Environ. Sci. Technol. 2008, 42, 2464–2469
Adsorption of Glyphosate on Goethite (r-FeOOH): Surface Complexation Modeling Combining Spectroscopic and Adsorption Data CAROLINE M. JONSSON, PER PERSSON, STAFFAN SJÖBERG, AND JOHN S. LORING* Department of Chemistry, Umeå University, SE-901 87 Umeå, Sweden
Received April 24, 2007. Revised manuscript received January 9, 2008. Accepted January 16, 2008.
N-(phosphonomethyl)glycine (glyphosate, PMG) is the most widely used herbicide, and its adsorption onto soil minerals plays a significant role in its mobility and rate of degradation. In this work, we present the results of the first serious effort to find a realistic surface complexation model that fits both adsorption and total proton concentration data for PMG on the common soil mineral, goethite. Special attention was focused on making sure that the final model was in good semiquantitative agreement with previously reported X-ray photoelectron spectroscopy and Fourier-transform infrared spectroscopic measurements. Electrostatic effects were accounted for using the Basic Stern model, and the charges of the PMGcontaining surface complexes were assumed to be distributed across the 0- and β-planes. The reactions for the protonation of the goethite surface were described using the 1 pK model. We optimized on the intrinsic formation constants and the charge distributions of the complexes, as well as the initial total proton concentration (I ) 0.1 M Na(NO3), 25.0 °C), and the following model was obtained. ≡FeOH0.5- + H3L h ≡FeHL1.5- + H+ + H2O Log10 β ) 4.70 ( 0.08, Q0 ) -0.18 ( 0.02 ≡FeOH0.5- + H3L h ≡FeL2.5- + 2H+ + H2O Log10 β ) -3.9 ( 0.1, Q0 ) -0.7 ( 0.1 Here, β is the intrinsic formation constant, Q0 is the charge at the 0-plane, and the errors are reported as one standard deviation. The charge distributions of the complexes are rationalized by considering intramolecular hydrogen bonding between the protons of the amine group and both the phosphonate and carboxylate groups.
Introduction N-(phosphonomethyl)glycine (glyphosate, PMG) is a widely used organophosphorous herbicide that outranked all other herbicides and pesticides used in the United States for agriculture in 2001, with an estimated 40 million kilograms applied to crops during that year (1). As a consequence of frequent usage, PMG has been found in surface waters and streams following agricultural, urban, and forestry applica* Corresponding author e-mail:
[email protected]; phone +46-90-786 63 28; fax: +46-90-786 91 95. 2464
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tions. PMG functions by inhibiting an enzyme in the shikimic acid pathway of aromatic amino-acid biosynthesis. Because this pathway is only found in plants and microorganisms, PMG has a low toxicity to mammals and birds (2). However, organisms may be indirectly affected by the use of PMG, because it could have a negative impact on the ecosystem by damaging nontarget vegetation that act as important habitats (3). One of the main ways that glyphosate is inactivated in soils is through its adsorption onto mineral surfaces (4), and this decreases its mobility and efficiency as a herbicide (5, 6). PMG adsorbs strongly to goethite (R-FeOOH) (7–9), manganite (γ-MnOOH) (10, 11), and aluminum (hydr)oxides, such as gibbsite (R-Al(OH)3) (5) and bayerite (β-Al(OH)3) (11). These minerals are all common components in soils and are therefore of great importance when discussing the impact and fate of PMG in the environment. The PMG molecule is strongly polar and has three proton-active groups; an amine, a carboxylate, and a phosphonate group. Like other phosphonates, PMG competes with inorganic phosphate for unoccupied binding sites (12–15). It adsorbs to mineral surfaces through its phosphonic acid moiety, and most studies show that the amine and carboxylate groups remain relatively free from complexation with goethite (7, 16) (but also see ref 17), leaving these groups potentially subject to degradation and/or complexation with