Environ. Sci. Technol. 2005, 39, 585-592
Aminomethylphosphonic Acid and Glyphosate Adsorption onto Goethite: A Comparative Study B. C. BARJA AND M. DOS SANTOS AFONSO* INQUIMAE and Departamento de Quı´mica Inorga´nica, Analı´tica y Quı´mica Fı´sica, Facultad de Ciencias Exactas y Naturales, Universidad de Buenos Aires, Ciudad Universitaria Pabello´n II, (C1428EHA) Buenos Aires, Argentina
Glyphosate is a non-selective, broad spectrum, postemergent herbicide widely used in weed control. Aminomethylphosphonic acid (AMPA) is one of the main products of biodegradation of glyphosate in natural systems before its ultimate mineralization and also the breakdown product of more complex phosphonates such as nitrilotris(methylenephosphonic acid). The adsorption isotherms and surface coverage of AMPA and glyphosate (N-phosphomethylglycine, PMG) in aqueous suspensions of goethite as a function of pH were measured. Electrophoretic mobility curves for the PMG/goethite system were also determined. The ATR-FTIR interfacial spectra of the surface complexes of AMPA and PMG onto goethite were analyzed as a function of the pH and the surface coverage. The phosphonate moiety of these two ligands coordinates to the iron oxide surface with similar structures as the methylphosphonic acid despite the presence of the amino and/or carboxylate groups of their molecules. Two predominating complexes have been identified where the phosphonate group in PMG or AMPA bonds monodentately or bridges bidentately to the surface of iron oxide in an inner sphere mode, while the carboxylate and amino group are noncoordinated to the surface. The stability constants of the surface complexes t FeOsP(O)(OH)sR, tFeOsP(O)2sR, and (tFeO)2sP(O)s R were calculated using the constant capacitance model.
Introduction Phosphonic acids and their derivatives are of interest due to their structural variety and great economic importance. Phosphonic acids derivatives are used as crop protection agents (weed control) in water treatment, in metal processing, and as flame-proofing agents (1). Phosphonates are molecules that contain one or more groups R-PO(OH)2. The dibasic phosphonic acids are mostly weaker acids than phosphoric acid. The P-C bond is generally very stable toward oxidation or hydrolysis so that many reactions can be carried out on the rest of the organic part of the molecule. These compounds not only possess a very high ability to form strong complexes (2, 3) with transition metals in aqueous solution but also show a large affinity for the surface of aluminum and iron oxides. All these properties play a very important role in the fate and rate of transport of these compounds in the environment. N-Phosphomomethylglycine (PMG, HOOCCH2-NH-CH2-PO3H2 commonly known as glyphosate) is * Corresponding author phone: +5411 4576 3380, ext 125; fax: +5411 4576 3341; e-mail:
[email protected]. 10.1021/es035055q CCC: $30.25 Published on Web 12/10/2004
2005 American Chemical Society
the active component of non-selective, post-emergent, and broad spectrum commercial herbicides widely used in agriculture (3). Significant amounts of herbicides may eventually reach the soil and remain in the soil or be transported to other areas before their ultimate decomposition due to the different application methods and the weather conditions. Aminomethylphosphonic acid (AMPA) is one of the main products of biodegradation of glyphosate in natural systems before its ultimate mineralization and is also the breakdown product of more complex phosphonates such as nitrilotris(methylenephosphonic acid). It is known that PMG is immobilized upon contact with soils and clay minerals due to the formation of surface complexes with metal ions (4-13). Mc Bride and Kung (14) first studied the adsorption of PMG onto goethite by transmission infrared spectroscopy measurements on dry films of the oxide. Even though strong bands appeared in the infrared frequency range of the phosphonate group, no band assignments were reported. Sheals et al. (15) examined the structures of the PMG onto goethite via spectroscopic techniques as a function of pH and band assignments were performed. The focus of this study is to examine the structures of the complexes of AMPA and PMG on the surface of goethite (R-FeOOH) on the basis of the previous results reported for the interfacial complexes of the more simple methylphosphonic acid (MPA) (16). In this work, we make a straightforward correlation between the interfacial phosphonate bands reported for the MPA/goethite and the ones of AMPA and PMG/goethite systems due to the similarities of their interfacial spectra given by the presence of the terminal -CH2-PO(OH)2 entity. In fact, adsorption experiments of different mono- and polyphosphonates performed with the iron(III) hydr(oxide) goethite (9) showed a striking similar adsorption behavior despite the different number of phosphonate moieties present in the ligand and the dissimilarity of their protonation level and net charge. In this sense, the study of AMPA permits us to evaluate the role of the carboxylic acid in the structures of the surface complexes of the herbicide with goethite as well as in the adsorption isotherms.
