Molecular Insights into Glyphosate Adsorption to Goethite Gained from

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Molecular Insights into Glyphosate Adsorption to Goethite Gained from ATR-FTIR, Two-Dimensional Correlation Spectroscopy, and DFT Study Wei Yan, and Chuanyong Jing Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.7b05643 • Publication Date (Web): 22 Jan 2018 Downloaded from http://pubs.acs.org on January 22, 2018

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Environmental Science & Technology

Molecular Insights into Glyphosate Adsorption to Goethite Gained from ATR-FTIR, Two-Dimensional Correlation Spectroscopy, and DFT Study

Wei Yana and Chuanyong Jinga,b*

a

State Key Laboratory of Environmental Chemistry and Ecotoxicology, Research

Center for Eco-Environmental Sciences, Chinese Academy of Sciences, Beijing 100085, China

b

University of Chinese Academy of Sciences, Beijing 100049, China

*Corresponding author: Dr. Chuanyong Jing; Tel: +86 10 6284 9523; Fax: +86 10 6284 9523; E-mail: [email protected]

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ACS Paragon Plus Environment


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Abstract

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Glyphosate (PMG) complexation on iron (hydr)oxides impacts its fate and transport in the

3

environment. To decipher the molecular-level interfacial configuration and reaction mechanism

4

of PMG on iron (hydr)oxides, the PMG protonation process, which influences the chemical and

5

physical properties of PMG, was first determined using ATR-FTIR spectroscopy. The FTIR

6

results reveal that the deprotonation occurs at carboxylate oxygen when pKa1< pH < pKa2, at

7

phosphonate oxygen when pKa2< pH < pKa3, and at amino nitrogen when pH > pKa3. PMG

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complexation on goethite was investigated using in situ flow-cell ATR-FTIR, two-dimensional

9

correlation spectroscopy (2D-COS), and density functional theory (DFT) calculations. The

10

results indicate that the phosphonate group on PMG interacts with goethite to form inner-sphere

11

complexes with multiple configurations depending on pH: binuclear bidentate (BB) and

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mononuclear bidentate (MB) without proton under acidic conditions (pH 5), mononuclear

13

monodentate (MM) with proton and BB without proton at pH 6-8, and MM without proton under

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alkaline conditions (pH 9). Phosphate competition significantly impacted the PMG adsorption

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capacity and its interfacial configurations. As a result, the stability of the adsorbed PMG was

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impaired, as evidenced by its elevated leachability. These results improve our understanding of

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PMG-mineral interactions at the molecular level and have significant implications for risk

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assessment for PMG and structural analog pollutants.

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Table of Contents (TOC)/Abstract Art

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43

Introduction

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Glyphosate (N-(phosphonomethyl)glycine, PMG) is a nonselective, post-emergence herbicide for

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weed control.1 Due to the worldwide application of PMG, its degradation, bioavailability, and

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transport in the environment are of great concern, and these environmental processes are mainly

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regulated by its chemistry at mineral/aqueous interfaces.2-5

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PMG interacts with mineral surfaces through its three polar functional groups, namely,

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phosphonate, amine, and carboxylate (Figure S1, in the Supporting Information (SI)). These

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functional groups are readily deprotonated and/or dissociated depending on pH. The dissociation

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constants have been accurately determined, but the deprotonation sequence is still a subject of

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controversy. Some earlier studies reported that the deprotonation of PMG occurred at the

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carboxyl group when pKa1< pH < pKa2, at the phosphonate group when pKa2< pH < pKa3, and

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at the amine group when pH > pKa3 (Figure S2, a).6-8 Recent DFT9 and NMR10 studies suggested

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a different deprotonation sequence which is carboxyl, amine, and phosphonate group as pH

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increase (Figure S2, b).

