Molecular Orbital Theory Study on Surface Complex Structures of

Surface Complex Structures of. Glyphosate on Goethite: Calculation of Vibrational Frequencies. LORENA TRIBE,* , †. KIDEOK D. KWON, ‡. CHAD C. TROU...
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Environ. Sci. Technol. 2006, 40, 3836-3841

Molecular Orbital Theory Study on Surface Complex Structures of Glyphosate on Goethite: Calculation of Vibrational Frequencies L O R E N A T R I B E , * ,† K I D E O K D . K W O N , ‡ CHAD C. TROUT,‡ AND JAMES D. KUBICKI‡ Division of Science, The Pennsylvania State University, Berks Campus, Tulpehocken Road, Reading, Pennsylvania 19610, and Department of Geosciences and Earth & Environmental Systems Institute, The Pennsylvania State University, University Park, Pennsylvania 16802

Six possible complexes of glyphosate (O-PO(OH)CH2NH2+CH2CO2H) with an Fe-hydroxide dimer were modeled with hybrid molecular orbital/density functional theory calculations to establish the nature of the bonds of glyphosate on goethite (R-FeOOH). Monodentate and bidentate coordination of the phosphonate moiety were considered, using three forms of the glyphosate molecule appropriate for different pH ranges: glyphosate with both phosphonate and amino moieties protonated, glyphosate with unprotonated phosphonates, and glyphosate with both unprotonated phosphonates and no hydrogen ion on the amino group. The calculated infrared vibrational modes were compared to experimental values, finding particularly good agreements with the monodentate complexes in all the pH ranges.

Introduction Glyphosate (N-(Phosphonomethyl)glycine) is the currently most used herbicide in the world. The interaction of glyphosate with soil influences the herbicide’s environmental impact, potential toxicity, and degradation (1-5, 28). It is, thus, an important area of research, especially in the light of recent studies about the carcinogenic potential of glyphosate (6). In particular, many experimental studies are available for the interactions of glyphosate with the mineral goethite (R-FeOOH) (7-13) which is present in almost every soil type. Infrared spectroscopy, among other techniques, was used in the most recent of these studies (7, 8) to establish the nature of the interaction between the phosphonate moiety of glyphosate and the iron oxy(hydr)oxide surface. Sheals et al. (7) concluded that glyphosate adsorbs via predominantly monodentate complexation, with a minor quantity of the bidentate coordination of the phosphonate moiety forming at near neutral pH and when the concentration of glyphosate is low (0.2 µmol PMG/m2). Similarly, Barja and dos Santos Afonso (8) found no indication of bonding through the amino and carboxyl moieties, and assigned IR spectral peaks to mono- and bidentate complexes involving the phosphonate moiety. * Corresponding author phone: (610) 396-6187; fax: (610) 396 6024; e-mail: [email protected]. † Division of Science, The Pennsylvania State University. ‡ Department of Geosciences and Earth & Environmental Systems Institute, The Pennsylvania State University. 3836

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Accurately constraining the surface speciation of this herbicide is an important step before performing surface complexation model calculations that will be, in turn, used to help predict transport and fate in the environment. In this study, we present quantum mechanical calculations of surface complex structures and vibrational frequencies for glyphosate adsorbed onto goethite. In our calculations, we have considered monodentate and bidentate coordination of the molecules via the phosphonate moiety and three glyphosate protonation states. Molecular orbital/density functional theory (MO/DFT) has been used to predict IR frequencies of possible complexes proposed in the experimental studies (7, 8). These frequencies were obtained by modeling the surface complexes that can occur at the interface and calculating the vibrational frequencies based on electronic structure. The calculated frequencies were compared to the experimental IR frequencies (7, 8) to suggest which type of surface complexes correspond to the experimental spectra.