Experimental Section Materials. All solutions and suspensions were prepared using reagent grade chemicals and Milli-Q water. Glyphosate and AMPA (both 99% pure) were a gift from Monsanto Argentina, Planta Za´rate. Sarcosine (99% pure) was provided by Sigma Co. Goethite was prepared following the Atkinson-Posner technique (17) by hydrolyzing of ACS grade ferric nitrate with NaOH with a 0.5 M Fe/OH ratio. The solution was aged 2 days at room temperature and then titrated 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. The surface areas obtained by N2 adsorption (BET analysis) were 60 and 75 m2/g for PMG and AMPA studies. Suspensions of goethite were prepared by re-suspending the oxide in Milli-Q water and sonicating intermittently for 2 d to ensure hydration. Adsorption Studies. Sample Preparations. Suspensions of goethite were brought to a fixed ionic strength of 0.01 M NaCl and desired pH conditions by adding microliter quantities of 1 M NaCl and NaOH or 0.01-1 M HCl. These samples were left to reach equilibrium for 24 h. Next, a given number of microliters of 0.010 M PMG or AMPA were added to these vigorously stirred pre-equilibrated goethite suspensions, and the pH was readjusted until constant values were VOL. 39, NO. 2, 2005 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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obtained within a few tenths of a pH unit. The pH was measured prior to any analysis. Adsorption Isotherms. PMG and AMPA adsorption was calculated from the difference between the total added ligand and that measured in the supernatant after 24 h of equilibrium. The suspensions (13.3 g/L) were centrifuged and filtered through a 0.05 µm Nucleopore filter, and the amount of PMG and AMPA present in the supernatant was measured as elementary phosphorus by ICP (inductively coupled plasma) emission spectrometry with a Perkin-Elmer Optima 3000XL atomic emission spectrometer with a detection limit lower than 0.1 µM. The typical experimental error is lower than 5% for all the experimental results. ATR-FTIR Studies. The ATR (Attenuated Total Reflection)-infrared spectra of PMG and AMPA/goethite suspensions were recorded interferometrically with a Nicolet Magna 560 Fourier transform infrared (FTIR) spectrometer equipped with a HgCdTe (MCT) detector and a Balston H2O/CO2 stripper. A horizontal boat plate (Spectra Tech.) with a ZnSe 45° crystal was used as the IRE (internal reflection element). The spectral resolution was 4 cm-1 for all measurements, and all single-beam IR spectra were the result of 2000 coadded interferograms. The system cutoff is near 800 cm-1 for work in water. The suspensions of goethite with ligand (PMG or AMPA) were centrifuged, and half of the supernatant was used as reference. After that, the solid was resuspended in the other half of the supernatant and was used as the sample (60 g/L). Both sample and reference ATR-FTIR spectra were run. The reference is the supernatant of the suspension of goethite with the ligand. The supernatant contains only the adsorbate that remains in solution, not adsorbed to the goethite. The sample contains the adsorbate that is in solution and the adsorbate adsorbed to the oxide, plus the oxide. The spectrum of the reference (supernatant) was subtracted from the spectrum of the corresponding sample (slurry) to obtain a final spectrum that has the interfacial bands of the adsorbate onto the oxide and, of course, the bands of the goethite itself. In all cases, the empty cell was used as the background. A detailed description of the ATR-FTIR spectrum of goethite suspended in water is already reported (18). The strong bands due to the Fe-O-Fe and /or Fe-O-H vibrations locate at 895 and 800 cm-1 are out of the frequency range considered in this work (1900 to 950 cm-1). Further description of this method can be found elsewhere (16, 18, 19). Electrophoretic Mobility. Measurements were performed with a PenKem System 3000 electrokinetics analyzer at 25 °C and constant ionic strength of 0.01 M NaCl. Suspensions of goethite loaded with different amounts of PMG were prepared at several pH values, and the content of total adsorbed PMG was measured. From these results, we selected those samples of similar PMG adsorption density (ΓPMG) and measured the electrophoretic mobilities of their particles. Finally, five curves were obtained for suspensions with 0, 0.4, 0.8, 1.3, and 1.7 µmol of PMG/m2 of oxide as a function of pH. The samples were prepared by diluting, with their own supernatant, preequilibrated aliquots of the 13.3 g/L samples of PMG/ goethite described previously to a concentration of 0.15 g/L.