metal ions in the aqueous phase (7, 18–20). It was recently shown that PMG retards microbial activity in soil but is still microbially degradable when adsorbed to goethite (18). There have been two recent studies that have investigated the adsorption of PMG on goethite using a combination of macroscopic and spectroscopic measurements (7, 16). The first was published in 2002 from our own department by Sheals et al. (7) and used a combination of adsorption measurements, X-ray photoelectron spectroscopy (XPS) and Fourier-transform infrared spectroscopy (FTIR). The XPS results confirmed that the amine group of adsorbed PMG is proton-active over the pH range 3-9 and is not coordinated to iron at the surface. The infrared results showed that any interaction of the carboxylate group with the surface is also negligible, but significant shifts in the P-O stretching bands compared with PMG in solution confirmed that the ligand is inner-spherically coordinated to the goethite surface through the phosphonate group. Furthermore, a significant downward shift of a particular P-O stretching band with decreasing pH was interpreted as due to intramolecular hydrogen bonding between the phosphonate and the increasingly protonated amine groups. The presence of this intramolecular hydrogen bonding and the direction of the shift of this P-O stretching band is explained by assuming that PMG is complexed to surface Fe(III) through only one oxygen of its phosphonate group. This is firm evidence for the predominance of monodentate complexation at the surface (7), and monodentate coordination through the phosphonate group is also supported by a recent ab initio quantum mechanical study (21). The second study was published in 2005 by Barja and Afonso (16) and concluded that PMG adsorbs onto goethite through the phosphonate group in a bidentate-bridging configuration at high pH and in a monodentate fashion at low pH. The only spectroscopic technique used was FTIR, and they partially based their conclusions by analogy with their previous study of methylphosphonic acid adsorption onto goethite (22). They suggested that the amine group was protonated over the pH range 3-9, the phosphonate group of the bidentate-bridging complex was unprotonated, and 10.1021/es070966b CCC: $40.75
2008 American Chemical Society
Published on Web 02/28/2008
the phosphonate group of the monodentate complex was (mostly) protonated. Because protonation of the amine over the entire pH range is disputed by the XPS results of Sheals et al. (7), we consider the infrared spectroscopic interpretations and the conclusions drawn by Barja and Afonso (16) as being questionable. The aim of this work was to develop a surface complexation model that explains the pH and coverage dependence of the adsorption of PMG onto the surface of goethite. Such a model could help to predict the distribution of PMG in natural waters, and this contributes to an understanding of its degradation and bioavailability in the environment. Although the constant capacitance model (CCM) was applied to the goethite-PMG system in both the Sheals et al. (7) and Barja and Afonso (16) articles, the CCM is limited because the charge of a surface complex can only be placed at the plane of the surface (23). PMG is a relatively large molecule, and the charge of a goethite-PMG surface complex is certainly not only located at the surface but also further in the electrical double layer. Thus, we apply a more realistic electrostatic model that incorporates charge distribution, the Basic Stern model (BSM) (23). This is the first serious effort to model the goethite-PMG system, and we fit to both adsorbed PMG concentrations and total proton concentration data. Furthermore, the model we present is based not only on data from macroscopic measurements but also on previous characterizations of glyphosate-goethite surface complexes based on molecular level probes. Here, we constrain our modeling efforts by the spectroscopic results of Sheals et al. (7).