Materials and Methods The program Gaussian 03 (14) was used to perform quantum mechanical calculations for glyphosate molecules bonded to Fe-hydroxide clusters. These structures were minimized for potential energy without any constraints, and B3LYP functionals (15, 16) with the 6-31G(d) basis set (17) were used to calculate the frequencies for the energy minimized structures. For Fe3+, the pseudopotential basis set CEP-121G (18-20) was chosen. Frequencies were scaled by a factor of 0.9613 (21) to correct for systematic errors, such as the neglect of anharmonicity and approximating electron correlation. Atomic movements corresponding to calculated frequencies were analyzed with the program MOLDEN (version 4.0) (22). Correlation analysis was performed between the calculated and experimental frequencies to determine which type of surface complexes correspond to the experimental spectra. Surface complexes of glyphosate binding with Fehydroxide clusters for bidentate and monodentate structures are shown in Figure 1. The protonation of the amino and the phosphonate groups was represented by changing the number of H+ in the clusters, reflecting the structures with both groups protonated (PMG-), with the phosphonate group unprotonated but amino group protonated (PMG2-), and with all groups unprotonated (PMG3-). These species are shown in Table 1, along with the corresponding pKa values (23) for reference. Nine H2O molecules were added in each case to include the solvation effect on the interface, which can affect vibrational frequencies of anionic species (2427).

Results Model Structures. The energy-minimized surface complexes have predicted Fe-P distances that vary slightly according to both the protonation of the glyphosate molecule and the nature of the surface complex: monodentate or bidentate. The results are summarized in Table 2. There are no readily available experimental data for the interatomic distances between Fe and P in the goethite-glyphosate system; however, these results can be used as predictions for each configuration. EXAFS has currently become possible for P-containing compounds; consequently, it should be possible to measure Fe- - -P distances for glyphosate adsorbed onto goethite. The present results are similar to those reported by Kwon and Kubicki (27) for orthophosphate adsorbed to model Fe-hydroxide surfaces (3.21-3.37 Å), as is the slight distortion of the Fe-hydroxide dimer structures after glyphosate binds 10.1021/es052363a CCC: $33.50

 2006 American Chemical Society Published on Web 05/17/2006

TABLE 2. Distances between the Phosphorus Atom in Glyphosate and Iron Atoms in Goethite for the Six Configurations Considered in This Work dFe1-P/Angstroms PMGPMG2PMG3-

FIGURE 1. (a) PMG- Monodentate complex. (b) PMG- Bidentate complex. Note protonated amine and phosphonate groups, but deprotonated carboxyl moiety corresponding to PMG- structure in both (a) and (b). (c) PMG2- Monodentate complex. (d) PMG2Bidentate complex. Note protonated amine group, but deprotonated phosphonate and carboxyl moieties corresponding to PMG2structure in both (c) and (d). (e) PMG3- Monodentate complex. (f) PMG3- Bidentate complex. All functional groups are deprotonated in e and f, corresponding to PMG3- structure. Fe: orange; O: green; H: red; P: pink; C: gray; N: blue.

TABLE 1. Condensed Molecular Formulas Indicating the Protonation of Glyphosate Species in Solution as a Function of pH.a pKa

molecular structure

2.09 5.52 10.28

+ 2H-CH2-NH2 -CH2-CO2H O--PO2H-CH2-NH2+-CH2-CO2O--PO2--CH2-NH2+-CH2-CO2O--PO2--CH2-NH-CH2-CO2-

a

PMG PMGPMG2PMG3-

O--PO

pKa values at 0.1 M ionic strength from ref 23.

with the dimers. Atomic force microscopy (AFM) indicates that the surface relaxes and reconstructs in solution in acidic conditions (27, 28), so the model relaxation may reflect similar distortions on goethite surfaces caused by adsorbed surface complexes. IR Vibrational Frequencies. The calculated vibrational frequencies for PMG- are listed, with the experimental frequencies, in Table 3. In Table 4 similar data are reported for PMG2-, and in Table 5, for PMG3-. The experimental values, to which the theoretical results are compared, come