Results and Discussion Adsorption Studies. The adsorption isotherms of equilibrated suspensions of PMG and AMPA/goethite are shown in Figure 1. These curves were calculated using a nonlinear regression fitting program (Solver, Excel 5.0) to approximate a Langmuir shape. The Langmuir constant (KL) and maximum coverage (Γmax) of every system are given in Table 1 together with the corresponding values of MPA (16). Γmax values were normalized with the area of the goethite for a better comparison. Figure 2 shows the pH dependence for MPA, AMPA, and 586
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FIGURE 1. Adsorption isotherms of PMG (a) and AMPA (b) in goethite suspensions at different pH values. Suspensions are 13.3 g of solid/L and I ) 0.01 M in NaCl. (a) (9) pH ) 3, ([) pH ) 4, (b) pH ) 5.1, (2) pH ) 6.2, (0) pH ) 7.3, (]) pH ) 8.2, and (O) pH ) 9.2. (b) (b) pH ) 5, (9) pH ) 3, (0) pH ) 7, and (O) pH ) 9. Experimental errors are less than 5%.
TABLE 1. Maximum Adsorption Densities (Γmax) and Langmuir Constants (KL) for PMG, AMPA, and MPA Adsorbed onto Goethite Γmax. (µmol/m2)
KL (L/µmol)
pH
PMG
AMPA
MPAa
PMG
AMPA
MPAa
3.5 4.0 5.0 6.1 7.2 8.2 9.2
2.4 2.2 2.1 1.9 1.7 1.0 0.6
2.3
2.3
0.18
0.33
2.5
0.19
1.8
2.1 1.9 1.7
0.07
0.15 0.13 0.09
0.4
1.0
0.3 0.24 0.11 0.07 0.03 0.03-0.02 0.01
0.03
0.01
a
Data obtained from ref 16.
PMG adsorption onto goethite. The curves show a similar profile for the three ligands. The curve patterns are also similar to those previously reported for anion adsorption on hydrous ferric oxides (20). Table 1 shows that the change in affinity of PMG for the surface of goethite (given by the value of ∆KL) for a constant ∆pH ∼ 1 is ∆KL/∆pH ) -0.06, -0.13, -0.04, -0.04, -0.01, and -0.01. For the ∆pH between 4.0 and 5.0, the affinities of PMG are approximately 3.25 and 2 times larger than for the ones between pH 5.0-7.2 and pH 3.1-4.0, respectively. KL values for PMG decreases 37% from pH ) 3.5 to pH ) 5. This was also observed for the KL values of MPA in the same range of pH (with a decrease equal to 45%). The ∆KL/∆pH values decrease at pH close to 8, reflecting the fact that the adsorption of PMG onto goethite is not favored due to the negatively charged oxide surface. With respect to the Γmax, the highest values for PMG are obtained at pH < 5.0 and its value decreases smoothly up
FIGURE 3. Electrophoretic mobility curves for PMG adsorbed on the surface of goethite at different pH values and I ) 0.01 M NaCl. Each curve represents the electrophoretic mobility of systems with a constant value of adsorption density of Γ ) 0 µmol of PMG/m2 (0), 0.4 µmol of PMG/m2 (9), 0.8 µmol of PMG/m2 (b), 1.3 µmol of PMG/m2 ([), and 1.7 µmol of PMG/m2 (2). Experimental errors are less than 5%.