Background Surface complexation theory treats adsorption of ions according to thermodynamic concepts of complexation in solution. However, the basic ideas regarding thermodynamic solution speciation must be extended to address (i) electrostatic effects at the surface-solution interface, (ii) surface-site heterogeneity, and (iii) the reactions for protonation of surface sites (23). Below, we describe the components of the surface complexation model used in this study. Equilibrium constants for the formation of surface complexes must be corrected to account for the electrostatic mean field potential of charged surfaces. There are several standard electrostatic models used for this correction (23), and here we have chosen to apply the BSM (24). The BSM is the simplest model that accounts for ionic-strength dependence and describes electrolyte specific behavior by introducing ion-pair formation between the ions of the electrolyte and the oppositely charged surface functional groups. It describes the electric double layer as an inner compact layer combined with a diffuse layer, and charges may be placed either at the plane defined by the mineral surface (0-plane) or the plane that separates the inner layer from the diffuse layer (β-plane) (23). In the BSM, an apparent formation constant (βapp) is the product of an intrinsic constant (βint) and the appropriate electrostatic corrections (eq 1). βapp ) βinte(-∆z0Ψ0F⁄RT)e(-∆zβΨβF⁄RT)
(1)
In eq 1, ∆z0 and ∆zβ are the differences in charge between the formed and reacting surface species at the 0- and β-planes, respectively, and Ψ0 and Ψβ are the potentials at these planes. The apparent formation constant is equal to the intrinsic formation constant when the surface is uncharged, i.e., whenΨ0 ) Ψβ ) 0 . The values of Ψ0 and Ψβ are calculated under several constraints, including electroneutrality of the particles, and the following relation exists (eq 2),
Ψ 0 - Ψβ )
σ0 CStern
(2)
where CStern is the capacitance of the charge-free Stern layer, located between the 0- and β-planes. In this study, we apply the concept of charge-distribution (CD) as introduced by Hiemstra et al. in 1996 (25). The CD model is a combination of the classical Gouy–Chapman treatment of the diffuse double layer (26) and the distribution of charge in a solid crystal as introduced by Pauling in 1929 (27). Adsorbed ligands are not being treated as point charges, but instead, their charge is distributed in the interfacial region. Thus, in the framework of the BSM, a fraction (f) of the charge of a surface complex is assumed to be located at the 0-plane while the remainder (1 - f) is placed at the β-plane; the actual charges at the 0- and β-planes are defined as Q0 and Qβ, respectively. This is a more realistic approach for treating the electrostatic contributions of bulky ligands with several functional groups, such as the PMG molecule investigated here. The simplest method to estimate f is to use Pauling’s bond valence theory (27), and in this study we optimize on the charge distribution by treating Q0 as an unknown parameter in our modeling. The heterogeneity of the surface is addressed using the MUSIC model developed by Hiemstra and co-workers (25, 28, 29), which uses crystallographic information to distinguish singly, doubly, and triply coordinated O(H) and OH(H) surface groups of (hydr)oxide minerals. Predictions of the reactivity of the different types of surface functional groups suggest ≡FeOH0.5- and ≡Fe3OI0.5- sites to be responsible for the acid-base properties of the goethite surface in the pH range from 1 to 11. On the basis of crystallographic data, the site densities of singly and triply coordinated sites are 3.64 and 2.73 sites/nm2 (30), respectively, which correspond to 10.6 µmol/m2 proton-active sites overall. The reactions for the protonation of the goethite surface are described using the 1 pK model (31), which invokes only one pKa value per surface group. Thus, the protonation reactions for the singly and triply coordinated sites, respectively, are as follows, ≡FeOH0.5- + H+ h ≡ FeOH20.5+ β≡FeOH2
(3)
≡Fe3O0.5- + H+ h ≡ Fe3OH0.5+ β≡FeOH
(4)
0.5+
0.5+
where the charges of these sites are assumed to be located in the 0-plane and are assigned according to Pauling’s bond valence theory (27). An advantage of the 1 pK approach is that the pKa values can be experimentally obtained as the pristine point of zero charge (pHppzc) (32). The previously determined values for the formation constants of Reactions 3 and 4 at zero ionic strength are listed in Table 1, as well as the formation constants for the ion-pairing reactions with Na+ and NO3- (30).