monodentate bidentate monodentate bidentate monodentate bidentate

3.32 3.26 3.26 3.00 3.17 3.30

dFe2-P/Angstroms 3.19 3.26 2.92

from two independent sources and correspond to different wavenumber ranges for a similar number of experimental points. The values taken from Sheals et al. (7) are provided from 950 to around 1700 cm-1, whereas those taken from Barja and dos Santos Afonso (8) range between 950 and 1200 cm-1. Both sets of data have been provided at three similar pH values, so the observed frequencies have been combined to provide a larger set of experimental points. The following criteria were used when integrating the sets of experimental data from the independent sources: Between 950 and 1080 cm-1, the data from Sheals et al. (7) gives no specific information, therefore the peaks provided by Barja and dos Santos Afonso (8) were included between these two values; between 1152 and 1624 cm-1, only Sheals et al. (7) provided experimental data, so those values were used as such; between 1080 and 1152 cm-1, the narrow range where both papers reported values, the values from both sources were similar and were listed independently only if significant differences were observed. The observed adsorption frequency listed at 1140 cm-1 (7) was assumed to be a combination of the 1129 and 1150 cm-1 peaks (8) observed in that range. Many calculated frequencies of the glyphosate molecule do not change significantly with bonding to the Fe-hydroxide dimer. Initial correlations of the observed and calculated frequencies over the full range of experimental data were very good for both the monodentate and bidentate coordination of the phosphonate moiety. The frequencies that are most sensitive to the P-O-Fe bonding are thought to exist in the 950-1200 cm-1 range, so the focus of the analysis is on that range. The PMG- structures (Figure 1a,b) corresponds to the most acidic conditions considered in this work. Correlations were performed between the theoretical results for both mono- and bidentate complexes with the combined experimental results from references (7) and (8). The experimental data corresponds to pH values of 4.2 and 3, respectively. To establish the best fit, the values of the slope (m), the intercept (b), R2 and standard deviation (SD) were considered. The graph is shown in Figure 2 and the values are reported in Table 6. Comparing the reported experimental values to the monodentate and the bidentate theoretical results for PMG-, the first three parameters are compatible with a better fit to the monodentate complex, whereas the SD is slightly better for the bidentate complex. For the intermediate pH complex, PMG2- (Figure 1c,d), the theoretical calculations were compared to the experimental values at pH 5 (7) and 5.7 (8). The correlation graph is shown in Figure 3 and the values are reported in Table 6. As in the previous case, the first three indicators considered are better for the fit with the monodentate complex when considering the aggregate experimental data although the SD is again slightly better for the bidentate complex. Finally, for PMG3- (Figure 1e,f), we compared the theoretical calculations with the data provided at pH 7 (8) and 7.4 (7). The correlation graph is shown in Figure 4, and the values are reported in Table 6. In this case as well, the VOL. 40, NO. 12, 2006 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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TABLE 3. Calculated vs Experimental IR Vibrational Frequencies in cm-1 for the Surface Complexes of Glyphosate on Goethite. Adsorbed Ligand in the Form of PMG-.a source pH model

theoretical monodentate

theoretical bidentate

PMG942 (C-Oas + C-N) 977 (C-Oas + CH2 twist) 1001 (P-O(Fe) + C-N + CH2 twist) 1020(P-O(Fe) + POH) 1030 (P-O) 1045 (C-C + H2O) 1052 (CH2 wag) 1057 (CH2 rock) 1063 (P-OH + CH2 rock) 1118 (P-Os + CH2 twist) 1167 (C-N + CH2 twist) 1241 (C-Os + CH2, NH2 twist) 1267 (P-O-H bend + CH2 wag) 1279 (P-O-H bend + CH2 rock) 1311 (C-Os + CH2 wag)

PMG949 (CH2 + NH2 rock) 968 (P-Oas + C-C) 990 (P-O-H bend + CH2, NH2 rock) 1000 (P-O(Fe) + POH) 1005 (C-N)

1376 (C-C + C-Os) 1417 (C-Os + NH2 twist) 1438 (CH2 bend + NH2 rock) 1459 (CH2 bend) 1529 (NH2 bend) 1628 (C-Oas) 1629 (C-Oas) This mode coupled with H2O bend out to 1730 a

1031 (P-Os) 1068 (P-Oas) 1076 (CH2, NH2 rock) 1088 (P-O-H bend) 1209 (CH2, NH2 rock) 1221 (P-O-H bend) 1239 (P-O-H bend + CH2 twist) 1328 (C-Os + CH2 wag) 1336 (C-Os + NH2 wag) 1374 (C-Os + C-C) 1412 (CH2 bend + NH2 wag) 1448 (CH2 bend + NH2 twist) 1460 (CH2 bend) 1481 (CH2 bend) 1600 (C-Oas + NH2 bend) 1610 (NH2 bend)

exp.-refs 7 and 8 3 to 4.2 950 973 988 1008 1028 1050 1080 1129 1150 1327 1399

1576 1612

Experimental values in bold correspond to ref 7.

