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Modeling of glyphosate and metal-glyphosate speciation in solution and at solution-mineral interfaces
Umeå University, Faculty of Science and Technology, Chemistry.
2007 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Glyphosate (N-(phosphonomethyl)glycine, PMG, H3L) is a widely used organophosphorous herbicide. It interacts with metal ions and mineral surfaces, which may affect its mobility, degradation and bioavailability in the environment. However, these interactions are far from fully understood. This thesis is a summary of five papers discussing the complexation of PMG with metal ions in aqueous solution and the adsorption of PMG and/or Cd(II) on different mineral surfaces.

The complexation of PMG with the metals Cd(II) or Al(III) in aqueous solution was investigated with macroscopic and molecular scale techniques. Potentiometric titration data were combined with EXAFS, ATR-FTIR and NMR spectroscopic data to generate solution equilibrium models. In the PMG-Cd(II) system, only mononuclear complexes were formed, while both mono and binuclear complexes were observed in the PMG-Al(III) system.

EXAFS, ATR-FTIR, and XPS measurements showed that PMG adsorbs to the surfaces of goethite (α-FeOOH), aged γ-alumina (γ-Al2O3) and manganite (γ-MnOOH) through one oxygen of its phosphonate group to singly-coordinated surface sites. Surface complexation models consistent with these spectroscopic results were fit to adsorption data using the 1pK reaction formalism. Electrostatic effects were accounted using either the Extended Constant Capacitance Model (ECCM) or the Basic Stern Model (BSM), and the charge of the surface complexes was distributed over the different planes. The formation of the surface complexes was described according to the following reactions:

≡MeOH(0.5-) + H3L <=> ≡MeHL(1.5-) + H2O + H+

≡MeOH(0.5-) + H3L <=> ≡MeL(2.5-) + H2O + 2H+

The coadsorption of PMG and Cd(II) on the surfaces of goethite and manganite results in the formation of ternary mineral-PMG-Cd(II) surface complexes, as suggested from EXAFS results. Previous EXAFS measurements have also established the coordination geometries for the binary goethite-Cd(II) and manganite-Cd(II) surface complexes. In addition to the surface reactions in the binary mineral-Cd(II) and mineral-PMG systems, a single ternary complex with the stoichiometry ≡MeLCd(OH)(1.5-) was sufficient to explain coadsorption data:

≡MeOH(0.5-) + H3L + Cd2+ <=> ≡MeLCd(OH)(1.5-) + 3H+

It was concluded that the affinity of PMG for the three mineral systems decreases within the series: goethite > aged γ-Al2O3 > manganite. The formation of the ternary surface complex is more significant on goethite surfaces than on manganite surfaces.

Place, publisher, year, edition, pages
Umeå: Kemi , 2007. , 80 p.
Keyword [en]
herbicide, glyphosate, surface complexation model, speciation, goethite, aged gamma-alumina, bayerite, manganite, cadmium(II), aluminium(III), potentiometric titration, adsorption
National Category
Chemical Sciences
Identifiers
URN: urn:nbn:se:umu:diva-1074ISBN: 978-91-7264-277-5 (print)OAI: oai:DiVA.org:umu-1074DiVA: diva2:140120
Public defence
2007-04-27, KB3B1, KBC-huset, Umeå Universitet, 901 87 Umeå, 10:00 (English)
Opponent
Supervisors
Available from: 2007-04-03 Created: 2007-04-03 Last updated: 2009-11-18Bibliographically approved
List of papers
1. Thermodynamic and spectroscopic studies of cadmium(II)–N-(phosphonomethyl)glycine (PMG) complexes
Open this publication in new window or tab >>Thermodynamic and spectroscopic studies of cadmium(II)–N-(phosphonomethyl)glycine (PMG) complexes
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2004 (English)In: Inorganica Chimica Acta, ISSN 0020-1693, E-ISSN 1873-3255, Vol. 357, no 4, 1185-1192 p.Article in journal (Refereed) Published
Abstract [en]

Speciation and equilibria in the H+–Cd2+N-(phosphonomethyl)glycine (PMG, H3L) system have been studied in 0.1 M Na(Cl) medium at 25.0 °C. Formation constants for a series of mononuclear complexes, CdHL, CdL, CdL2 4− and CdL(OH)2− were determined from potentiometric titrations. The structures of the predominating species CdL, and CdL2 4− in solution were investigated using EXAFS and IR spectroscopic techniques. In the 1:1 complex bonds are formed between the Cd(II) ion and all three donor groups (amino, carboxylate, phosphonate) of the PMG molecule resulting in two 5-membered chelate rings. At the remaining three of the corners of the distorted Cd(II) octahedra oxygens were found which are replaced by donor groups of a second PMG molecule in the 1:2 complex. Furthermore, a solid phase consisting of Cd9(PMG)6(H2O)12 · 6H2O crystals was synthesized and the crystal structure was determined. The structure consists of six CdL octahedra connected through a seventh Cd–O octahedron in the centre of the entity, with two additional Cd–O octahedra located at the apices of the unit formed.