Experimental Section In this current study, we used some adsorption and total proton concentration data previously collected in our department. The adsorption data has already been reported in Sheals et al. (7), but the total proton concentration data is first published here. For the sake of clarity, we repeat the experimental information that is relevant to this data. Chemicals. All solutions and suspensions were made from deionized and boiled water (resistance ) 18.2 MΩ/cm), and NaNO3 (Merck p.a., dried at 80 °C) was used to provide a constant ionic medium of 0.1 M Na(NO3). The parentheses around NO3- indicate that the nitrate concentration was allowed to vary while the sodium ion concentration was held constant. Stock solutions of HNO3 (Fisher, p.a.) were standardized coulometrically or against tris(hydroxymethyl)VOL. 42, NO. 7, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 1. Proposed Surface Complexation Model for the Adsorption of PMG onto Goethitea reaction
Log10 βb
Q0
Qβ
c
deprotonation of PMG in solution -2.44 -8.33 -19.14 protonation of goethite surfaced ≡FeOH0.5- + H+ h ≡FeOH20.5+ 9.4 ≡FeOH0.5- + Na+ h ≡FeOH0.5--Na+ -0.69 ≡FeOH0.5- + H+ + NO3- h ≡FeOH20.5+-NO38.51 ≡Fe3O0.5- + H+ h ≡Fe3OH0.5+ 9.4 ≡Fe3O0.5- + Na+ h ≡Fe3O0.5--Na+ -0.69 ≡Fe3O0.5- + H+ + NO3- h ≡Fe3OH0.5+-NO38.51 adsorption of PMG onto goethitee ≡FeOH0.5- + H3L h ≡FeHL1.5- + H2O + H+ 4.70 ( 0.08 ≡FeOH0.5- + H3L h ≡FeL2.5- + H2O + 2H+ -3.9 ( 0.1 H3L h H2L- + H+ H3L h HL2- + 2H+ H3L h L3- + 3H+
+0.5 -0.5 +0.5 +0.5 -0.5 +0.5
0 +1 -1 0 +1 -1
-0.18 ( 0.02 -0.7 ( 0.1
-1.32 -1.8
a The BSM was used to account for electrostatic effects at the mineral surface. The capacitance of the Stern layer is 0.94 F/m2. b Constants at zero ionic strength. Constants for surface complexes are intrinsic. c Constants recalculated to zero ionic strength from values in ref 19. d Reference 30. e Log10 β and Q0 values were optimized in this work. The errors are one standard deviation (σ). ∆z0≡FeHL1.5- ) 0.32, ∆zβ≡FeHL1.5- ) -1.32, ∆z0≡FeL2.5- ) -0.2, and ∆zβ≡FeL2.5- ) -1.8
aminomethane (Trizma base). NaOH (Merck, p.a.) solutions were standardized against these standardized HNO3 solutions. N-(phosphonomethyl)glycine (PMG) (Sigma-Aldrich, purity >95%) and glyphosate-phosphonomethyl-14C (SigmaAldrich) were used without further purification. The actual concentration of PMG in our stock solutions was determined by potentiometric titrations with a precision of (0.3%. Goethite Synthesis and Characterization. Goethite was synthesized according to the method described by Hiemstra et al. (29). Briefly, 2.4 L of 2.5 M KOH (EKA p.a.) was added to 10 L of 0.5 M Fe(NO3)3 · 9H20 (Merck p.a.) at a rate of 10 mL min-1. The suspension was stirred with a propeller at room temperature. The precipitates were aged for 100 h at 60 °C and then dialyzed for two weeks to remove dissolved potassium nitrate and excess potassium hydroxide. X-ray powder diffraction and electron microscopy were used to confirm that the resulting particles were goethite. A specific surface area of 85 m2 g-1 was determined using the BET N2 adsorption method (33). Adsorption Experiments. A total of 82 batch samples were prepared with a solid concentration of 9.3 g/L and with a total concentration of 14C-labeled PMG ranging from 0.2 to 3.5 µmol/m2 (7). pH was adjusted by adding precise volumes of standardized HNO3 or NaOH in order to cover the pH range 3–10. The lower pH limit was chosen to avoid dissolution of the mineral, whereas the upper limit was dictated by the uncertainties in pH measurements of the glass electrode. Argon gas was constantly purged through the suspensions to avoid contamination by CO2. The samples were put on an end-over-end rotation test tube holder for equilibration overnight at 25 ( 0.5 °C. -Log10 [H+] was then measured using a combination glass electrode that was calibrated in solutions of known H+ concentrations. pH was calculated from -Log10 [H+] using the Davies equation (eq 5). The samples were centrifuged at a relative centrifugal force of 2880g, and the concentration of PMG in the supernatant was measured by liquid scintillation counting (Beckman LS6500). The quantity of PMG adsorbed at the surface of goethite was calculated as the difference between the known total concentration and the concentration remaining in the aqueous phase. To monitor the dissolution of goethite, the total concentration of Fe(aq) in the supernatant was analyzed using flame atomic absorption spectroscopy (Perkin-Elmer). No iron was detected; thus, the solid phase was assumed to remain stable during the experiments (7). Calculations. The computer program MAGPIE (34) was used to fit a surface complexation model to the experimental 2466