TABLE 4. Calculated vs Experimental IR Vibrational Frequencies in cm-1 for the Surface Complexes of Glyphosate on Goethite. Adsorbed Ligand in the Form of PMG2-.a source pH model

theoretical monodentate

exp.-refs 7 and 8 5

PMG2928(P-O(Fe)) 940(NH2,CH2 rock) 952 (C-C) 977(CH2, NH2 rock) 989 (CH2, NH2 rock) 1005 (C-N) 1041(C-N) 1085(P-Oas) 1191(CH2 twist) 1284 (CH2 wag) 1300(CH2 wag) 1363 (C-Os + NH2 twist) 1410 (C-Os + NH2 twist) 1423 (CH2 bend) 1455 (CH2 bend) 1546 (NH2 wag) 1627 (C-Oas)

a

theoretical bidentate PMG2906(P-Os + C-N) 942(P-Oas) 954(P-Oas + CH2, NH2 rock) 969 (P-O-Feas + C-N) 1018 (C-N) 1030 (C-N + CH2 rock) 1075 (P-Oas) 1189 (P-Oas) 1191 (CH2, NH2 twist) 1209 (CH2, NH2 twist) 1281 (CH2, NH2 twist) 1305 (C-Oas + CH2 wag) 1354 (C-Os) 1396 (C-C + C-Oas) 1434 (CH2 bend) 1456 (CH2 bend) 1473 (NH2 wag) 1601 (C-Oas) 1642 (C-Oas + NH2 bend)

950 972 988 1009 1026 1055 1080 1134

1327 1401 1491 1590 1624

Experimental values in bold correspond to ref 7.

monodentate complex fits the data well with better value of slope, intercept, and R2 than the bidentate complex. There is only a slightly better value of standard deviation for the bidentate complex.

Discussion The calculated infrared vibrational spectra of the monodentate complexes are in good agreement with the experimental values, as shown above. An issue of interest in the theoretical calculations is the inclusion of H2O molecules to 3838

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adequately simulate the presence of water in the physical system. In all cases calculated in this work, nine water molecules were included to prevent the exchange of H+ ions as established in previous studies (27, 29). In addition, it was necessary to take into account the effect of the increasing number of negative charges with pH, due to the successive deprotonation of the phosphonate and the amino groups. Another interesting feature of the results obtained here is the consistent agreement of the experimental data with the calculations for the monodentate complex model. There

TABLE 5. Calculated vs Experimental IR Vibrational Frequencies in cm-1 for the Surface Complexes of Glyphosate on Goethite. Adsorbed Ligand in the Form of PMG3-.a source pH model

a

theoretical monodentate

theoretical bidentate

exp.-refs 7 and 8 7

PMG3923 (P-O(Fe)) 937 (C-N) 957 (CH2, NH rock) 973 (P-Oas) 979 (P-Os) 1032 (P-Os, CH2 rock) 1036 (CH2, NH rock) 1046 (CH2 rock) 1068 (P-Os) 1084 (P-Os) 1102 (P-Os) 1144 (C-N) 1203 (CH2, NH twist) 1294 (CH2 rock) 1318 (CH2 wag + C-Oas) 1397 (C-Os) 1412 (CH2 bend) 1424 (CH2 bend) 1510 (NH wag) 1558 (C-Oas) 1651 (C-Oas)

PMG3927 (C-N) 951 (CH2, NH rock) 964 (C-Oas, C-P) 971(P-Oas) 1026 (P-O + C-N) 1042 (CH2 rock) 1068 (P-Oas) 1095 (P-Oas + C-N) 1151 (C-N) 1208 (C-N) 1292 (CH2 wag) 1293 (CH2 wag) 1320 (CH2 wag + C-Oas) 1393 (C-C + C-Os) 1416 (CH2 bend) 1423 (CH2 bend) 1537 (NH wag) 1550 (C-Oas) 1630 (C-Oas + HOH bend)

950 980 997 1026 1060 1080 1101 1126

1400

1569 1612

Experimental values in bold correspond to ref 7.

FIGURE 2. Calculated vs observed frequencies for surface complexes of glyphosate (as PMG-) on goethite and experimental data (7, 8) at pH 3. Circles: monodentate; squares: bidentate. See Table 3 for frequencies and Table 6 for linear fit parameters.