FIGURE 2. pH dependence of the binding of the ligands (MPA, AMPA, and PMG) on the goethite surface. Suspensions are 13.3 g of solid/L and I ) 0.01 M in NaCl. ([) 200 µmol of ligand/g of goethite; (2) 150 µmol of ligand/g of goethite; (O) 100 µmol of ligand/g of goethite; (b) 75 µmol of ligand/g of goethite; (9) 50 µmol of ligand/g of goethite. In all the cases, dots are experimental points, and solid lines were calculated using MINEQL 3.01 and the surface complexes established using the IR band assignments. Experimental errors bars are included. to pH ) 7. At pH ) 8.2 and 9.2, Γmax changes in almost 60% and 35% of its value at pH ) 7.2. This trend in the values of the Γmax was also observed in the MPA/goethite system. The values of their Γmax are identical for similar pH. In view of these results, it is reasonable to think that the amino and carboxylate groups, which are present in the molecule of PMG, are not directly involved in the adsorption of PMG to the surface of the oxide. For the case of the AMPA/goethite system, the values for the Γmax are also very close to the ones of the PMG and MPA at the measured pH but the highest value for the ∆KL/∆pH ) 0.05 is observed between pH 5.0 and pH 7.2. The highest values of KL ) 0.19 and Γmax ) 2.5 are obtained at pH ) 5.0 and not 3.5 contrary to what should be expected for the adsorption of anions. The maximum affinity and adsorption density are not attained at the lowest pH values as is the case of the MPA and PMG/goethite systems. This behavior can be explained in terms of the electrostatic forces involved between the surface of the oxide and the total charge of the molecule of AMPA. For pH below 5.0, the AMPA molecule is in the form of a zwitterion and has no net charge while the surface of the oxide is positively charged (pHzpc of goethite ) 8.8, see Figure 3 for Γ ) 0 µmol/g). At pH ∼ 5, the phosphonate group of the AMPA deprotonates (21) (pKa1 ) 5.38 at I ) 0.1 M) into a negatively charged anion while the
surface of the goethite is still positive. It seems that AMPA will have a higher tendency to reach the surface of the oxide at pH values higher than its pKa due to the attractive electrostatic interactions of the opposite charges of the surface and the AMPA molecule. Although PMG also forms a zwitterion in aqueous solution, the pKa of the carboxylic acid is approximately 2 leaving a total negative charge in the molecule of PMG at pH ) 5 and higher (22). Finally, the striking result that the values of Γmax of the three ligands are comparable at similar pH values indicate that the number of surface sites of the goethite that are being occupied by the PMG, AMPA, and MPA are the same. The main mechanism of ligand adsorption is ligand exchange; the surface hydroxyl is exchanged by another ligand (20). The extent of surface coordination and its pH dependence can be explained by considering the affinity of the surface sites for ligands and the pH dependence of the activity of surface sites and ligands. Since the adsorption of anions is coupled with a release of OH- ions, adsorption is favored by lower pH values. Also, bidentate ligands surface chelates are formed (20). PMG, AMPA, and MPA forms inner-sphere surface complexes on the goethite surface as is suggested by electrophoretic measurements. Thus, the adsorption of PMG, AMPA, and MPA on the goethite surface as a function of pH is shown in Figure 2. In this figure the solid lines are calculated using the constant capacitance model and MINEQL 3.01 software. The type of surface complexes considered in this model for the calculations were those suggested by using the spectroscopic results (see ATR-FTIR Spectroscopy Section). The adsorption process is characterized by the following reactions:
tFeOH + RsP(O)(OH)sO- + H+ T tFeOsP(O)(OH)sR + H2OKs1 tFeOH + RsP(O)(OH)sO- T tFeOsP(O)2sR + H2OKs2 2tFeOH + RsP(O)(OH)sO- + H+ T (tFeO)2sP(O)sR + 2H2OKs3 R means CH3, NH3+CH2, and -OOCCH2(NH2)+CH2 group for MPA, AMPA, and PMG, respectively. The stability constant of surface complexes of MPA, AMPA, and PMG are detailed VOL. 39, NO. 2, 2005 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 2. Stability Constants for Surface Complexes of MPA, AMPA, and PMG onto Goethitea ligand
Ks
log 1 log Ks2 log Ks3
MPA
AMPA
PMG
12 6 17
12.8 5 17
10.5 4.5 16.5
a Surface complexation constants were obtained using the Constant Capacitance Model. For the corresponding chemical reactions, see text.