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The disagreement in PMG deprotonation seriously hinders the molecular-level understanding

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of PMG adsorption on minerals including goethite,11-13 and divergent phosphonate group

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involved mechanisms have been proposed on PMG surface complexation. For example, Sheals et

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al6 speculated that PMG was adsorbed on goethite surfaces via a primary monodentate complex

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and a secondary bidentate complex at neutral pH. Tribe et al13 found that PMG only formed

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monodentate surface complexes in all the pH ranges. In contrast, Barja et al14 identified two

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predominant surface species co-existing on goethite, namely, monodentate protonated and

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bridging bidentate complexes. Recent studies suggest that adsorbed ions on goethite may exhibit 4


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a variety of surface configurations depending on the exposed crystal faces, adding complexity to

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the structural identification.15-17

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The ambiguities in deprotonation sequence and surface molecular structure may largely be

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attributed to the lack of effective approach for the structural determination. As a widely-used

69

technique to probe the solid-liquid interface reactions, attenuated total reflectance (ATR) Fourier

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transform Infrared (FTIR) spectroscopy often results in equivocal peak assignment due to its

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intrinsic low resolution to resolve convoluted peaks,18 which may impair the accurate

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identification of surface complexes. Thus, complementary techniques such as two-dimensional

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correlation spectroscopy (2D-COS) analysis, and DFT calculations are central to re-examine the

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PMG deprotonation process and the surface complexation.

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The application of PMG is usually accompanied by phosphorus fertilization. The abundant

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phosphate in the soil can compete with PMG for the available surface sites on minerals, leading

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to the leaching and runoff of PMG to deeper layers of the soil.19 Nevertheless, the

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molecular-level impacts of phosphate on the PMG interaction with surfaces and the

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corresponding structures of PMG surface complexes are far from well understood.

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The objective of this study was therefore to explore the molecular-scale interaction of PMG on

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Fe(III) (hydr)oxide surfaces using multiple complementary techniques, including in situ

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flow-cell ATR-FTIR spectroscopy (in H2O and D2O), 2D-COS analysis, and DFT calculations.

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The insights gained from this study improve our knowledge in predicting the environmental fate

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of PMG and structural analog pollutants.

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Material and Methods

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Materials. All chemicals were analytical reagent grade or higher and were used without further

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purification. Milli-Q water (18.2 MΩ) for ATR-FTIR analysis was boiled for 60 min and cooled

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with N2 gas purging to remove CO2.

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Goethite was selected as a typical iron (hydr)oxide, considering its ubiquity in the

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environment and highly reactive surfaces. Goethite was synthesized via iron nitrate hydrolysis as

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detailed in our previous report (Figure S3).20,

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determined with an ASAP2000 instrument (Micromeritics Instrument Corp., USA). The point of

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zero charge (PZC) of goethite was determined to be 8.9 with a Zetasizer Nano ZS (Malvern

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Instruments, U.K.).

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ATR-FTIR

Measurements.

ATR-FTIR

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The BET surface area (84.7 m2/g) was

measurements

were

performed

using

a

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Thermo-Nicolet Nexus 6700 FTIR spectrometer equipped with a liquid-nitrogen cooled MCT

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detector. Both H2O and D2O (99.9 atom % D, Sigma) were used as solvent in the FTIR

98

experiments. The IR spectra were collected using a horizontal attenuated total reflectance (HATR)

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cell (PIKE Tech) with a 45°ZnSe (pH/pD 5-9) or Ge (beyond pH/pD range of 5-9) ATR crystal

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in 0.01-0.2 M NaCl (Figure S4). A total of 1000 and 256 scans were recorded for static and in

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situ flow cell ATR-FTIR spectrum, respectively, over 1800-800 cm−1 at a resolution of 4 cm−1.22

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The details of static and in situ flow cell ATR-FTIR measurements are provided in the SI.

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For better identification of peaks, the peak fittings of all IR spectra were carried out with

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Peakfit v.4.12 software using the second derivative fitting algorithm. The goodness of fit R factor

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in all of the fitted spectra was greater than 0.999. The details are provided in the Figure S5.

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Frequency Calculations. The IR frequency of the PMG molecule and PMG surface

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complexes with different configurations were calculated using the Gaussian 09 program with the

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B3LYP hybrid DFT method.23 The cluster models of goethite, PMG, and PMG surface

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complexes were constructed and the details are provided in the SI. The cluster model results

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could only be used as a qualitative guide in IR peak analysis rather than as an absolute energy

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value. Because PMG surface complexes are not dispersion-dominated and no interaction

112

energies are involved, the dispersion corrections to standard DFT functions are not applied.

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2D-COS Analysis. In order to determine the origin of the IR bands in the obtained IR spectra,

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2D-COS analysis was performed using 2Dshige software (Shigeaki Morita, Japan). Compared to

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conventional FTIR analysis, 2D-COS has the advantage of being able to distinguish one band

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from another by its origins, which has been successfully applied to probe complicated surface

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interaction processes in different systems.20-22 A detailed description of 2D-COS analysis is

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provided in the SI.

