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Langmuir 1999, 15, 2316-2321
Complexation of Methylphosphonic Acid with the Surface of Goethite Particles in Aqueous Solution Beatriz C. Barja,† M. Isabel Tejedor-Tejedor,* and Marc A. Anderson Water Chemistry Program, University of Wisconsin, 660 North Park Street, Madison, Wisconsin 53706 Received May 7, 1998. In Final Form: January 12, 1999 Adsorption of methylphosphonic acid (MPA) onto goethite (R- FeOOH) was studied as a function of pH and the concentration of MPA in solution. In situ attenuated total reflectance-FTIR (ATR-FTIR) techniques were used to identify the type of surface complexes formed under each of the solution conditions used in the adsorption studies. Additionally, electrophoretic mobility measurements were performed in each of these systems. At low pH and high Γ, MPA was bound to the surface of goethite predominantly as a monodentate protonated species, while at high pH and low Γ the prevailing structure was that of a bridging bidentate complex. The change in denticity of the surface MPA under different solution conditions may explain the charge reversals shown in the electrophoretic mobility curves.
1. Introduction Methylphosphonic acid (MPA) is one terminal moiety of the more complex molecule N-phosphonomethylglycine (or glyphosate). The latter compound is widely used as a broad spectrum and postemergent herbicide.1 It is rapidly immobilized upon contact with soil,2,3 which indicates adsorption onto soil components.4 In aqueous solutions, Motekaitis et al.5 already reported the formation of complexes of divalent and trivalent metal ions with glyphosate and related ligands (iminodiacetic and iminodimethylenephoshonic acids) and determined their stability constants by potentiometric titration. McBride et al.6 also studied the complexation of Fe(III) with glyphosate, iminodiacetic acid, aminomethylphosphonic acid, and glycine by ESR and UV-vis spectroscopy. The results of their investigation showed that glyphosate and aminomethylphosphonic acid form sufficiently strong complexes with this ion to at least suppress the metal hydrolysis and precipitation in the pH range 2-4. Moreover, the above-mentioned studies as well as other literature data7,8 conclude that those ligands containing the phosphonate moiety in their structure are capable of forming stronger complexes with most metals. Glyphosate adsorption onto goethite was studied by transmission infrared spectroscopy measurements on dry films of the oxide.6 Although strong bands appearing in the infrared frequency range of phosphonate provided some evidence of surface complexation, no band assignments or tentative structures were reported for these interfacial iron-glyphosate complexes. In fact, the complexity of the glyphosate spectrum makes it difficult to assign spectral bands to the vibrational modes conclu* To whom correspondence should be addressed. † INQUIMAE, Facultad de Ciencias Exactas y Naturales, Universidad de Buenos Aires, Ciudad Universitaria. Pabellon II, 1428 Buenos Aires, Argentina. (1) Franz, J. E.; Mao, M. K.; Sikorski, J. A. Glyphosate: A unique global herbicide; ACS Monograph 189; American Chemical Society: Washington, DC, 1997. (2) Hensley, D. L.; Beuerman, D. S. N.; Carpenter, P. L. Weed Res. 1978, 18, 287. (3) Sprankle, P.; Megitt, W. F.; Penner, D. Weed Sci. 1975, 23, 224. (4) Glass, R. L. J. Agric. Food Chem. 1987, 35, 497. (5) Motekaitis, R. J.; Martell, A. E. J. Coord. Chem. 1985, 14, 139. (6) McBride, M.; Kung King-Hsi. Soil Sci. Soc. Am. J. 1989, 53, 1668. (7) Wozniak, M.; Nowogrocki, G. Talanta 1979, 26, 1135. (8) Carter, R. P.; Carroll, R. L.; Irani, R. R. Inorg. Chem. 1967, 6 (5), 939.
