Environ. Sci. Technol. 2002, 36, 3090-3095
Adsorption of Glyphosate on Goethite: Molecular Characterization of Surface Complexes JULIA SHEALS,* STAFFAN SJO ¨ BERG, AND PER PERSSON Department of Chemistry, Inorganic Chemistry, Umea˚ University, S-901 87 Umea˚, Sweden
As a component of herbicides, the fate of glyphosate (PMG) in the environment is of significant interest. The nature of PMG adsorption on mineral surfaces plays a significant role in the degradation of PMG. The adsorption of PMG on goethite (R-FeOOH) has been studied as a function of pH and PMG concentration. Adsorption was investigated with batch experiments, attenuated total reflectance Fourier transform infrared spectroscopy (ATRFTIR), and X-ray photoelectron spectroscopy (XPS). The N 1s line in XPS spectra showed deprotonation of the amine group of PMG (NH2+) with increasing pH. IR analyses showed no evidence for the interaction of PMG’s carboxylate group with the goethite surface, while the phosphonate group formed inner-sphere complexes. There is evidence for intramolecular hydrogen bonding between NH2+ and both the carboxylate and the phosphonate groups at low pH. Intramolecular hydrogen bonding is lost when the amine group is deprotonated, and the trend in intramolecular hydrogen bonding between NH2+ and phosphonate shows that PMG adsorbs via predominantly monodentate complexation. A minor quantity of bidentate complexes is thought to form both at near-neutral pH and when the surface concentration of PMG is low. While the phosphonate group of PMG binds directly, the carboxylate group remains relatively “free” from complexation with goethite, leaving it subject to degradation and/or complexation with metal ions present in the environment.
Introduction N-(Phosphonomethyl)glycine (PMG), also known as glyphosate, has been receiving an increasing amount of attention in the literature, with emphasis on its environmental chemistry. As a component of organophosphorus herbicides, it is important to investigate the interactions of PMG with both naturally occurring and anthropogenic species in order to determine its fate in the environment. One very significant aspect is the role of the solid-water interface. The nature of PMG adsorption onto mineral surfaces may determine how easily PMG lends itself to degradation, an important factor when assessing the bioavailability and potential toxicity of PMG. PMG forms a pH-dependent zwitterion in solution and possesses three donor groups (an amine group, a carboxylate group, and a phosphonate group) that are responsible for * Corresponding author telephone: +46 90 786 6846; fax: +46 90 786 9195; e-mail:
[email protected]. 3090
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complexation reactions with metal ions and mineral surfaces. There are several papers discussing the adsorption of solely PMG or related ligands on mineral surfaces (1-4) and several with PMG and trace metal ions adsorbed on mineral surfaces (5-7). Some authors have focused on more quantitative aspects of adsorption (2, 4, 5, 7) while others have attempted to understand the interactions of PMG and related ligands at a molecular level (1, 3, 6). This paper is designed to contribute new and complementary information for a better understanding of the structure of PMG complexes formed at the goethite-water interface. In the present study, we examine the adsorption of PMG on the surface of goethite (R-FeOOH) as a function both of pH and of PMG concentration. The structures of surface complexes formed are primarily investigated with Fourier transform infrared (FTIR) spectroscopy. X-ray photoelectron spectroscopy (XPS) and a speciation model (8) are used to investigate the protonation state of the amine group.