in Table 2. These values are comparable to those obtained by Nowack and Stone (9) and Sheals et al. (15). Electrophoretic Mobilities. The electrophoretic mobility curves for the goethite particles with and without adsorbed PMG at different pH values are shown in Figure 3. The data are plotted in such a way that each curve represents the electrophoretic mobility of a system with a constant value of adsorption density (Γ ) 0, 25, 50, 75, and 100 µmol of PMG/g of goethite). This figure shows how the pHiep shifts to lower pH values as the amount of adsorbed PMG increases from Γ ) 0 (pHiep ) 8.8) to Γ ) 25 (pHiep ) 7.4) to Γ ) 50 (pHiep ) 6.7) to Γ ) 75 (pHiep ) 5.8) and to Γ ) 100 (pHiep ) 4.7). The magnitude of the shift in the pHiep to lower values increases with increasing amounts of adsorbed PMG indicating that the PMG adsorbs to the surface of the oxide as an anionic species. Also, at constant pH, the change of the surface charge is negative and increases as the amount of adsorbed PMG increases showing that the surface complexes formed are more negative than those formed during the surface protonation. Thus, PMG surface complexes contain a higher charge density than the protonated or deprotonated surface complexes formed during surface/water interaction. These results provide conclusive evidence about the formation of inner-sphere surface complexes between the surface of the goethite particles and PMG. ATR-FTIR Spectroscopy. All the spectra of goethite and AMPA or goethite and PMG suspensions were obtained by subtracting the spectrum of the supernatant (bulk solution) from the suspension spectrum of each sample. In this way, the difference spectrum contains only the adsorption bands of the interfacial species and the bands of the species in the bulk of the oxide. Due to the strong and broad absorption bands of the stretching vibration of the hydroxyl ions of the water molecules in liquid phase, the high-frequency IR region (3500-3000 cm-1) was not considered in the analysis of the bands of the samples. In this region, a reliable difference spectrum is difficult to obtain because a slight difference in surface hydration of the suspensions causes significant errors when the reference spectrum is subtracted and artifacts might be mistaken as bands. From the results reported for the MPA/ goethite system, it was shown that the speciation of MPA varies with both the pH and adsorption densities and that all spectra could be described as a linear combination of mainly two types of surface complexes. The bridging bidentate surface complex [(tFeO)2sP(O)sCH3]2(x-1) (type II) predominates from low to moderate surface coverage in almost all the range of pH under study. In [(tFeO)2sP(O)s CH3]2(x-1), x represents the residual surface charge of the metal ion. In this complex, a band at 1095 cm-1 was assigned to the stretching mode of the PdO bond (νPdO) while two more bands at 1015 and 984 cm-1 were assigned to the antisymmetric and symmetric stretching modes of the PsOFe bonds, (νPOFe). At low pH values and high surface coverage the monodentate protonated complex [(tFeO)sP(OH)(O)sCH3]x-1 is the preponderant surface species with νPdO, 1108 cm-1; νPOFe, 991 cm-1; and νP-OH, 971 cm-1. At pH ) 9 or higher, the deprotonated monodentate surface complex [(tFeO)s 588
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FIGURE 4. ATR-FTIR spectra of adsorbed PMG (lower panel) and AMPA (upper panel) onto the surface of goethite at different pH values and low surface coverage (solid line). Goethite suspensions are 60 g/L and I ) 0.01 M in NaCl. The corresponding spectra in aqueous solution at the same pH values (dotted lines) are superimposed to the interfacial ones. (PO2)sCH3]x-2 (type III) was proposed, and a band at 1032 cm-1 was assigned to the νs(PO2) mode. The bands of both types of complexes are observed simultaneously in the MPA/goethite spectra with different intensities according to the conditions of the experiment; however, all spectra show mainly one band in the highfrequency zone (PdO bond) which differentiates from the two or three bands in the low-frequency region (single PsO bonds). The assignment of the bands was performed with the assumption that the same symmetry for MPA in solution and the MPA complexed on the surface of goethite should produce similar spectral profiles. AMPA and PMG Adsorbed onto Goethite. Figure 4 shows spectra of aqueous suspensions of goethite with AMPA and PMG together with the correspondent aqueous solution of the ligand at the same pH values for the phosphonate frequency range (1200-950 cm-1). Differences in either the position or the number of bands can be observed when comparing the spectra of the suspensions with the spectra of the ligand in aqueous solution. Since, under the same pH conditions, the spectra of the ligands in solution and at the surface of the oxide are different, it can be concluded that AMPA and PMG are coordinated to the surface of the goethite particles. Similar interfacial spectra of PMG have previously been reported (15). To obtain more detailed information from this set of spectra, we used the program Origin 5.1 (Microcal) to deconvolute them into several peaks. Figures 5 and 6 show the deconvoluted spectra of aqueous suspensions of goethite with AMPA at different pH and adsorption densities. It is interesting to note the similarity in the general patterns of the bands of the spectra of the AMPA/goethite system when compared with the corresponding spectra of the PMG/ goethite system in the 1200-950 cm-1 range.