sively. Changes in the spectral bands (number, broadening, or shifts) in the phosphonate adsorption region could be due not only to surface metal-ligand complexation but also to the different states of protonation of the adsorbed phosphonate itself as well as to changes in the structure of other functional groups present in the molecule. An investigation of the structure of interfacial metalglyphosate complexes using IR spectroscopy can greatly be aided by studying the structure of complexes of metal ions with smaller surrogated moieties of the glyphosate molecule. MPA would seem to be a good choice for simulating a moiety of glyphosate that only has the phosphonate group. In both molecules, MPA and glyphosate, the PO3H2 has the same group symmetry and therefore the absorption spectra are expected to have the same number of bands and profiles with similar relative intensities. In this paper, we study the complexation of MPA onto the surface of goethite in aqueous solution. Adsorption density studies were performed as a function of the pH and concentration of MPA in solution in order to evaluate the affinity of MPA for the oxide surface and also to characterize the systems studied by IR. The structures of the interfacial complexes of the MPA-goethite system in aqueous suspensions were examined using in situ cylindrical internal reflection-Fourier-transformed infrared (CIR-FTIR) spectroscopy9 as a function of pH and adsorption density. Moreover, we studied the charging characteristics of the MPA-goethite particles by measuring the electrophoretic mobilities on systems with the same adsorption density at different pH values. The approach in this paper will be similar to the one used to study the adsorption of phosphate on goethite published elsewhere.10 In comparing the results from both studies, one can learn how the substitution of an oxygen ligand (or OH) in the central atom, P, for a methyl group influences the adsorption strength and the structure of these surface complexes. 2. Experimental Section 2.1. Goethite Properties. Goethite was prepared by hydrolyzing ACS grade ferric nitrate with NaOH (Fe/OH ) 0.5).11 This solution was aged 2 days at room temperature and then titrated (9) Tejedor-Tejedor, M. I.; Anderson, M. A. Langmuir 1986, 2, 203. (10) Tejedor-Tejedor, M. I.; Anderson, M. A. Langmuir 1990, 6, 602. (11) Atkinson, R. J.; Posner, A. M.; Quirk, J. P. J. Inorg. Nucl. Chem. 1968, 30, 2371.
10.1021/la980540y CCC: $18.00 © 1999 American Chemical Society Published on Web 03/05/1999
Complexation of Methylphosphonic Acid to pH 12.5 with NaOH. The resulting precipitate was aged for 6 more days at 60 °C. The suspension was washed with Milli-Q water and dried in an oven at 40 °C. A surface area of 80.3 m2/g was obtained by N2 adsorption analysis. Stock suspensions of goethite (99.6 g/L) were prepared by resuspending the oxide in Milli-Q water and sonicating intermittently for 2 days to ensure hydration. 2.2. Adsorption Studies. Sample Preparation. Batch equilibrium adsorption experiments were conducted at 25 °C for 24 h. Prior to adding any methylphosphonic acid, aliquots of goethite suspensions (99.6 g/L) were brought to a fixed ionic strength of 0.01 M NaCl and desired pH conditions by using microliter quantities of 1 M NaCl and 0.01-1 M NaOH or HCl. These suspensions were then left to reach equilibrium for 24 h in a temperature-controlled shaker (New Brunswick). Next, a given number of milliliters of a 1.0 × 10-2 M MPA solution was added to vigorously stirred, pre-equilibrated goethite suspensions. The pH was readjusted until constant values were obtained within a few tenths of a pH unit. Prior to any analysis, the pH was measured again. Adsorption Isotherms. MPA adsorption was calculated from the difference between that of the added MPA and that measured in the supernatant after 24 h of equilibration. To separate the particles from bulk solution, samples were first centrifuged and then filtered through a 0.05 µm Nuclepore filter. The MPA concentrations in the supernatant were measured as elementary phosphorus by ICP (inductively coupled plasma) emission spectrometry with a Perkin Elmer Plasma 1000/2000 atomic emission spectrometer. 