Experimental Section Materials. Boiled Milli-Q plus 185 water (resistance ) 18.2 MΩ) was used in all H2O-based solutions and suspensions. D2O (99.9 atom % D; Aldrich) and DCl (diluted from 35% DCl in D2O; Aldrich) were used for D2O-based samples. N(phosphonomethyl)glycine was from Sigma, and its purity (95%) has been verified with potentiometry. NaNO3 (Merck p.a., dried at 353 K) and NaCl (Merck p.a., dried at 453 K) were used to provide constant ionic media of 0.1 M. Glyphosate-phosphonomethyl-14C (Sigma) and OptiPhase Hi-Safe 3 (Wallac Oy) scintillation cocktail were used in determining PMG concentrations. Goethite with a BET surface area of 85 m2 g-1 was prepared and characterized as described by Boily et al. (9). The main crystal plane of goethite has been previously identified as the {110} plane (10). Transmission electron microscopy imaging performed by Boily et al. (9) showed the {110} plane to represent more than 90% of the goethite surface, while the remaining 10% (terminations of the goethite particles) could be approximated to the {001} plane. pH Measurements. A combination glass electrode was used to measure pH (and pD) in all samples. The electrode was externally calibrated using HNO3, HCl, or DCl solutions of known concentrations, with a constant ionic medium of 0.1 M Na(NO3) or Na(Cl). Adsorption Studies. To produce adsorption isotherms for PMG adsorbed at the goethite surface in a constant medium of 0.1 M Na(NO3), at 25 °C, 10-mL samples were prepared in polypropylene centrifuge tubes. The 5-mL aliquots of 20 g L-1 goethite suspension were mixed with varying quantities of a 10 mM 14C-labeled PMG solution, NaOH or HNO3, and NaNO3 solution. Samples were purged with Ar to eliminate carbon dioxide. The samples were equilibrated overnight, pH was measured, and the samples were centrifuged. pH ranged from 3 to 9.6, and total concentration ranged from 0.2 to 2.7 µmol of PMG/m2 of goethite. For IR analyses, similar adsorption studies (total concentration ) 2.3 µmol of PMG/m2 of goethite) were carried out in 0.1 M Na(Cl) to avoid interference from nitrate in IR spectra. To monitor dissolution of goethite, the Fe content of the supernatants was analyzed with flame atomic absorption spectrometry (Perkin-Elmer). Samples of PMG adsorbed on goethite (also 2.3 µmol of PMG/m2 of goethite) were prepared in the range 4 < pD < 9 to examine the effect of D2O on the IR frequencies. The surface concentration of PMG was studied as a function of total PMG concentration at three different 10.1021/es010295w CCC: $22.00
2002 American Chemical Society Published on Web 06/07/2002
constant pH values (pH 4.3, 6.0 and 8.5) with a constant ionic medium of 0.1 M Na(Cl). A PHM290 pH stat controller was used to maintain constant pH in a goethite suspension while aliquots of a 24.6 mM 14C-labeled PMG solution were added, one aliquot per day. The total concentration in the samples ranged from 0.7 to 4.6 µmol of PMG/m2 of goethite. The suspension was continuously purged with N2 to eliminate carbon dioxide. Prior to each addition, a 3-mL sample was removed from the suspension and centrifuged. The PMG remaining in the aqueous phase was measured for all adsorption samples by adding 5 mL of supernatant to 10 mL of scintillation cocktail and measuring the activity with a Beckman LS6500 multipurpose scintillation counter. X-ray Photoelectron Spectroscopy. PMG adsorbed on goethite as a function of pH was examined with XPS using a Kratos Axis Ultra spectrometer. The samples were prepared with a total concentration of 2.3 µmol/m2 of goethite. After centrifugation, a drop of wet goethite paste was loaded onto a sample holder and placed on the precooled (20 min at -160 °C) end of the transfer rod within the analysis chamber. There was a 20-s delay before pumping, and the pressure in the analysis chamber during the experiments was 10-8.310-8.5 Torr. The temperature was maintained at -160 °C throughout the measurements. The precooling procedure aims to minimize chemical changes at the mineral/solution interface due to loss of H2O (11). Wide spectra (pass energy 160 eV) and spectra of individual photoelectron lines (pass energy 20 eV) P 2p, Cl 2p, C 1s, N 1s, O 1s, Fe 2p, and Na 1s were acquired using an Al KR source operated at 225 W. To compensate surface charging, a low energy electron gun was used. Spectra were processed with Kratos software, and the binding energy scale was referenced to the C 1s line of aliphatic carbon contamination set at 285.0 eV. Attenuated Total Reflectance Fourier Transform Spectroscopy. Attenuated total reflectance (ATR) FTIR spectra were collected using a Perkin-Elmer 2000 FTIR spectrometer fitted with a deuterated triglycine sulfate (DTGS) detector. The spectra were recorded with a horizontal ATR accessory and a diamond crystal as the reflection element (SensIR Technologies). The sample cell was purged with nitrogen gas throughout data collection to exclude carbon dioxide and water vapor. The angle of incidence for the setup was approximately 45°, which is far from the critical angle. This and the fact that the bands analyzed are weak (