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Batch Competitive Adsorption Isotherms. To explore the impact of phosphate on PMG

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adsorption and the corresponding structures of PMG complexes, competitive adsorption

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experiments of PMG and phosphate on goethite surfaces were conducted at pH 7. The details of

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batch adsorption experiments and the sample analysis, including HPLC-ESI-MS/MS and

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colorimetric assay of PMG and phosphate, are provided in the SI.

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Results and Discussion

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PMG deprotonation process. Because of its central importance, the PMG deprotonation

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process was determined first by analyzing the IR spectra for various dissolved PMG species

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(Figure 1a). According to the dissociation constants (pKa1=2.3, pKa2=6.0, pKa3=11.0),7 the 7



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predominant species of PMG at pH 1.5, 4.2, 9.0, and 12.5 are neutral (PMG), monoanion

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(PMG−), dianion (PMG2−), and trianion (PMG3−), respectively (Figure S1). It worth note that the

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phosphonate group can protonate to form a positively charged species at pH 0.99) between the results of

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DFT calculations and deconvoluted ATR-FTIR spectra (Figure S6, S8, S9, Table S3-S6).

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Specifically, the DFT results for the PMG2− species are aligned with the deprotonated

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phosphonate group rather than the deprotonated amine group (slope: 0.99 vs 0.96, SD: 30 vs 59,

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Figure S10), supporting our FTIR results that the second deprotonation of PMG occurs at the

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phosphonate oxygen rather than the amino nitrogen.

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Surface Complexation of PMG on Goethite. The protonation process of PMG has a

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substantial impact on its solid-water interfacial interactions in the environment. Since PMG and

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PMG3− exist, respectively, under extremely acidic and alkaline conditions, only environmentally

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relevant PMG species, i.e. PMG− and PMG2− at pH 5-9, were investigated regarding their surface

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complexation.

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Though PMG is a tridentate chelating ligand, mounting evidence suggests that the

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carboxylate and amine groups do not directly interact with goethite.6, 13, 14 As shown in Figures 2

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and S6, adsorption did not shift the peak of υas(COO−) at 1400 cm−1, indicating either no or weak

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outer-sphere carboxylate surface interactions.6 In addition, the XPS analysis by Sheals et al.

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showed no evidence of direct interaction between the amine group and goethite.6 Conversely, the

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IR vibrations of the phosphonate group (1200-900 cm−1) exhibited dramatic changes upon

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adsorption at different pH values (Figure 2), indicating the formation of inner-sphere surface

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complexes on goethite via the phosphonate group.

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For an in-depth understanding of the PMG-goethite interaction mechanism, DFT

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calculations were performed to provide the theoretical IR bands of various PMG surface

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complexes. The possible PMG surface configurations included mononuclear monodentate (MM),

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mononuclear bidentate (MB), and binuclear bidentate (BB) structures. In addition, each structure

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considered two protonation states of phosphonate moiety: protonated (Type I) and deprotonated

210

(Type II) species, leading to an overall of six PMG complexes (Figure 3).

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The comparison of DFT-calculated and experimental IR spectra showed that none of the single

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configuration match the experimental IR data. The disagreement can be attributed to two facts: 1)

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more peaks appear in the calculated spectra than in the experimental spectra due to the

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asymmetry of cluster models, and 2) the DFT-calculated spectrum only corresponds to one

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specific surface configuration, whereas more than one PMG surface configurations may coexist

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at certain pH.

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To solve these problems, we first categorized the observed IR peaks into different groups by

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2D-COS analysis due to its merits in distinguishing one band from another by its origins upon

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external perturbations.20 The 2D-COS analysis results in Figure 4 contoured the synchronous and

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asynchronous correlation maps for PMG-goethite complexes, which were obtained from the

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dynamic IR spectra of PMG on goethite at pH 5, 7, and 9 (Figure S11). According to the detailed

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discussion in the SI, the IR peaks can be classified into the following five groups at pH 5-9: a)

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1142, 1018, 987 cm−1 and b) 1122, 1052, 956 cm−1 at pH 5; c) 1025, 987, 938 cm−1 and d) 1128,

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1094, 1060 cm−1 at pH 7; while only e) 1102, 1004, and 980 cm−1 at pH 9. Compared to the

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DFT-calculated spectra, group a) and c) belong to BB II; group b), d), and e) belong to MB II,

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MM I, and MM II, respectively (Figure 3, Figure S12). This 2D-COS result highlights the

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diversity of PMG configurations on goethite surfaces.