2.3. CIR-FTIR Analysis. Infrared spectra of both MPA aqueous solutions and MPA-goethite suspensions were recorded interferometrically with a Nicolet 60 SX Fourier transform infrared (FTIR) spectrometer and a HgCdTe (MCT) detector. The sampling method was attenuated total reflection with cylindrical optics (CIR) (Spectra Tech CIRCLE System) and a 39° ZnSe crystal rod. Spectral resolution was 4 cm-1 for all measurements. Single-beam IR spectra were the result of 2000 (MPA solutions) or 4000 (MPA-goethite suspensions) coadded interferograms. The system cutoff is near 800 cm-1 for work in water. All final spectra were the result of subtracting the spectrum of a reference (supernatant of a suspension or aqueous solutions having the ionic strength and pH equal to those of the corresponding sample) from the spectrum of the sample (suspensions or MPA in solution). In every instance, the empty cell was used as the background. Further description of this method is presented elsewhere.9 In the study of MPA dissolved in water, the sample composition was 0.1 M MPA and 1 M NaCl for the different values of pH, while 1 M NaCl solutions, matching the sample pH, were used as references. In these CIR studies, aliquots of MPA-goethite suspensions, prepared as described above, were used as samples (50 g/L), and the supernatant, obtained by centrifugation, was used as the corresponding reference. 2.4. Electrophoretic Mobility. The electrophoretic mobility measurements were performed with a PenKem system 3000 electrokinetics analyzer at 25 °C and constant ionic strength (0.01 M in NaCl). Suspensions of MPA-goethite with adsorption densities of 0, 50, 100, 130, and 140 µmol/g were the systems measured as a function of pH. Samples of 13.0 g/L of MPAgoethite were prepared as described in section 2.2. When equilibrium was reached, a small volume of these suspensions was diluted to a concentration of 0.15 g/L with their own supernatant.
3. Results and Discussion 3.1. Adsorption Studies. Results of measuring the adsorption densities of MPA on goethite at equilibrium with different MPA concentrations in solution and for different values of suspension pH are reported in the form of adsorption isotherms (Figure 1). These adsorption isotherms approximate a Langmuir shape for every pH studied. As expected, the value of the apparent adsorption maximum decreases as pH increases. The Langmuir constant (KL) and the maximum coverage (ΓM) of these isotherms were calculated using a nonlinear regression
Langmuir, Vol. 15, No. 7, 1999 2317
Figure 1. Adsorption isotherms of MPA adsorbed onto goethite at different pH values. Goethite concentrations are 13.3 g/L, and the ionic strength I ) 0.01 M in NaCl in all isotherms. Table 1. Maximum Adsorption Densities Γmax (µmol/g) and Langmuir Constants KL (L/µmol) of Methylphosphonic Acid Adsorbing onto Goethite at Several pH Values pH
ΓM (µmol/g)
K L (L/µmol)
3.5 5.0 6.5 7.5 9.0
182 168 156 142 84
0.33 0.15 0.13 0.09 0.01
fitting program (Solver, Excel 5.0), and the results of these calculations are reported in Table 1. As shown, values for KL change from 0.33 to 0.01 L/µmol as pH varies between 3.5 and 9.0. Values for ΓM vary from 182 to 85 µmol/g over this same pH range. When we repeated the isotherms at pH ) 3.5, for different batches of goethite having slightly different surface areas, KL values were nearly the same. In an earlier study performed in this laboratory involving the adsorption of aromatic carboxylates on goethite, KL values were determined to be 0.37 L/µmol for salicylic acid and 0.35 L/µmol for dehydroxybenzoate at pH ) 5.5.12 These two organic species form chelate complexes with the surface. On the other hand, weaker binding, nonchelating parahydroxybenzoate and benzoate had KL values of 0.002 and 0.0005 L/µmol (pH ) 5.5), respectively.12 In another study, we also showed that KL values for phosphate adsorption over almost the equivalent pH range (4.0-8.4) varied between 0.10 and 0.01 L/µmol.10 While the magnitude of the KL change is almost the same for the adsorption of phosphate and MPA, the change in ΓM values for phosphate is not as significant (213-146 µmol/g) as that for MPA. This represents a 30% decrease in surface coverage for phosphate but a 50% decrease in MPA adsorption over this same pH range. Table 1 also shows that the value for ∆KL/∆pH in the pH range 3.5-5.0 is five times larger than the one between pH 5.0 and 7.5, while for the former pH range the value for ∆ΓM/∆pH is slightly smaller. The lack of parallel response of ΓM (number of sites) and KL (affinity) to pH may suggest a change in the denticity of the MPA surface complexes. More specific information regarding the structure of these surface complexes will come from the CIRFTIR spectra of suspensions of goethite particles loaded with MPA. The spectra were measured on an array of systems having either different pH or adsorption density values. (12) Tejedor-Tejedor, M. I.; Yost, E. C.; Anderson, M. A. Langmuir 1992, 8, 525.