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results. MAGPIE minimizes the sum of the squares of the deviations (U) between calculated and experimental data using a modified Levenberg–Marquardt (35) nonlinear leastsquares algorithm where the Jacobian is calculated by a finite difference approach. In the course of the minimization, unknowns are iterated upon until the smallest value of the sum of the squares of the deviations is found. Calculated values are evaluated using the SGW algorithm by Eriksson (36), and activity coefficients (γi) are determined according to the Davies equation (26),
(
)
√I (5) - 0.2I 1 + √I where I is the ionic strength in units of molarity, and zi is the charge of species i. A residual sum of squares was calculated for both total proton concentration (UTotH) and adsorption (UAds) data, and the total residual sum of squares (U) is Log10 γi ) -0.509zi2
(6) U ) UTotH + wUAds where w is a weighting factor. The weighting factor is set to a value so that the data from the two techniques span the same range of numerical values to avoid bias in the optimization; the weighting factor is not based on experimental errors of the observations. This method for determining the weighting factor is model-independent and assumes that the two data types have approximately the same relative error. Total proton concentrations were analyzed by minimizing the following sum of the squares of the deviations, N
UTotH )
∑ (H
2 Tot Tot i,Calc - Hi,Meas
)
(7)
i)1
where Hi,TotCalc and Hi,TotMeas are the calculated and measured total proton concentrations, respectively, for the ith data point, and N is the overall number of data points. Similarly, adsorption data were treated according to eq 8, N
UAds )
∑ (C
2 Ads Ads i,Calc - Ci,Meas
)
(8)
i)1
where Ci,AdsCalc and Ci,AdsMeas are the calculated and measured concentrations of PMG adsorbed on the goethite surface, Ads respectively. Values of HTot i, Calc and Ci, Calc were calculated from the surface complexation model.
Results and Discussion The total proton concentration and adsorption data are shown in Figure 2. PMG adsorbs strongly in the interval 3 e
FIGURE 1. (a) Total proton concentrations and (b) adsorbed PMG concentrations versus pH from batch adsorption experiments in 0.1 M Na(NO3). The solid lines are the fits to the data based on the surface complexation model in Table 1. The legend in panel a gives the total PMG concentrations (in µmol/ m2) for each batch series, and this legend also applies to the adsorption data in panel b. Note that total proton concentrations were not correctly monitored in the batch series with total PMG concentrations of 2.6 µmol/m2 and 3.5 µmol/m2; hence, these data were excluded from the total proton data in panel a. The adsorption data in this figure were previously published in Sheals et al. (7). pH < 5, and it is not until the total PMG concentration is higher than 1.6 µmol/m2 that PMG is detected in solution. The data indicate that the surface is nearly saturated below pH 5 with about 2.7 µmol/m2 of PMG adsorbed. Adsorbed concentrations decrease with pH values above 5 as the surface becomes less positively charged due to the deprotonation of surface sites. Only a maximum of ∼0.5 µmol/m2 is adsorbed at pH values at or above the pHpzc of goethite (9.3 in 0.1 M Na(NO3) ionic medium). This reflects the unfavorable electrostatic conditions for a negatively charged PMG to adsorb to a negatively charged surface, yet the fact that it still adsorbs demonstrates the covalency of the adsorbate–surface bond. The spectroscopic evidence suggests that there are two major surface complexes, both of which are inner sphere where PMG is bound in a monodentate fashion to a surface iron through one of the phosphonate oxygens. Neither the phosphonate nor the carboxylate groups are protonated, and the complexes differ only by the protonation state of the amine group. Because rates of oxygen exchange at doubly and triply coordinated oxygen sites are several orders of magnitude lower than for singly coordinated sites (37), we only consider singly coordinated sites to participate in innersphere complexation. Hence, we propose the stoichiometries ≡FeHL1.5- and ≡FeL2.5- for these complexes, and their adsorption can be attributed to the following reactions: ≡FeOH0.5- + H3L h ≡ FeHL1.5- + H+ + H2O β≡FeHL
1.5-
≡FeL2.5-
≡FeOH0.5- + H3L h ≡ FeL2.5- + 2H+ + H2O β where β is the intrinsic formation constant.