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To confirm the existence of multiple PMG surface complexes, ATR-FTIR spectra in D2O were

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collected and compared with those in H2O. As shown in Figure 5, most IR peaks at pD 5 and 9 in

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D2O had the same positions to those in H2O. This observation implied that the phosphonate

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group of the PMG surface complexes at pH 5 and 9 were without proton.28 At pH/pD=7,

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however, the group d as classified by the above 2D-COS analysis, namely, 1128, 1094, and 1060 12


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cm−1, had a significant shift to lower frequencies, whereas the peaks in the group c (1025, 987,

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and 938 cm−1) remained approximately the same positions upon the deuterium exchange. This

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result strongly suggested that two PMG surface complexes coexisted at pH 7. Integrating IR

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spectra, 2D-COS analysis, and DFT results concluded that BB II and MB II complexes were

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predominant under a slightly acidic condition (pH 5), MM I and BB II under neutral condition

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(pH 6–8), while only MM II under alkaline condition (pH 9). The comparison of the calculated

239

and experimental IR vibrational frequencies and the assignment of possible PMG complexes was

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summarized in Table 1.

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The pH dependence of PMG surface complexes can be attributed to the ligand-exchange

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mechanism. Under acidic conditions, the ligand, phosphonate group on PMG, is concentrated to

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the goethite surface (pHpzc=8.9) by the electrostatic attraction, which increases the energetic

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favorability to replace the surface hydroxyl group to form bidentate structures, including BB and

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MB configurations. As the pH increases to neutral, the competition of OH− hinders the ligand

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exchange reaction of PMG with surfaces, resulting in a successive disappearance of the MB

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configuration. MB is a four-membered ring structure which has a relatively unfavorable energy

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compared with the BB configuration.14 Under alkaline conditions, the MM complex without

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proton becomes the predominant configuration due to the strong competition with OH−.

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Competition of phosphate with PMG for Surface Complexation. Because PMG forms

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surface complexes through the phosphonate group, the competitive adsorption of PMG and

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phosphate should be of great environmental significance considering the co-application of

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phosphate fertilizers and the herbicide PMG.19

13



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Both PMG and phosphate adsorption conformed to the Langmuir isotherm, and the

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adsorption capacity of PMG (42.3 mg/g or 0.25 mM/g, Figure 6a) was much lower than that of

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phosphate (159.3 mg/g or 1.021 mM/g, Figure 6b). In the competitive system, phosphate

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dramatically reduced PMG adsorption by up to about 50% when the molar ratio of phosphate to

258

PMG increased to 5:1 (Figure 6a). In contrast, PMG had negligible effect on phosphate

259

adsorption, even when the molar ratio was as high as 10:1 (Figure 6b).

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The difference in the macroscopic competitive adsorption behaviors of PMG and phosphate

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can be explained by the FTIR results (Figure 6c). The spectrum of the PMG and phosphate

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coexisting system (molar ratio = 1:1, bottom of Figure 6c) was almost identical to that of

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phosphate adsorption alone (middle of Figure 6c), both of which were in stark contrast to that of

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PMG adsorption alone (top of Figure 6c). This observation indicates that phosphate has a

265

stronger affinity for goethite surfaces. The unchanged profile and peak positions at 1590, 1400,

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and 1324 cm−1 in the spectrum of PMG and phosphate coadsorption (Figure S13) indicate that

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the carboxyl group of PMG may not directly interact with goethite.

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Further quantitative analysis on the area of deconvoluted interfacial peaks can provide insights

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into the structural information. Compared to the phosphate-only system (middle of Figure 6c),

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the relative peak areas of several peaks increased in the competitive system, especially the peak

271

at 1124 cm−1 (28% increase relative to 1050 cm−1, bottom of Figure 6c), the position of which is

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close to the strongest peak (1128 cm−1) in the PMG-only system (top of Figure 6c). This result

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indicated that a small amount of PMG complexes, though less than phosphate, contributed to the

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inner-sphere complexation in the competitive system.