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Barja et al. Table 2. IR Frequencies and Band Assignments for Methhylphosphonic Acid Species in Aqueous Solution in the Range 1400-700 cm-1 IR frequency (cm-1) band assignment
CH3-PO32-
δ(CH3 ) ν(PdO) ν(PO2)
1304 (A1)
ν(PO3)
1055 (E) 979 (A1)
ν(PsOH) ν(PC)
CH3-PO3H1313 (A′)
CH3-PO3H2 1319 (A′) 1166 (A′)
1144 (A′′) 1058 (A ′) 922 (A′) 752
756
1006 (A′′) 950 (A′) 758
Table 3. MPA-Goethite Systems Examined by CIR-FTIR
Figure 2. CIR-FTIR spectra of 0.1 M MPA in aqueous solution at different pH values. For pH < 1 the spectrum was obtained by difference (pH 1.75 - pH 5.3) Scale: h1006(pH < 1) ) 0.021 au; h1058(pH ) 1.75) ) 0.014 au; h1058(pH ) 5.3) ) 0.045 au; h1055(pH ) 7.1) ) 0.057 au; h1055(pH ) 9.3) ) 0.094 au.
3.2. CIR-FTIR Spectroscopy. Methylphosphonic Acid in Solution. The infrared spectra of MPA in aqueous solution at several pH values are shown in Figure 2. Spectral changes are interpreted in terms of the acid base equilibria reactions of MPA:
CH3-PO3H2 S CH3-PO3H- S CH3-PO32where the pK1 and pK2 values are 2.19 and 7.55 in 0.1 M KNO3.13 According to these pKa values, the spectra in Figure 2 at the pH values 9.3, 5.3, and pH < 1 should be attributed to the totally deprotonated, monoprotonated, and diprotonated MPA in solution, respectively. At pH ) 9.3, the vibrational spectrum of the dibasic anion can be interpreted on the basis of a tetrahedral group having C3v symmetry which is lowered to Cs symmetry for the remaining pH values. To isolate the adsorption bands of the totally protonated acid (Figure 2, pH < 1), the spectrum at pH ) 5.3, multiplied by the quotient I1058 cm-1 pH 1.75/I1058 cm-1 pH 5.3, was subtracted from the one at pH ) 1.75. This approach was necessary, since at pH ) 1.75 there is still a significant amount of the monobasic MPA and a solution of pH < 1 would result in damage to the ZnSe crystal. At the intermediate pH values 1.75 and 7.1, the spectral bands correspond to the vibrations of two species in equilibrium. The band assignments are shown in Table 2. As was the case for other oxyanions in aqueous media,10,12 these assignments were made on the basis of differences in the (13) Wozniak, M.; Nowogrocki, G. Talanta 1979, 26, 381.