(9)
(10)
Our strategy was to model the adsorption and total proton data in Figure 2 using the BSM, and assuming the reactions in eqs 9 and 10, as well as the reactions describing deprotonation of PMG in solution and the protonation of the goethite surface in 0.1 M Na(NO3) medium listed in Table 1. The calculation involves a determination of the formation constants (β≡FeHL1.5- and β≡FeL2.5-) and Q0 values (Q0≡FeHL1.5and Q0≡FeL2.5-) for the two surface species. The sum of Q0 and Qβ was constrained to equal the total charge of the surface complex, so the calculation of Q0 amounts to the determination of the charge distribution between the 0- and β-planes in the BSM. The initial total proton concentration was corrected to account for the fact that the pH of the stock goethite suspension after purging with argon was about 8.5, although this pH should be at the pHpzc of 9.3 (0.1 M ionic medium). The reason for this pH difference is probably due to adsorbed carbonate and/or the presence of residual nitrate that were not removed during the goethite synthesis. A correction of 0.23 mM was used and is based on the protonation reactions for the goethite surface (see Table 1), as well as the solid concentration and the surface area of the mineral. Initial guesses for Log10 β≡FeHL1.5- and Log10 β≡FeL2.5were 6 and -6, respectively, and their Q0 values were guessed to be -0.25, as estimated from Pauling’s bond valence theory (27). The adsorption data were weighted by a factor of 2.77 (see eq 6) so that the adsorption and total proton data spanned the same range of numerical values. We optimized on the formation constants and charge distributions simultaneously, and although a good fit to the data was obtained, the species distribution was directly contradicted by semiquantitative infrared spectroscopic results. In an attempt to improve the agreement with spectroscopy, we also chose to set the initial total proton concentration as an unknown. All five unknowns were optimized simultaneously, and the final results for the formation constants and Q0 values are reported in Table 1. That the model explains the data well is demonstrated in Figure 2 by the excellent agreement between the calculated and the experimental results. The initial total proton concentration was calculated to be 0.15 ( 0.01 mM, which is close to the estimated value of 0.23 mM (see above). The disagreement between these calculated and estimated values could be due to a carbonate contaminant that is consumed and released as carbon dioxide when the goethite suspension is acidified. Charge distribution is important when describing the adsorption of a bulky PMG molecule to the surface of goethite. Here, we have used a very simplistic model that assumes the charge of an adsorbed PMG molecule is distributed at only two surface planes. The results show that the charge in the 0-plane (Q0) of the protonated surface complex is less negative than the corresponding charge of the unprotonated surface complex, which may be explained by the previous infrared spectroscopic results (7). The spectra indicate that within the ≡FeHL1.5- complex there is intramolecular hydrogenbonding between the protons of the amine group and both the phosphonate and carboxylate groups (see Figure 1). This hydrogen bonding relocates negative charge away from the 0-plane and toward the outer β-plane. At higher pH, the amine group is deprotonated and the hydrogen bond is lost. Thus, the charge at the 0-plane becomes more negative, which is consistent with our modeling results. Figure 3 shows distribution diagrams at total PMG concentrations of 0.8 and 2.3 µmol/m2 according to the model in Table 1 and covering the pH range of our experimental data. For both total PMG concentrations, ≡FeHL1.5- is the dominant surface complex below pH ) 5. At the lower total PMG concentration of 0.8 µmol/m2, ≡FeL2.5- begins to form at about pH ) 4 and dominates above pH ) 8, yet a mixture of the protonated and unprotonated forms exists even as high as pH ) 9.5. At a total PMG concentration of 2.3 µmol/ VOL. 42, NO. 7, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 2. Proposed stoichiometries and structures of PMGgoethite surface complexes based on previous results from infrared spectroscopic and XPS measurements. This figure was modified from Sheals et al. (7).