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Interestingly, phosphate preferentially occupied the goethite sites that originally were prone to

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form the BB II configuration in the PMG-only system. For example, for the peak at 980 and 942

277

cm−1 in the competitive system, corresponding to 987 and 938 cm−1 of BB II in the PMG-only

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system (top of Figure 6c), the relative peak areas only increased 8% and 1%, respectively

279

(bottom of Figure 6c). By comparison, for the peaks at 1124 and 1088 cm−1 in the competitive

280

system, which are close to the peaks at 1128 and 1094 cm−1 of MM I in the PMG-only system,

281

the relative peak areas appreciably increased by 28% and 21%, respectively. The uneven increase

282

in peak areas showed that phosphate not only displayed a stronger affinity than PMG for goethite

283

surfaces, but also preferentially occupied the limited surface sites which originally formed the

284

stable PMG surface complex, namely, BB II. As a result of phosphate competition, a great

285

proportion of PMG molecules were compelled to form relatively unfavorable MM I.

286

Notably, phosphate surface complexes may also consist of multiple species.17,

29

Because

287

phosphate dramatically reduced PMG adsorption whereas PMG had a negligible effect on

288

phosphate adsorption (Figure 6a, b), it is unlikely that PMG has a great impact on phosphate

289

surface structure. Therefore, the configurations of phosphate complexes were relatively stable in

290

the presence of PMG and the uneven increase in peak areas should be mainly attributed to the

291

change in the relative proportion of PMG surface species.

292

The impact of surface structures on the mobility of adsorbed PMG and phosphate motivated

293

our further study using extraction experiments. The release of PMG and phosphate followed the

294

pseudo-second-order kinetics model (Figure S14, Table S8-S9). The leaching results show that

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19.1-29.5% of PMG was released in the ternary PMG-phosphate-goethite system, appreciably

296

higher than that in the binary PMG-goethite system (9.6-18.6%, Table S8). This result clearly 15



297

demonstrates that phosphate competition altered the PMG surface complex to a less stable

298

configuration. Conversely, PMG resulted in an insignificant impact on phosphate desorption

299

(3.8-4.0% in ternary system vs 6.1-6.3% in binary system, Table S9)，in agreement with their

300

adsorption behaviors (Figure 6).

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Environmental Significance. PMG has become an agricultural panacea due to its high

302

efficiency and broad-spectrum properties, while its environmental behaviors is still unclear. Our

303

results here provide new insights into the molecular-level surface complexation of glyphosate on

304

goethite. The phosphonate group plays a key role in the formation of inner-sphere complexes on

305

goethite surfaces, while the carboxyl group may only take part in outer-sphere surface

306

interactions via electrostatic attraction. Notably, multiple PMG surface configurations may

307

coexist and transform depending on pH. Considering that the BB structure is more stable than

308

MB and MM, we should pay particular attention to the pH factor in evaluating the behavior,

309

bioavailability, and fate of PMG in the environment.

310

Our work also demonstrates that the presence of phosphate inhibits PMG adsorption by

311

competing for surface sites. Moreover, phosphate competition impairs the affinity of PMG

312

toward goethite surfaces by restricting PMG to forming an energetically unfavorable MM rather

313

than a more stable BB structure, resulting in higher mobility for adsorbed PMG. Given the

314

widespread phosphorus fertilization and its strong affinity to soil components, phosphate, the

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structural analog to the active moiety of PMG, should be carefully considered in future studies

316

on PMG risk assessment.

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ASSOCIATED CONTENT

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Supporting Information. The Supporting Information is available free of charge on the ACS

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Publications website at http://pubs.acs.org.

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Details of in situ flow cell ATR-FTIR measurements; Peak fitting procedure; 2D-COS analysis;

321

PMG and phosphate Determination; analytical procedure; dynamic spectra of PMG on goethite

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at pH 5, 7, and 9; and additional figures.

323 324

Notes The authors declare no competing financial interest. ACKNOWLEDGEMENTS

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We acknowledge the financial support of the National Key Basic Research Program of China

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(2015CB932003, 2014CB441102), the Strategic Priority Research Program of the Chinese

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Academy of Sciences (XDB14020302), the National Natural Science Foundation of China

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(21477144, 41425016, and 21321004).