FIGURE 3. Calculated vs observed frequencies for surface complexes of glyphosate (as PMG2-) on goethite and experimental data (7, 8) at pH 5. Circles: monodentate; squares: bidentate. See Table 4 for frequencies and Table 6 for linear fit parameters.

TABLE 6. Correlation Parameters between Experimental Data and Theoretical Results for Monodentate and Bidentate Complexes in Three Ranges of pH

the glyphosate molecule changes the IR spectra as a function of pH. This behavior contrasts with modeling results for orthophosphate where calculations indicate that a change from bidentate to monodentate bonding occurs (27). This difference is expected in organophosphate molecules such as glyphosate because the P-C bond in glyphosate takes the place of what would either be a P-OH or P-O-Fe group for orthophosphate adsorbed onto Fe-hydroxides. Previous studies indicate that degradation of adsorbed glyphosate is slower than free glyphosate (3, 4). Moreover, photodegradation of glyphosate in water under natural conditions occurs with DT50 values e28 days, but no substantial photodegradation in soil was recorded in a study lasting 31 days (5). The model presented above provides a theoretical framework that advances the understanding of the mechanisms by which glyphosate is adsorbed to the surface of goethite, and therefore, degrades more slowly than when it is free in aqueous solution. It is interesting to note that the calculated vibrational frequency for the P-O(Fe) bond decreases monotonically with increasing pH values, suggesting a weakening of the adsorption of glyphosate to

PMG-

PMG2-

PMG3-

monodentate

m ) 0.99 b)9 R2 ) 0.97 SD ) 12

m ) 0.95 b ) 50 R2) 0.98 SD ) 9

m )1.05 b ) -52 R2 )0.98 SD ) 8

bidentate

m ) 0.70 b ) 293 R2 ) 0.96 SD ) 10

m ) 1.34 b ) 343 R2 ) 0.97 SD ) 7

m ) 1.2 b ) -196 R2 ) 0.96 SD ) 7

is no indication in the present results that leads to propose the formation of bidentate complexes (7, 8), as the correlations between the results for the bidentate model and the experimental data are not as good as those for the monodentate model for any of our calculations. Consequently, it appears that glyphosate adsorbs as a monodentate species at all relevant pHs and only protonation/deprotonation of

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(6)

(7)

(8)

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(10)

FIGURE 4. Calculated vs observed frequencies for surface complexes of glyphosate (as PMG3-) on goethite and experimental data (7, 8) at pH 7. Circles: monodentate; squares: bidentate. See Table 5 for frequencies and Table 6 for linear fit parameters. goethite in neutral to basic soil conditions. This is the expected behavior as increasingly negative surface charges would tend to limit the amount of adsorption that can occur via the phosphonate moiety. Because the currently modeled surface complex (Figure 1e) reproduces the observed IR spectrum at this pH, we do not believe that a change in bonding mechanism occurs under basic conditions. The adsorption of glyphosate to goethite was studied with quantum mechanical calculations. The equilibrium interatomic distances and relaxation of the surface were compatible with previous calculations and experimental data. The comparison of calculated to experimental values of the infrared vibrational frequencies of glyphosate molecules adsorbed to goethite surfaces supports the idea that in a range of acidic to neutral pHs the molecule is primarily monodentate through the phosphonate moiety. The results provide a theoretical framework for the molecular level interpretation of the adsorption mechanisms of glyphosate on goethite.

Acknowledgments J.D.K. and C.C.T. acknowledge the support of the Center for Environmental Molecular Sciences at Stony Brook (CEMS), an NSF/DOE Environmental Molecular Sciences Institute. Computational support was provided by CEMS, the Center for Materials Simulation, a PSU MRSEC facility, and the Center for Environmental Kinetics Analysis (CEKA) at PSU, an NSF/DOE Environmental Molecular Sciences Institute. K.D.K. acknowledges the support of ACS Petroleum Research Fund grant “Experimental and Theoretical Investigation on Adsorption of Extracellular Compounds onto Mineral Surfaces”.

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Received for review November 23, 2005. Revised manuscript received April 15, 2006. Accepted April 19, 2006. ES052363A

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