Fig. 9 at reaction time ⬎ 30 s and approximately 70% of the adsorption.

the aqueous–alumina interface with pH. The results are consistent with a sorption reaction mechanism that is controlled by the leaving group lability at the surface sites (Al–OH⫹ 2 and Al–OH). The distribution of surface sites with pH, based on the triple-layer model (Davis et al., 1978; Davis and Leckie, 1978) was calculated using FITEQL (Westall, 1982). For the purpose of these calculations the total concentration of surface sites and density are 1.3 ⫻ 10⫺3 mol/L and 8 sites/nm2 (surface area). Surface acidity constants and zero point charge (ZPC) of the solid are as reported in Ainsworth et al. (1998). The concentration of the total (⬎Al–OH⫹ 2 ) sites are calculated to be 2.23 ⫻ 10⫺4, 1.05 ⫻ 10⫺4, and 1.95 ⫻ 10⫺5 mol/L at pH 4, 6, and 8, respectively. Likewise, the concentration of (⬎Al–OH) sites over this same pH range are calculated to be 1.1 ⫻ 10⫺3, 1.23 ⫻ 10⫺3, and 1.32 ⫻ 10⫺3 mol/L. At pH 4 the (⬎A–OH⫹ 2 ) site concentration is about 100-fold greater than [sal]tot. However, this excess decreases to slightly greater than equal molar concentrations at pH 8. Although the protonated sites are expected to be the most active salicylate adsorption sites, clearly the concentration of these sites affects the reaction rate. On the other hand, the concentration of the neutral (⬎Al–OH) sites remain almost constant and well in excess of [sal]tot. Larger concentrations of hydronium sites, generated by either lowering the suspension pH or increasing the alumina solid to salicylate ratio, will favor the reaction that is expressed by the Elovich

pathway, which is typically faster than reaction following the pseudo-first order pathway. These results suggest that there are multiple sites active in salicylate adsorption. Conceptually, we view the protonated aluminol sites as good candidates for the rapid Elovich pathway s ⫺ Al(OH)(OH2)⫹ ⫹ HA⫺ 3 s ⫺ Al:A ⫹ 2H2O

(30)

s ⫺ Al(OH2)22⫹ ⫹ HA⫺ 3 s ⫺ Al:A ⫹ 2H2O ⫹ H⫹

(31)

or

because protonation of the hydroxyls will increase both the substitutional lability of the surface aluminol bond and the variation of its binding (and activation energy) at heterogeneously distributed sites on the alumina surface. The pseudofirst order rate constant varies little with initial salicylate concentration, ionic strength, or pH ⬎ 4. A plausible interpretation for the slow pseudo-first order reaction pathway involves ligand exchange between salicylate and the nonprotonated surface hydroxyl groups s ⫺ Al(OH)2 ⫹ HA⫺ 3 s ⫺ Al:A ⫹ H2O ⫹ OH⫺

(32)

for which the Al–OH binding and activation energy are much higher than those of Al–OH⫹ 2 . We hypothesize that the much less labile Al–OH bond is less affected by site heterogeneity or charge density as compared with the Al–OH⫹ 2 bond. In addi-

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Fig. 11. Elovich plot of salicylate adsorption kinetics from Fig. 9 at reaction time ⬍ 30 s. In the data fit, t0 ⫽ 0.16 s; ␣ ⫽ 1.06 䡠 10⫺6 mol/L s⫺1.

the rate at zero surface coverage. Results in Table 2 show that the initial rate decreases as pH increases from 4.0 to 8.0 and as ionic strength increases from 0.01 to 1.0 mol/L. Electrostatic attraction between cations and anions is responsible for the formation of an outer sphere complex, which is the first step in the Eigen-Wilkens-Werner mechanism (Eqn. 17). Conversion of hydronium sites to hydroxyl sites decreases the density of the cation sites on the alumina surface that may cause the initial reaction rate to decrease. At a fixed pH, the density of hydronium sites is relatively constant with respect to ionic strength. However, an increase of ionic strength results in (1) a shrinking of the electric double layer and (2) local charge neutralization on the solid surface due to competition with perchlorate anion. Both of these effects lead to reduced electrostatic interaction between the surface hydronium sites and salicylate anion and reduce the surface concentration of the outer sphere complex, thus leading to the reduction of initial reaction rate.