329 330

References

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(7) Li, W.; Wang, Y.-J.; Zhu, M.; Fan, T.-T.; Zhou, D.-M.; Phillips, B. L.; Sparks, D. L. Inhibition mechanisms of Zn precipitation on aluminum oxide by glyphosate: A 31P NMR and Zn EXAFS study. Environ. Sci. Technol. 2013, 47, 4211-4219. (8) Harris, W. R.; Sammons, R. D.; Grabiak, R. C.; Mehrsheikh, A.; Bleeke, M. S. Computer simulation of the interactions of glyphosate with metal ions in phloem. J. Agric. Food Chem. 2012, 60, 6077-6087. (9) Peixoto, M. M.; Bauerfeldt, G. F.; Herbst, M. H.; Pereira, M. S.; da Silva, C. O. Study of the stepwise deprotonation reactions of glyphosate and the corresponding pKa values in aqueous solution. J. Phys. Chem. A 2015, 119, 5241-5249. (10) Liu, B.; Dong, L.; Yu, Q.; Li, X.; Wu, F.; Tan, Z.; Luo, S. Thermodynamic study on the protonation reactions of glyphosate in aqueous solution: potentiometry, calorimetry and NMR spectroscopy. J. Phys. Chem. B 2016, 120, 2132-2137. (11) Gimsing, A. L.; Borggaard, O. K.; Sestoft, P. Modeling the kinetics of the competitive adsorption and desorption of glyphosate and phosphate on goethite and gibbsite and in soils. Environ. Sci. Technol. 2004, 38, 1718-1722. (12) Mazzei, P.; Piccolo, A. Quantitative evaluation of noncovalent interactions between glyphosate and dissolved humic substances by NMR spectroscopy. Environ. Sci. Technol. 2012, 46, 5939-5946. (13) Tribe, L.; Kwon, K. D.; Trout, C. C.; Kubicki, J. D. Molecular orbital theory study on surface complex structures of glyphosate on goethite: Calculation of vibrational frequencies. Environ. Sci. Technol. 2006, 40, 3836-3841. (14) Barja, B. C.; dos Santos Afonso, M. Aminomethylphosphonic acid and glyphosate adsorption onto goethite: A comparative study. Environ. Sci. Technol. 2005, 39, 585-592. (15) Villalobos, M.; Perez-Gallegos, A. Goethite surface reactivity: A macroscopic investigation unifying proton, chromate, carbonate, and lead(II) adsorption. J. Colloid Interface Sci. 2008, 326, 307-323. (16) Villalobos, M.; Cheney, M.A.; Alcaraz-Cienfuegos, J. Goethite surface reactivity: II. A microscopic site-density model that describes its surface area-normalized variability. J. Colloid Interface Sci. 2009, 336, 412-422. (17) Kubicki, J.D.; Paul, K.W.; Kabalan, L.; Zhu, Q.; Mrozik, M.K.; Aryanpour M.; Pierre-Louis, A.M.; Strongin, D.R. ATR-FTIR and density functinal theory study of the structures, energetics, and vibrational spectra of phosphate adsorbed onto goethite. Langmuir 2012, 28, 14573-14587. (18) Barja, B. C.; dos Santos Afonso, M. An ATR−FTIR study of glyphosate and its Fe(III) complex in aqueous solution. Environ. Sci. Technol. 1998, 32, 3331-3335. (19) Sasal, M. C.; Demonte, L.; Cislaghi, A.; Gabioud, E. A.; Oszust, J. D.; Wilson, M. G.; Michlig, N.; Beldoménico, H. R.; Repetti, M. R. Glyphosate loss by runoff and its relationship with phosphorus fertilization. J. Agric. Food Chem. 2015, 63, 4444-4448. (20) Yang, Y.; Yan, W.; Jing, C. Dynamic adsorption of catechol at the goethite/aqueous solution interface: A molecular-scale study. Langmuir 2012, 28, 14588-14597. 18


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Table 1. The Calculated IR Vibrational Frequencies (cm−1) and Assignment of Six Possible Phosphonate Group involved PMG Complexes with Goethite Surfaces. Solvent = H2O Assignment MM Ia MM IIa MB I MB II BB I BB II 1208 – 1181 – 1194 – δ (PO–H) of phosphonate group b 1129(1128) – 1126 (1122) – 1148(1142) υ (P=O) of phosphonate group – 1108(1102) – – – – υas (PO2) of uncomplexed P–O – 1037 – – – – υs (PO2) of uncomplexed P–O – – 1099 1048 (1052) 1105 1014(1018), υas (P–(OFe)2) of bidentate complexes – – 1002 957 (956) 1048 984(987), υs (P–(OFe)2) of bidentate complexes υ (P–OFe) of monodentate complexes

