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Thermodynamic and spectroscopic studies of cadmium(II)–N-(phosphonomethyl)glycine (PMG) complexes
Umeå University, Faculty of Science and Technology, Department of Chemistry.
Umeå University, Faculty of Science and Technology, Department of Chemistry.
Umeå University, Faculty of Science and Technology, Department of Chemistry.
Umeå University, Faculty of Science and Technology, Department of Chemistry.
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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. Vol. 357, no 4, 1185-1192 p.
Identifiers
URN: urn:nbn:se:umu:diva-2219DOI: doi:10.1016/j.ica.2003.10.029OAI: oai:DiVA.org:umu-2219DiVA: diva2:140115
Available from: 2007-04-03 Created: 2007-04-03 Last updated: 2017-12-14Bibliographically 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.
Keyword
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: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)
Opponent
Supervisors
Available from: 2007-04-03 Created: 2007-04-03 Last updated: 2009-11-18Bibliographically approved
2. Chemical Processes at the Water-Manganite (γ-MnOOH) Interface
Open this publication in new window or tab >>Chemical Processes at the Water-Manganite (γ-MnOOH) Interface
2004 (English)Doctoral thesis, comprehensive summary (Other academic)
Alternative title[sv]
Kemiska Processer vid gränsytan mellan vatten och manganit (γ-MnOOH)
Abstract [en]

The chemistry of mineral surfaces is of great importance in many different areas including natural processes occurring in oceans, rivers, lakes and soils. Manganese (hydr)oxides are one important group to these natural processes, and the thermodynamically most stable trivalent manganese (hydr)oxide, manganit (γ-MnOOH), is studied in this thesis.

This thesis summarises six papers in which the surface chemistry of synthetic manganite has been investigated with respect to surface acid-base properties, dissolution, and adsorption of Cd(II) and the herbicide N-(phosphonomethyl)glycine (glyphosate, PMG). In these papers, a wide range of analysis techniques were used, including X-ray photoelectron spectroscopy (XPS), extended X-ray absorption fine structure (EXAFS) spectroscopy, Fourier transform infra-red (FTIR) spectroscopy, atomic force microscopy (AFM), scanning electron microscopy (SEM), X-ray diffraction (XRD), potentiometry, electrophoretic mobility measurements and wet chemical techniques, in order to obtain a more complete understanding of the different processes occurring at the manganite-water interface.

From the combined use of these techniques, a 1-pKa acid-base model was established that is valid at pH>6. The model includes a Na+ interaction with the surface:

=MnOH2+½ --> =MnOH-½ + H+ log β0 (intr.) = -8.20 = -pHiep

=MnOH2+½ + Na+ --> =MnOHNa+½ + H+ log β0 (intr.) = -9.64

At pH<6 the manganite crystals dissolve and disproportionate into pyrolusite (β-MnO2) and Mn(II)-ions in solution according to:

2 γ-MnOOH + 2H+ --> β-MnO2 + Mn2+ + 2H2O log K0 = 7.61 ± 0.10

The adsorption and co-adsorption of Cd(II) and glyphosate at the manganite surface was studied at pH>6. Cd(II) adsorption displays an adsorption edge at pH~8.5. Glyphosate adsorbs over the entire pH range, but the adsorption decreases with increasing pH. When the two substances are co-adsorbed, the adsorption of Cd(II) is increased at low pH but decreased at high pH. The adsorption of glyphosate is increased in the entire pH range in the presence of Cd(II). From XPS, FTIR and EXAFS it was found that glyphosate and Cd(II) form inner sphere complexes. The binary Cd(II)-surface complex is bonded by edge sharing of Mn and Cd octahedra on the (010) plane of manganite. Glyphosate forms inner-sphere complexes through an interaction between the phosphonate group and the manganite surface. The largest fraction of this binary glyphosate complex is protonated throughout the pH range. A ternary surface complex is also present, and its structure is explained as type B ternary surface complex (surface-glyphosate-Cd(II)). The chelating rings between the Cd(II) and glyphosate, found in aqueous complexes, are maintained at the surface, and the ternary complex is bound to the surface through the phosphonate group of the ligand.

Publisher
74 p.
Keyword
Inorganic chemistry, manganite, γ-MnOOH, mineral surface, acid-base properties, adsorption, Cd(II), N-(phosphonomethyl)glycine, glyphosate, PMG, surface complex, dissolution, disproportionation, XPS, EXAFS, infrared spectroscopy, SEM, AFM, potentiometry, electrophoresis, Oorganisk kemi
National Category
Inorganic Chemistry
Research subject
Inorganic Chemistry
Identifiers
urn:nbn:se:umu:diva-253 (URN)91-7305-634-0 (ISBN)
Public defence
2004-05-19, KB3B1, KBC, Umeå University, Umeå, 13:00 (English)
Opponent
Supervisors
Available from: 2004-04-29 Created: 2004-04-29 Last updated: 2009-12-04Bibliographically approved

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Ramstedt, MadeleineNorgren, CarolineSheals, JuliaBoström, DanSjöberg, StaffanPersson, Per
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