tion, as the results of the surface site speciation calculations show, the (⬎Al–OH) sites are in enormous excess under the experimental conditions of this study. Thus, there appear to be two parallel (competing) reaction paths for the formation of the bidentate salicylate species at the alumina–water interface. The major pathway is rapid and follows Elovich kinetics, and the minor pathway is much slower following second order (pseudo-first order) kinetics. Such interpretations are supported by the calculated extent of reaction (Table 2). As pH increases from 4.0 to 8.0, the extent of reaction following the Elovich rate equation decreases from 98% to 68%, which appears to be a direct consequence of the conversion of the Al–OH⫹ 2 sites to Al–OH sites as pH increases. Although the pseudo-first order rate constant changes little, the initial reaction rate a for the fast Elovich pathway showed a profound change within the pH and ionic strength range explored. In the Elovich equation, the initial reaction rate a is

Table 1. Elovichian and pseudo-first order kinetic data for salicylate adsorption on alumina at different initial salicylate concentration.a [Sal]Total (10⫺6 mol/L) 1 2 4 8 16

[(Sal)]Eb (10⫺6 mol/L)

[(Sal)]E/[Sal]Total (%)

t0 (s)

␣ (106 L mol⫺1)

a (10⫺6 mol L⫺1 s⫺1)

kobsd (䡠10⫺2 s⫺1)

0.837 1.584 3.075 5.638 11.46

83.7 79.2 76.9 71.2 71.6

0.16 0.28 0.15 0.16 0.09

7.57 3.26 1.88 1.06 0.54

0.83 1.10 3.54 5.87 20.8

0.38 0.43 0.39 0.35 0.41

a pH 6.0, I ⫽ 0.01 mol/L, 25°C, 1 g/L alumina suspension; the error limits on t0 ␣ and a are estmated to be ⱕ10%; k error limits are estimated to be ⱕ4%. b [(Sal)]E ⫽ the adsorbed salicylate concentration described by the Elovich equation.

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Table 2. Elovichian and pseudo-first order kinetic data for salicylate adsorption ([Sal]Total ⫽ 2 ⫻ 10⫺6 mol/L) on alumina at different pH and ionic strength.a

pH 4.0 6.0 8.0 IS (mol/L) 0.01 0.10 1.00 a b

[(Sal)]E (10⫺6 mol/L)

[(Sal)]E/[Sal]Totalb (%)

t0 (s)

␣ (106 L mol⫺1)

a (10⫺6 mol L⫺1 s⫺1)

kobsd (10⫺2 s⫺1)

1.96 1.58 1.37

98.0 79.2 68.2

0.00 0.28 1.55

3.58 3.26 2.64

— 1.10 0.25

— 0.43 0.47

1.58 1.59 1.44

79.2 79.7 72.0

0.28 0.35 1.50

3.26 3.01 3.183

1.10 0.948 0.203

0.43 0.42 0.43

At 25°C, 1 g/L alumina suspension; the error limits on T0, ␣ and a are estimated to be ⱕ10%; k error limits are estimated to be ⱕ4%. [(Sal)]E ⫽ the adsorbed Sal-conc. described by the Elovich equation.