FIGURE 5. Deconvoluted ATR-FTIR spectra of AMPA adsorbed onto the surface of goethite at different pH values for Γ ) 0.6 µmol/m2 (pH ) 5 and 7) and Γ ) 0.3 µmol/m2 (pH ) 9). Goethite suspensions are 60 g/L and I ) 0.01 M in NaCl: (solid line) experimental spectra, (dot-dashed line) individual peaks, (dotted line) fitted spectra. Phosphonate Group (1200-950 cm-1). From the deconvoluted spectra of the AMPA/goethite (Figures 5 and 6) and PMG/goethite spectra (Figures 7 and 8) a pattern of interfacial bands similar to those observed for the MPA/goethite system still remains. The band assignment for the phosphonate moiety of the MPA/goethite system will be the main tool to understand the AMPA/goethite and PMG/goethite spectra, and for that the first step is to discard from the spectra those bands that are not strictly associated with the surface phosphonate bonds. In the low frequency range of the interfacial spectra of the AMPA/goethite and PMG/goethite there are three bands (approximately at 993-1010, 980, and 970 cm-1 in Figures 5 and 6) located at similar frequency values as those reported for the MPA/goethite indicating that these bands must be associated with the vibrations of the single PsO bonds. This observation is in accordance with the fact that in aqueous solution the stretching vibration mode of the PsOH group is located at similar frequency values in the three ligands (922, 916, and 917 cm-1 for the MPA, AMPA, and PMG, respectively). On the contrary, in the high-frequency zone of the spectra of the PMG/goethite system we find two or three bands (∼1130-1120, 1100, and 1150 cm-1) instead of one as in the MPA/goethite system. Before any attempt of assignment of these bands, we performed several studies with the N-methylglycine or sarcosine CH3sNHsCH2sCOOH. This molecule contains the same functional groups as the PMG except for the terminal phosphonic acid and also forms a zwiterion in aqueous solution, CH3sNH2+sCH2sCOO-. Several attempts were performed to obtain the complex with Fe(III) such as was made with MPA, AMPA, and PMG, but no results were obtained in accordance with what was reported for Fe(III)/ glycine and Fe(III)/iminodiacetic acid in which the ESR spectra of these systems were similar to the free Fe(III) (14).
FIGURE 6. Deconvoluted ATR-FTIR spectra of AMPA adsorbed onto the surface of goethite at different pH values for Γ ) 1.3 µmol/m2 (pH ) 5 and 7) and Γ ) 2.5 µmol/m2 (pH ) 5). Goethite suspensions are 60 g/L and I ) 0.01 M in NaCl: (solid line) experimental spectra, (dot-dashed line) individual peaks, (dotted line) fitted spectra. The adsorption of sarcosine onto goethite was also determined by measuring total organic carbon at similar total ligand concentrations as in the case of MPA, AMPA, and PMG, but the results suggested that adsorption occurred only at very high concentrations and low pH values. Several spectra of sarcosine adsorbed onto goethite were measured in different conditions showing a band at ∼1150 cm-1 for those samples of high concentration of sarcosine. Figure 9 shows the spectra of two samples of PMG and sarcosine adsorbed onto goethite demonstrating that the band at 1150 cm-1 is not a band associated with the vibrations of the phosphonate group. This band could be assigned to the out-of and in-plane vibrations of the methylene and methyl groups as indicated in bibliography for sarcosine (23). Another band to be discarded is the one located at ∼1100 cm-1 (Figures 5-8) as it can be assigned to the amino moiety (see next section). AMPA: Amino Group. It was previously reported that the rocking vibration of the terminal protonated amino group -NH3+ of the AMPA molecule (FNH3+) locates at 1113 cm-1 in solid state (24) and at ∼1100 cm-1 in aqueous solution (2). If the amino group is protonated and not coordinated to the goethite, the asymmetric and symmetric in-plane deformations of this group (δNH3+) should be observed at similar frequency values as the corresponding ones for the free ligand in aqueous solution. Figure 10 shows the interfacial spectra of AMPA in suspensions of goethite in the 1900-1200 cm-1 range at pH ) 5, 7, and 9 for a surface coverage of 1.3 µmol of AMPA/m2 of oxide. In these spectra, the band located at 1790 cm-1 correspond to the bulk oxide (FesOH bending). Two bands corresponding to the asymmetric and symmetric in-plane deformations of the protonated amino group δNH3+ are observed at similar frequency values as the corresponding ones for the free ligand in aqueous solution at similar pH VOL. 39, NO. 