sample
pH
adsorption density (µmol/g)
sample
pH
adsorption density (µmol/g)
3.5-1 3.5-2 3.5-3 3.5-4 5.5-1 5.5-2 5.5-3 5.5-4 6.5-1 6.5-2
3.5 3.5 3.5 3.5 5.5 5.5 5.5 5.5 6.5 6.5
50.0 100.0 150.0 180.2 50.0 100.0 143.7 169.0 50.0 99.8
6.5-3 6.5-4 7.5-1 7.5-2 7.5-3 7.5-4 9.0-1 9.0-2 9.0-3 9.0-4
6.5 6.5 7.5 7.5 7.5 7.5 9.0 9.0 9.0 9.0
142.2 155.4 50.0 98.2 129.3 146.4 44.8 68.9 76.5 87.6
spectra of methylphosphonate for the three states of protonation, in conjunction with literature data on frequency values for CH3 and P-C groups. They are in agreement with those reported in the literature for the Raman and IR transmission spectra of MPA species.14 Methyphosphonic Acid on the Surface of Goethite. In the spectra of goethite suspensions, spectral features of interfacial species can be enhanced by subtracting the supernatant spectrum (bulk solution) from the suspension spectrum of each sample. These difference spectra contain absorption bands only of interfacial species and species in the bulk of the goethite.9 Table 3 describes the suspensions of goethite loaded with MPA studied by IR in terms of adsorption density and pH conditions. Figures 3-5 show difference spectra of some of the systems of Table 3. These figures only show the spectral region of MPA adsorption (900-1200 cm-1). Spectra in Figure 3 belong to suspensions at pH ) 3.5 with different quantities of adsorbed MPA. The set of spectra in each of the other two figures (4 and 5) represent systems that have the same adsorption density (50 and 146 µmol/g, respectively) but different pH values. Large differences in either the position or the number of bands can be observed when comparing the spectra of these three figures with the spectra of MPA in aqueous solution for the same pH value (Figure 2). Since, under the same pH conditions, the spectra of solution and interfacial MPA are different, it can be concluded that interfacial MPA is coordinated to the surface iron of goethite. In addition, Figures 3-5 show that the speciation of surface MPA varies with both pH and adsorption density. Figure 3 (suspension pH ) 3.5) illustrates changes in the structure of surface species as a function of adsorption density. A qualitative examination of the spectra suggests that they could be the result of combining two sets of bands, each representing a different surface complex. Spectral changes induced by pH are shown in Figures 4 and 5 for low and high MPA adsorption density, respectively. Again, inspection of the spectra in these figures provides evidence (14) Van Der Veken, B. J.; Herman, M. A. J. Mol. Struct. 1973, 15, 225.
Complexation of Methylphosphonic Acid
Figure 3. CIR-FTIR spectra of different amounts of MPA adsorbed onto goethite at pH ) 3.5 and I ) 0.01 M in NaCl. Goethite suspensions are ∼50 g/L. Scale: h1099 ) 0.0027 au; h1102 ) 0.0069 au; h1105 ) 0.0068 au; h1108 ) 0.0046 au.
that low pH and high MPA surface coverages favor the formation of a MPA surface species with absorption bands at 971, 991, and 1108 cm-1. On the contrary, higher pH values and low MPA surface coverages favor the formation of surface species with absorption near 984, 1015, and 1095 cm-1. Spectra of Figures 3-5 were deconvoluted into several peaks using the Nicolet Peaksolver program. Typical results for the deconvolution of these spectra are shown in Figure 6. Spectra of suspensions with higher Γ and lower pH values resemble that of Figure 6a (Γ ) 180.2 µmol/g and pH ) 3.5). In contrast, spectra of suspensions with lower Γ and higher pH values are more similar to the one of Figure 6b (Γ ) 50 µmol/g and pH ) 7.5). All the spectra in Figures 3-5, except the ones associated with suspensions of pH 9.0 or higher, can be described as a linear combination of the spectra in parts a and b of Figure 6. This claim is illustrated in Figure 6d using the spectrum of a suspension of pH 5.5 and Γ ) 100 µmol /g. In summary, the mathematical analysis of spectra of MPA-loaded goethite suspensions, having various pH values and adsorption densities, shows that, below pH 9, MPA forms two types of complexes on the surface of goethite. One of the complexes absorbs at 974, 992, and 1108 cm-1 and is associated with low pH and high Γ values; the other absorbs at 983, 1011, and 1095 cm-1 and is the prevailing species at high pH and low Γ. Band Assignment. Next, we attempted to assign these observed bands to vibrational modes of MPA surface complexes in order to obtain information on their structure. Normal coordination analysis for surface MPA is out of the question, since several of its vibrational modes cannot be experimentally observed and, furthermore, there is not enough information concerning the geometry of these complexes. Therefore, in this pursuit we will be guided by
Langmuir, Vol. 15, No. 7, 1999 2319
Figure 4. CIR-FTIR spectra of adsorbed MPA onto goethite for Γ ) 50 µmol/g at different pH values. Goethite suspensions are ∼50 g/L and I ) 0.01 M in NaCl. Scale: h1099(pH ) 3.5) ) 0.0027 au; h1096(pH ) 5.5) ) 0.00516 au; h1096(pH ) 6.5) ) 0.0020 au; h1095(pH ) 7.5) ) 0.0025 au; h1093(pH ) 9.0) ) 0.0019 au; h1091(pH ) 10) ) 0.0010 au.