or

4. SUMMARY AND CONCLUSIONS

O-salicylate anion adsorption kinetics were investigated by stopped-flow with fluorescence detection over a range in pH, IS, and [sal]tot. The results of these studies indicate that the adsorption reaction path has both similarities to and differences from the path deduced for the homogeneous reaction of aqueous Al3⫹ and salicylate. The aqueous reaction is characterized as an Eigen-Wilkens-Werner mechanism (Eqns. 17–19), and the slow step is the elimination of H2O in the formation of an intermediate monodentate species. Although the surface reaction does form a monodentate carboxylate complex at the surface followed by ring closure, the slow step of this process appears to be ring closure rather than the monodentate species formation. This is seen clearly in the transient build up of the 423-nm species, and its subsequent conversion to the 383-nm bidentate surface species. Although we did not observe the formation kinetics of the outer sphere complex in alumina suspension or the aqueous systems, the presence of an outer sphere complex has been indicated (albeit at higher salicylate concentrations) in equilibrated salicylate–alumina systems (Ainsworth et al., 1998). Hence, we conclude that the major product of salicylate adsorption is a bidentate Al:A complex whose formation closely follows that observed for aqueous Al complexation mechanism: (1) formation of an outer sphere complex, (2) formation of an inner sphere monodentate carboxylate complex accompanied by the loss of H2O, and (3) followed by ring closure and the formation of bidentate complex. However the monodentate complex is stabilized through the interaction between the phenolic oxygen and adjacent aluminol groups, making ring closure the slow step in the adsorption process rather than monodentate formation as in aqueous solution. The kinetics of salicylate adsorption is dependent on surface site speciation. Conceptually, we view the adsorption of salicylate to alumina to consist of two competing (parallel) reactions. The rapid pathway is sensitive to surface site heterogeneity (the protonated sites) producing Elovich kinetic growth. The slower second order reaction is pseudo-first order under the conditions of low surface coverage (surface aluminol in excess over the sorbate concentration). These two pathways are represented by the overall reactions: Rapid Elovich: s ⫺ Al(OH)(OH2)⫹ ⫹ HA⫺ 3 s ⫺ Al:A ⫹ 2H2O

(33)

s ⫺ Al(OH2)22⫹ ⫹ HA⫺ 3 s ⫺ Al:A ⫹ 2H2O ⫹ H⫹

(34)

Slower pseudo first order: s ⫺ Al(OH)2 ⫹ HA⫺ 3 s ⫺ Al:A ⫹ H2O ⫹ OH⫺

(35)

In this chemical reaction scheme, transient intermediates may form (such as the monodentate complex suggested by the spectra excited at 300 nm) but are not shown in the overall reactions. The proposed mechanism suggests the hypothesis that site heterogeneity has a significant influence on lability and activation energy of the protonated sites Al–OH⫹ 2 , which results in rapid Elovich kinetics over those sites, whereas the lability and activation energy of the neutral aluminol sites Al–OH are so little affected by surface site heterogeneity that the ligand substitution reaction on the colloidal particles is simply second order with a single pseudo-first order rate constant. We are currently studying the role of activation heterogeneity in producing Elovich-like kinetic behavior. Acknowledgments—The authors thank Dr. Donald M. Camaioni for a loan of the stopped-flow kinetics apparatus and Dr. Stephan Joyce for helpful comments regarding the interpretation of the Elovich rate law. Also the authors wish to gratefully acknowledge an anonymous reviewer for his/her helpful comments, careful review of the data analysis, and constructive criticism that greatly contributed to improving this manuscript. This research was supported by the Molecular Science Research Initiative at Pacific Northwest National Laboratory. Pacific Northwest National Laboratory is operated for the U.S. Department of Energy by Battelle under Contract DE-AC06-76RLO 1830. REFERENCES Adamson A. W. (1990) In Physical Chemistry of Surfaces, 5th ed., chapter XIIV, Chemisorption catalysis, pp. 699 –703. Wiley Interscience. Aharoni C. and Sparks D. L. (1991) Kinetics of soil chemical reactions—A theoretical treatment. In Rates of Soil Chemical Processes (ed. D. L. Sparks), SSSA special publication, no. 27. Soil Sciences Society of America. Aharoni C., Sparks D. L., Levinson S., and Israela R. (1991) Kinetics of soil chemical reactions: Relationships between empirical equations and diffusion models. Soil Sci. Soc. Am. J. 55, 1307–1312. Ainsworth C. C., Sumner M. E., and Hurst V. J. (1985) Effect of aluminum substitution in goethite on phosphorus adsorption: I. Adsorption and isotopic exchange. Soil Sci. Soc. Amer. 49, 1142–1153. Ainsworth C. C., Friedrich D. M., Gassman P. L., Wang Z., and Joly

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