can be calculated to be 8.6 from our modeling results in Table 1. However, in solution the pKa for the deprotonation of the amine group is 10.81 (see Table 1), which is significantly higher. In other words, PMG has increased acidity at the goethite surface compared with the corresponding species in solution. This could be explained in terms of electrostatics in that the ≡FeL2.5- complex is more favorable on a positively charged mineral surface at lower pH compared with the solution species. With increased coverage, the surface becomes more negatively charged, and the ≡FeHL1.5- is more favored. Hence, the conditional pKa shifts to a higher value. This phenomenon seems to be a typical feature with respect to the adsorption of weak acids onto mineral surfaces and their complexation to metal ions in solution (38–40). Given that we optimized on five parameters, it is perhaps not surprising that such a good fit was obtained to our data. In fact, there are other sets of values for the formation constants and charge distributions that result in a fit nearly as good as the one shown in Figure 2. The merit of the surface complexation model presented in Table 1 is that it is in excellent semiquantitative agreement with spectroscopic results, whereas alternate models fail in this respect. This study demonstrates the value of using spectroscopy to apply constraints and to obtain more realistic values for model parameters. The rate at which PMG degrades in the environment depends on the extent to which it adsorbs onto soil minerals such as goethite. The reactivity of PMG is probably also influenced by its surface speciation, that is, whether it exists on goethite in the protonated or unprotonated form. The surface complexation model we have determined can be used to predict both the amount and the speciation of PMG at the goethite surface and in solution at different solid/solution ratios, mineral surface areas, total PMG concentrations, pH values, etc. This information could be useful for understanding its degradation and bioavailability in soils. Furthermore, the results presented here form the basis for a forthcoming publication that concerns the modeling of the ternary systems of PMG, goethite, and the heavy metals Cu(II) and Cd(II).
Acknowledgments
FIGURE 3. Distributions of PMG in solution and at the surface of goethite modeled according to the parameters in Table 1 and assuming a 10 g/L suspension, a specific surface area of 85 m2/ g, and a 0.1 M Na(NO3) ionic medium. FPMG is the fraction of the total concentration of PMG found in respective species. m2, ≡FeL2.5- starts to form at pH ) 6 and dominates above about pH ) 9, but it also coexists with the protonated complex at our highest pH. These results are in excellent semiquantitative agreement with previous infrared spectroscopic measurements. Spectra collected at a total PMG concentration of 2.3 µmol/m2 and covering the range 4.2 < pH < 8.5 show that ≡FeHL1.5- is the dominant surface complex until at least pH 5.7. The ≡FeL2.5- complex is clearly detected at pH ) 6.0, but a mixture of the protonated and unprotonated forms exists at pH ) 8.5. Infrared data at a total PMG concentration of 0.8 µmol/m2 also show that the ≡FeL2.5- is detected at pH ) 6.0. Furthermore, the spectra indicate that the ratio of the concentrations of ≡FeL2.5- and ≡FeHL1.5- is larger at a total PMG concentration of 0.8 µmol/m2 compared to the concentration of 2.3 µmol/m2 at pH ) 6, which is also predicted by our model. It is also interesting to note from the distribution diagrams in Figure 3 that the apparent pKa for the deprotonation of the amine group for adsorbed PMG is between 8 and 9, depending on the coverage. Additionally, the intrinsic pKa 2468
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The authors thank Dr. Julia Jönsson for performing the experimental work. Dr. Johannes Lützenkirchen is gratefully acknowledged for his valuable comments. Dr. Bernd Nowack is also thanked for his helpful comments that significantly improved the quality of this manuscript. This work was financed by the Swedish Research Council.
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