band assignments for MPA in aqueous solution, in the three stages of protonation as already shown above. The main differences between the spectra of MeP(OH)nO3-n and MeP(OFe)nO3-n complexes are associated with the covalent character of the O-X bond. Since the electron density of a P-OFe bond is higher than that for a P-OH bond, the frequency value of the P-OFe vibration should be higher than that for the P-OH mode. Furthermore, frequency values for the PO3-n vibrations (n ) 1 or 2) should be lower in the Fe-containing complexes. We also assumed that the same symmetry for the MPA in solution and the complexed MPA on the surface of goethite should produce similar spectral profiles. These profiles are related to the number of bands and their relative positions. The discussion of the structure of interfacial MPA will be performed on the basis of these considerations. In principle, there exist three possible structures for surface complexes of MPA on goethite: a protonated monodentate complex, (FeO)PO(OH)(CH3)(x-1) (type I), a bridging bidentate complex, (FeO)2PO(CH3)2 (x-1) (type II), and a deprotonated monodentate complex, (FeO)(PO2)(CH3)(x-2) (type III) (x is the residual surface charge of the metal ions). Out of those three possibilities, the spectrum of type I will exhibit the highest frequency value, ν(PdO), due to the concurrent presence in the structure of PdO and OH groups. The spectrum of this complex will exhibit two more bands at lower frequency values; the one with the highest value is associated with ν(PsOFe), and the lowest frequency one is associated with ν(PsOH). The ∆ν between the latter vibrations should be much smaller than the ∆ν between the ν(PdO) and ν(PsOFe) modes. We also know,
2320 Langmuir, Vol. 15, No. 7, 1999
Figure 5. CIR-FTIR spectra of adsorbed MPA on goethite for Γ ) 146 µmol/g at different pH values. Goethite suspensions are ∼50 g/L and I ) 0.01 M in NaCl. Scale: h1108(pH ) 3.5) ) 0.0068 au; h1107(pH ) 5.5) ) 0.0044 au; h1105(pH ) 6.5) ) 0.0028 au; h1101(pH ) 7.5) ) 0.0018 au.
from the spectra of MPA in aqueous solution (Figure 2), that the ν(P-OH) mode in the type I complex should be between 1006 and 922 cm-1, since the frequency of this mode should decrease in the following order of species: CH3PO(OH)2 in solution (1006 cm-1) > type I complex > CH3PO2OH in solution (922 cm-1) The spectrum of type II is expected to exhibit two bands in the 1000 cm-1 region, attributed to νs and νa of PsOFe bonds, in addition to a third one at a higher frequency value, associated with ν(PdO). The νa(PsOFe) mode should appear at higher frequencies than the ν(PsOFe) mode of type I. Moreover, one expects the νs(PsOFe) mode to appear at higher frequencies than the ν(PsOH) mode of type I. Finally, as we mentioned before, ν(PdO) of type II should have a lower frequency value than the one of type I. The spectrum of the type III structure should show two bands associated with the PO2 group, having frequency values lower than ν(PdO) and higher than νa(PsOFe) of the type II structure. A third band due to the ν(PsOFe) vibration should exhibit the lowest frequency of the three. The symmetry of type III is the same as that for MPA in solution at pH 5.3 (see Figure 2). However, in the type III structure, ν(PO2) modes will be shifted to lower frequency values and their ∆ν should be smaller than that in solution. According to the previous analysis, we assigned the set of bands at 1108, 992, and 974 cm-1 to an interfacial complex with type I structure (protonated monodentate). The 1108 cm-1 band, the highest one in all the spectra, is attributed to the ν(PdO) vibration of the type I structure. The 974 and 992 cm-1 bands would then be assigned to the ν(PsOH) and ν(PsOFe) vibrations, respectively. This assignment is also consistent with the fact that this group of bands predominates at low pH and high MPA coverage (Figure 3).