δ (PO–H) of phosphonate group υ (P=O) of phosphonate group υas (PO2) of uncomplexed P–O υs (PO2) of uncomplexed P–O υas (P–(OFe)2) of bidentate complexes υs (P–(OFe)2) of bidentate complexes υ (P–OFe) of monodentate complexes

1092(1094), 1054(1060)

1008(1004), 972(980)

MM I

MM II

1186

–

1118(1105)

–

–

–

935(936) –

Solvent = D2O MB I MB II

BB I

BB II

1160

–

1178

–

–

1130 (1118)

–

1146(1137)

–

1118(1102)

–

–

–

–

–

1030

–

–

–

–

–

–

1074

1044 (1052)

1096

1008(1014),

–

–

997

957 (956)

1040

1078(1073), 1044(1041)

1011(1005), 981(980)

–

–

–

981(984), 938(931) –

a

“I” and “II” represent protonated and deprotonated type, respectively. Values in the parentheses are experimental IR vibrational frequencies. MM: mononuclear monodentate; MB: mononuclear bidentate; BB: binuclear bidentate. All proposed interfacial configurations are shown in Figure 3.

b

20


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419 420 421 422 423 424 425 426 427 428 429 430 431 432 433 434 435 436 437 438 439 440 441 442 443 444 445 446 447 448 449 450


Figure Captions

Figure 1. ATR-FTIR spectra of 1000 mg L−1 dissolved PMG (a) and sarcosine (b) in 0.1 M NaCl for pH values 1.5, 4.2, 9.0 and 12.5. Figure 2. Deconvoluted ATR-FTIR spectra of adsorbed PMG on goethite surfaces at pH 5, 6, 7, 8, and 9 at equilibrium. Figure 3. Phosphonate group involved PMG complexes with goethite surfaces. The possible interfacial configurations, including mononuclear monodentate (MM), mononuclear bidentate (MB), and binuclear bidentate (MB), were optimized. Each configuration has two types, namely, the phosphonate group with (I) and without (II) proton. The explicit H2O molecules were not shown for clarity. Fe: cyan; C: grey; O: red; N: blue; P: orange; H: white.

Figure 4. Synchronous (a, c, e) and asynchronous (b, d, f) correlation contour maps of dynamic IR spectra for the adsorption of PMG on goethite surface at pH 5, 7, and 9. The blue (red) regions were defined as negative (positive) correlation intensities. Figure 5. IR spectra of adsorbed PMG in H2O and D2O for pH/pD 5 (left), pH/pD 7 (middle), and pH/pD 9 (right). Figure 6. Adsorption isotherms of PMG (a) and phosphate (b) in the competitive systems at pH 7. Symbols represent the experimental results, and the solid lines are Langmuir adsorption model simulations. (c) Deconvoluted ATR-FTIR interfacial spectra of PMG (top), phosphate (middle), and the competition of phosphate with PMG (bottom) in 0.1 M NaCl solution at pH 7. The red values in the parentheses are relative peak areas, which are normalized to the most intense peak located at 1050 cm−1.

451 452 453 21



454 455 456 457 458 459 460 461 462 463 464 465 466

Figure 1.

467 468 469 470 471 472 473 474 22


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475 476 477 478 479 480 481 482 483 484 485 486 487 488 489 490

Figure 2.

491 492 493 494 495 23



496 497 498 499 500 501 502 503 504 505 506 507 508 509 510 511

The “I” and “II” after the abbreviation of configurations represent the phosphonate group with and without proton, respectively. Salient bonds are highlighted by red circles.

512 513

Figure 3.

514 515 516 517 24


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518 519 520 521 522 523 524 525 526 527 528 529 530 531 532 533 534 535 536

Figure 4.

537 538 25



539 540 541 542 543 544 545 546 547 548 549

Wavenumber (cm−1)

550 551 552 553 554

Figure 5.

555 556 557 558

26


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559 560 561 562 563 564 565 566 567 568 569 570 571 572 573 574 575 576 577 578

Figure 6.

579 27



580

28


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Molecular Insights into Glyphosate Adsorption to Goethite Gained from

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