2, 2005 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 7. Deconvoluted ATR-FTIR spectra of PMG adsorbed onto the surface of goethite at different pH values for Γ ) 0.8 µmol/m2 (pH ) 3, 5, and 7) and Γ ) 0.6 µmol/m2 (pH ) 9). Goethite suspensions are 60 g/L and I ) 0.01 M in NaCl: (solid line) experimental spectra, (dot-dashed line) individual peaks, (dotted line) fitted spectra. values. The presence of these bands confirm that this group is protonated and not coordinated to the iron atoms of the surface of goethite. A protonated amino group was also reported for the complex of Fe(III)/AMPA in aqueous solution (2). PMG: Carboxylate and Amino Groups. As in the case of AMPA, the occurrence of the deformation modes of the amino group (δNH2+) were useful to show that the amino group of the PMG is not coordinated to the surface of the goethite particles and that the interaction of the carboxylate moiety of the PMG with the surface of the oxide does not exist or is extremely weak (15). These results suggest that the trend observed for the decreasing values of the νa stretching of the carboxylate group in aqueous solution is similar to what happens in the presence of the oxide; the amino group remains protonated until the pH of the suspension gets closer to its pKa value. These results also agree with what was observed for the 1:1 Fe(III)/PMG complex in aqueous solution in which no important changes or shifts were observed for the νa COO-, νs COO, and δNH2 modes when compared with the free ligand at similar pH values showing no evidence for amino or carboxylate coordination. The four bands centered at approximately 1135-1120, 993-1010, 980, and 970 cm-1 in Figures 5-8 correspond to vibration modes of the phosphonate moiety of adsorbed AMPA and PMG ligands. It is possible to see that some other bands do not seem to be affected by changes in pH or in the surface density. In the PMG/goethite system, the bands at 1060 and 1030 cm-1 in the interfacial spectra (Figures 7 and 8) can be assigned to the CCNC skeletal vibration of the adsorbed ligand as these modes locate at 1079 and 1031 cm-1 in the spectrum of the PMG in solid state (25). With respect to the AMPA, the same is observed for the band at 1070 cm-1 in Figures 5 and 6 and would not correspond to a phosphonate moiety. The band located at 970 cm-1 (indicative of 590
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FIGURE 8. Deconvoluted ATR-FTIR spectra of PMG adsorbed onto the surface of goethite at different pH values for Γ ) 2.4 µmol/m2 (pH ) 3), Γ ) 2 µmol/m2 (pH ) 5), and Γ ) 1.6 µmol/m2 (pH ) 7). Goethite suspensions are 60 g/L and I ) 0.01 M in NaCl: (solid line) experimental spectra, (dot-dashed line) individual peaks, (dotted line) fitted spectra.
FIGURE 9. Deconvoluted ATR-FTIR spectra of adsorbed PMG and sarcosine onto the surface of goethite at pH ) 5.1 and for a total added ligand concentration of 2.4 µmol/m2. The spectrum of sarcosine is not deconvoluted (dashed line). Goethite suspensions are 60 g/L and I ) 0.01 M in NaCl (solid line) experimental spectra, (dotdashed line) individual peaks, (dotted line) fitted spectra. a monodentated protonated surface complex in MPA/ goethite) is observed mainly at low pH values and high to intermediate surface coverage of AMPA (Figure 6 for Γ ) 2.5 and 1.3 µmol/m2) and at low pH values for PMG (Figure 7 for Γ ) 0.8 µmol/m2 at pH ) 3 and 5 and Figure 8 for Γ ) 2.4 at pH ) 3 and Γ ) 2 µmol/m2 for pH ) 5). This band at 970 cm-1 tends to be associated with the band of highest wavenumber at 1135 cm-1 in the spectra of both ligands, which shifts to lower values as the pH is increased. For the PMG/goethite, the broad band at 1135
FIGURE 10. ATR-FTIR spectra of AMPA adsorbed onto the surface of goethite at different pH values for Γ ) 1.3 µmol/m2 (pH ) 5 and 7) and Γ ) 0.3 µmol/m2 (pH ) 9). Goethite suspensions are 60 g/L and I ) 0.01 M in NaCl. cm-1 in the spectra of PMG/goethite at pH ) 3 in Figure 7 shifts to lower values as the pH of the medium increases, located at 1118 cm-1 at pH ) 9 but now the band at 980 cm-1 predominates. This behavior seems to repeat in Figure 8 (except for pH ) 3 at 2.4 µmol/m2 in which the spectrum is distorted) indicating that this band is associated with the phosphonate vibrations. This trend is also observed for the AMPA/goethite system (see Figures 5 and 6). Moreover, the highest value (1135 cm-1 at pH ) 3 in Figure 7 and pH ) 5 in Figure 6) for a phosphonate vibration of PMG and AMPA/ goethite differs from the correspondent one