Barja et al.
Figure 6. Deconvoluted spectra of selected systems of MPA adsorbed onto goethite. Goethite suspensions are ∼50 g/L and I ) 0.01 M in NaCl: (s) experimental spectra; - - - fitted spectra; - - - individual peaks. (a) pH ) 3.5 and Γ ) 180 µmol/g; (b) pH ) 7.5 and Γ ) 50 µmol/g; (c) pH ) 9.0 and Γ ) 50 µmol/g; (d) pH ) 5.5 and Γ ) 100 µmol/g.
The second set of bands at 1095, 1011, and 983 cm-1 we attributed to a bridging bidentate complex (described above as type II), instead of a monodentate deprotonated, type III structure. This option was favored mainly because ∆ν between the bands of lower frequency is smaller than the ∆ν between the two bands of higher frequency. In addition, the highest frequency of this group of bands (1095 cm-1) is not much lower than that in the former set (1108 cm-1). The bridging bidentate complex is the predominant MPA surface species for a wide range of pH values when the adsorption density is low (see Figure 4); its presence is also important in systems with moderate adsorption density values (see Figure 5) and for pH values between 6 and 9. The bands at 984 and 1015 cm-1 are assigned to the two ν(PsOFe) vibrations, and the band at 1095 cm-1 is assigned to ν(PdO). The shoulder near 1015 cm-1 in the spectra of Figures 3-5 is a visual indication of the presence of the bridging bidentate complex structure II. In the spectra of suspensions of pH 9 and 10, for which Γ was equal to 50 µmol/g or lower, there is a new band at 1032 cm-1. This band can be due to a νs(PO2) mode of a deprotonated monodentate complex (structure type III), since it is higher than the ν(PsOFe) vibrations of the bridging bidentate complex. Furthermore, its presence coincides with a shift to lower frequencies of the highest frequency band in the spectrum (see Figure 6c, 1095 cm-1 passes to 1093 cm-1). This downshift could be explained if the νa(PO2) mode of a type III complex was to be present in the spectrum. The 1093 cm-1 band is probably due to the unresolved absorption of the ν(PdO) and νa(PO2) modes of type II and III complexes, respectively. The position of the ν(PsOFe) for type III complexes is not apparent even
Complexation of Methylphosphonic Acid
Figure 7. Electophoretic mobility curves of goethite particles with adsorbed MPA. Each curve corresponds to a fixed value of adsorption density in micromoles of MPA per gram of goethite: (/) AD ) 0; (9) AD ) 50; (]) AD ) 100; (O) AD ) 130; (2) AD ) 140. The sample concentration is 0.15 g/L, and I ) 0.01 M in NaCl.
