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Adsorption of Glyphosate on Goethite (α-FeOOH): surface complexation modeling combining spectroscopic and adsorption data
Umeå University, Faculty of Science and Technology, Chemistry.
Umeå University, Faculty of Science and Technology, Chemistry.
Umeå University, Faculty of Science and Technology, Chemistry.
Umeå University, Faculty of Science and Technology, Chemistry.
2008 (English)In: Environmental Science and Technology, ISSN 0013-936X, 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.

Place, publisher, year, edition, pages
2008. Vol. 42, no 7, 2464-2469 p.
URN: urn:nbn:se:umu:diva-9528DOI: doi:10.1021/es070966bOAI: diva2:149199
Available from: 2008-04-17 Created: 2008-04-17 Last updated: 2009-11-18Bibliographically approved
In thesis
1. Modeling of glyphosate and metal-glyphosate speciation in solution and at solution-mineral interfaces
Open this publication in new window or tab >>Modeling of glyphosate and metal-glyphosate speciation in solution and at solution-mineral interfaces
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.
herbicide, glyphosate, surface complexation model, speciation, goethite, aged gamma-alumina, bayerite, manganite, cadmium(II), aluminium(III), potentiometric titration, adsorption
National Category
Chemical Sciences
urn:nbn:se:umu:diva-1074 (URN)978-91-7264-277-5 (ISBN)
Public defence
2007-04-27, KB3B1, KBC-huset, Umeå Universitet, 901 87 Umeå, 10:00 (English)
Available from: 2007-04-03 Created: 2007-04-03 Last updated: 2009-11-18Bibliographically approved

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