Speciation and equilibria in the H+–Cd2+N-(phosphonomethyl)glycine (PMG, H3L) system have been studied using potentiometry, and formation constants for CdHL, CdL, CdL2 4− and CdL(OH)2− were determined. The structures of CdL, and CdL2 4− in solution were investigated using EXAFS and IR spectroscopic techniques. The results indicated chelating PMG molecules forming two 5-membered chelate rings.

Place, publisher, year, edition, pages
Elsevier, 2004
Identifiers
urn:nbn:se:umu:diva-2219 (URN)doi:10.1016/j.ica.2003.10.029 (DOI)
Available from: 2007-04-03 Created: 2007-04-03 Last updated: 2017-12-14Bibliographically approved
2. Co-adsorption of cadmium(II) and glyphosate at the water–manganite (γ-MnOOH) interface
Open this publication in new window or tab >>Co-adsorption of cadmium(II) and glyphosate at the water–manganite (γ-MnOOH) interface
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2005 (English)In: Journal of Colloid and Interface Science, ISSN 0021-9797, E-ISSN 1095-7103, Vol. 285, no 2, 493-501 p.Article in journal (Refereed) Published
Abstract [en]

The co-adsorption of Cd(II) and glyphosate (N-(phosphonomethyl)glycine, PMG) at the manganite (γ-MnOOH) surface has been studied in the pH range 6–10 at 25 °C and with 0.1 M Na(Cl) as ionic medium. Batch adsorption experiments, Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS) and extended X-ray absorption fine structure (EXAFS) spectroscopy were used for the quantitative analysis and the determination of the molecular structure of the surface complexes. The adsorption of Cd(II) and PMG in the ternary Cd(II)–PMG–manganite system was compared with the adsorption in the binary Cd(II)–manganite and PMG–manganite systems. The formation of three inner sphere surface complexes was observed, a ternary Cd(II)–PMG–manganite complex, a binary Cd(II)–manganite complex and a binary PMG–manganite complex. The surface concentration of the ternary complex and the Cd(II)–manganite complex was more or less constant throughout the pH range studied. However, the surface concentration of the binary PMG–manganite complex decreased with increasing pH. The major part of the binary PMG–surface complex was protonated. The ternary surface complex displayed a type B structure (Cd(II)–PMG–manganite). The average Cd–Mn distance obtained from EXAFS (3.26 Å) indicates that the binary and ternary Cd(II)–surface complexes are formed by edge-sharing of Mn and Cd octahedra on the (010) plane of the manganite crystals.

Keyword
N-(phosphonomethyl)glycine, Glyphosate, Manganite, γ-MnOOH, Cadmium, EXAFS, XPS, FTIR, Ternary surface complexes
Identifiers
urn:nbn:se:umu:diva-2220 (URN)doi:10.1016/j.jcis.2004.12.003 (DOI)
Available from: 2007-04-03 Created: 2007-04-03 Last updated: 2017-12-14Bibliographically approved
3. Adsorption of Glyphosate on Goethite (α-FeOOH): surface complexation modeling combining spectroscopic and adsorption data
Open this publication in new window or tab >>Adsorption of Glyphosate on Goethite (α-FeOOH): surface complexation modeling combining spectroscopic and adsorption data
2008 (English)In: Environmental Science and Technology, ISSN 0013-936X, E-ISSN 1520-5851, Vol. 42, no 7, 2464-2469 p.Article in journal (Refereed) Published
Abstract [en]

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 PMG-containing 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.

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.

Identifiers
urn:nbn:se:umu:diva-9528 (URN)doi:10.1021/es070966b (DOI)
Available from: 2008-04-17 Created: 2008-04-17 Last updated: 2017-12-14Bibliographically approved
4. Glyphosate complexation to aluminium(III) in solution and to the surface of aged gamma-Al2O3: chemical speciation models combining spectroscopic and macroscopic data
Open this publication in new window or tab >>Glyphosate complexation to aluminium(III) in solution and to the surface of aged gamma-Al2O3: chemical speciation models combining spectroscopic and macroscopic data
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(English)Manuscript (Other (popular science, discussion, etc.))
Identifiers
urn:nbn:se:umu:diva-2222 (URN)
Available from: 2007-04-03 Created: 2007-04-03Bibliographically approved
5. Surface complexation modeling of cadmium(II) and glyphosate adsorption at the surface of manganite (gamma-MnOOH) and goethite (alpha-FeOOH)
Open this publication in new window or tab >>Surface complexation modeling of cadmium(II) and glyphosate adsorption at the surface of manganite (gamma-MnOOH) and goethite (alpha-FeOOH)
(English)Manuscript (Other (popular science, discussion, etc.))
Identifiers
urn:nbn:se:umu:diva-2223 (URN)
Available from: 2007-04-03 Created: 2007-04-03Bibliographically approved

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