of MPA/goethite system (reported at 1108 cm-1 for similar pH values) in 27 cm-1, which is consistent with the fact that the values for the stretching vibrations of the free -PO2 and -PO3 groups in aqueous solution for the PMG and AMPA are always higher (approximately 40 cm-1) than those of the MPA. For the rest of the spectra, the bands centered at 980 and 1000-1010 cm-1 dominates the spectral profiles in the lowfrequency region for both ligands. According to the previous analysis for AMPA and PMG interfacial bands, and considering the results obtained for the vibrational bands of interfacial MPA/goethite system, we conclude that bridging bidentate and the monodentate protonated complex are at least the main species in the solid/water interface. Thus, a tentative band assignment for both ligands would be as follows: monodentate protonated complex [tFeOs P(O)(OH)sR]: νPdO, 1135-1128 cm-1; νPOFe, 982 cm-1; and νP-OH, 970 cm-1; bridging bidentate complex [(tFeO)2sP(O)s R]: νPdO, 1118-1126 cm-1; νPOFe(assym), 993-1015 cm-1; and νPOFe(sym), 980 cm-1 where R refers to the rest of the AMPA or PMG molecules. Contrary to what was observed for the adsorption of the MPA, the bands of these two complexes cannot be isolated without having important contributions of other bands arising from the rest of the structure of the PMG.
There are two possible structures for the bridging bidentate complex. The first of them would involve one phosphonate group bonded to two neighboring surface iron(III). The O-O distance between the apexes of two adjacent iron octahedra is 2.95 Å (26), and the distance between two oxygen donors of the phosphonate group is approximately 2.6 Å (calculated using data from ref 3). It would take a certain extent of distortion to form this bond, but it may be balanced by the energy stabilization due to the formation of a six membered ring. Another possible structure for the bridging bidentate complex would be a bond where one phosphonate group binds to the same iron(III) center. In this case, the OsOH distance between two singly coordinated oxygens situated at the corners of the same goethite octahedron is approximately 2.76 Å, which is favorable for the formation of a mononuclear bridging bidentate complex, but the stabilization energy will be unfavorable because it would lead to a four-membered ring formation. In conclusion, both AMPA and PMG form surface complexes on goethite with similar maxima surface coverage and the extent of the complexation is dependent on the ligand concentration in solution and pH. The interfacial spectra of AMPA and PMG in aqueous suspensions of goethite were analyzed as a function of the pH and the surface coverage. The results strongly suggest that the phosphonate moiety of AMPA and PMG coordinates to the surface of the oxide with similar structures as the MPA, despite the presence of the amino and/or carboxylate groups of their molecules. Two predominating complexes have been identified in which the phosphonate group of PMG or AMPA bonds monodentately or bridging bidentately to the surface in an inner-sphere mode, while the carboxylate and amino groups are noncoordinated to the surface. This result is in accordance with the similar adsorption behavior observed for different phosphonates onto goethite (9). It is worth noting that the experiments with sarcosine in which no phosphonate moiety is present showed no complexation with Fe(III), indicating that the phosphonate moiety is the responsible for the adsorption of the herbicide and its degradation product AMPA to goethite. These results are potentially important when elucidating the bioavailability and mechanisms of PMG degradation in soils. Despite its high water solubility, most papers dealing with the fate of glyphosate in soils emphasize its low mobility, as a result of sorption processes (27-30). Adsorption of glyphosate to soil components is responsible, according to several authors (27, 31), for the fast inactivation of the glyphosate in the soil rather than its degradation. Degradation of adsorbed glyphosate is notably slower than that of free glyphosate (32, 33); thus the adsorption process makes herbicide more persistent in soil.
Acknowledgments The authors acknowledge Universidad de Buenos Aires, Secretarı´a de Ciencia y Te´cnica through Project UBACyT TW99 for financial support of this work.
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Received for review September 24, 2003. Revised manuscript received August 12, 2004. Accepted August 12, 2004. ES035055Q