in the deconvoluted spectra. One should assume that this band should appear at a higher frequency than that for ν(PsOFe) for a type II species. 3.3. Electrophoretic Mobilities. Electrophoretic mobility curves for goethite particles loaded with MPA, and also for the neat goethite (curve AD ) 0) used in these experiments are shown in Figure 7. The representation is illustrated in such a way that each curve represents the mobility of goethite particles loaded with the same quantity of MPA, as a function of pH. The curves of Figure 7 represent four levels of loading densities: adsorption densities of 130 and 140 µmol/g are considered to be very close to the monolayer coverage, and 50 µmol/g is considered to be a low adsorption density. This figure shows that the pHiep of the MPA-loaded goethite is shifted to lower values in relation to the one of goethite without MPA. Also, the larger the adsorption density, the larger the magnitude of the shift. These results provide still more evidence for the formation of inner-sphere surface complexes, in agreement with data obtained from IR spectroscopy. The shapes of the curves for Γ ) 50 and 100 µmol/g are typical of changes in the potential of metal oxides as the degree of protonation changes with pH. In contrast, suspensions of MPA goethite having higher adsorption densities (Γ ) 130-140 µmol/g) show two isoelectric points, one at a pH between 5 and 5.5 and a second one at pH 6. This charge reversal (as here defined by a change in the electrophoretic mobility curve from a negative to a positive slope with increasing pH) indicates that the acid-base titration of the MPA-loaded goethite surface not only causes a change in the degree of surface protonation but also changes the average pK of the surface. Charge reversals have often been observed in the electrophoretic mobility curves of metal oxide colloids in the presence of hydrolyzable metal ions,15 and are attributed to changes in the chemical nature of adsorbed metal ion species as
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a function of pH. To our knowledge, this is the first reported charge reversal of colloidal metal oxides in aqueous suspensions induced by the presence of protolyzable anions. However, as indicated by the IR data, the charge reversal in this system also can be attributed to a change in surface acidity as a function of pH, associated with a change in the denticity of the surface complex (MPA changes from monodentate to bridging bidentate type of complex with increasing pH at higher adsorption densities). The infrared studies showed that when the adsorption density is 50 µmol/g, the bridging bidentate complex is the prevalent surface species from pH 3.5 to 8. Since changes in pH do not alter the type of surface complex, one should not expect the presence of charge reversal in the electrophoretic mobility curve of this system. This prediction agrees with the experimental electrophoretic mobility data shown in Figure 7. For systems in which Γ ) 100 µmol/g at pH 3.5, both types of complexes coexist, although the prevalent one is the monodentate complex (see Figure 3). Surface MPA, however, is mostly transformed into a bridging bidentate species below pH ) 5.5. This change of surface species with pH is not reflected in the electrophoretic mobility curve for the 100 µmol/g system (see Figure 7). We believe that this lack of response is due to the fact that this transformation takes place at pH values far from the pHiep of these systems (pHiep ) 7), in a region where the slope of the electrophoretic mobility curve is very small and therefore very insensitive to surface changes. As mentioned before, in systems in which Γ ) 130-140 µmol/g, IR data show that surface MPA is mainly in the form of a monodentate protonated complex up to pH 5.5, and at pH 7.5 is already a bridging bidentate complex (see Figure 5). The transformation of one type of complex into the other takes place between pH 5.5 and 7.5 at high adsorption densities. This is the pH interval in which the electrophoretic mobility curve of these systems shows charge reversal (Figure 7), and it also contains the pHiep of this system. 4. Conclusion This paper shows that there is a lack of parallel response between KL and Γmax which may be attributed to a change in the denticity of MPA complexes. Infrared studies definitively show this change in denticity with changes in pH and are supported both by the infrared spectra and by changes in Γ. Changes in the slope of the electrophoretic mobility curve over this pH range may also reflect this phenomenon. To our knowledge, this is the first time that an adsorbed anion has been shown to “reverse” the charge of an oxide surface. Our evidence suggests that, below a pH of 9, MPA forms two types of complexes on the surface of goethite. One complex is formed at low pH and high Γ, and the other is formed at high pH and low Γ. On the basis of ATR-FTIR spectra, we have assigned the former complex to a protonated monodentate structure and the second one to a bridging bidentate structure. LA980540Y (15) Schindler, P. W. Surface complexes at oxide water interfaces. In Adsorption of Inorganics at Solid-Liquid Interfaces; Anderson, M. A., Rubin, A. J., Eds.; Ann Arbor Science Publisher: Ann Arbor, MI, 1981.
