Glyphosate is a non-selective, broad spectrum, post-emergent herbicide widely used in weed control. Aminomethylphosphonic acid (AMPA) is one of the main products of biodegradation of glyphosate in natural systems before its ultimate mineralization and also the breakdown product of more complex phosphonates such as nitrilotris-(methylenephosphonic acid). The adsorption isotherms and surface coverage of AMPA and glyphosate (N-phosphomethylglycine, PMG) in aqueous suspensions of goethite as a function of pH were measured. Electrophoretic mobility curves forthe PMG/goethite system were also determined. The ATR-FTIR interfacial spectra of the surface complexes of AMPA and PMG onto goethite were analyzed as a function of the pH and the surface coverage. The phosphonate moiety of these two ligands coordinates to the iron oxide surface with similar structures as the methylphosphonic acid despite the presence of the amino and/or carboxylate groups of their molecules. Two predominating complexes have been identified where the phosphonate group in PMG or AMPA bonds monodentately or bridges bidentately to the surface of iron oxide in an inner sphere mode, while the carboxylate and amino group are noncoordinated to the surface. The stability constants of the surface complexes (triple bond)FeO-P(O)(OH)--R, (triple bond)FeO--P(O)2--R, and ((triple bond)FeO)2--P(O)--R were calculated using the constant capacitance model.

The influence of the solvent on the structure and IR spectrum of the [Fe(CN)(5)NO](2)(-) ion is investigated by using gradient corrected density functional theory. IR spectra are also measured on different solvents and the results obtained are compared with the predicted ones. We have treated the solvent effects with a continuum model, based on the Onsager's reaction field approach; in order to mimic strong specific interactions, calculations were also performed on the complex protonated at the cyanide trans to the nitrosyl group. The reaction field calculations predict only qualitatively the most important observed trends, e.g., the shifts in the nitrosyl stretching wavenumber, but fail in accounting quantitatively for the differences between the spectra in water and acetonitrile. The possible role of specific interactions is consistently accounted for by interpreting the experimental shifts of the NO stretching wavenumber nu(NO), as well as the visible absorption energies, when changing the Lewis acidity of the solvent, as measured by the Gutmann's acceptor number. Ligand population analysis was performed to relate the solvent effects with the sigma donor and pi acceptor behavior of cyanide and nitrosyl ligands. The significance of nu(NO) shifts as a result of changes in the medium is discussed in view of the physiological relevance of transition-metal nitrosyl chemistry.

Adsorption of methylphosphonic acid (MPA) onto goethite (α- FeOOH) was studied as a function of pH and the concentration of MPA in solution. In situ attenuated total reflectance-FTIR (ATR-FTIR) techniques were used to identify the type of surface complexes formed under each of the solution conditions used in the adsorption studies. Additionally, electrophoretic mobility measurements were performed in each of these systems. At low pH and high Γ, MPA was bound to the surface of goethite predominantly as a monodentate protonated species, while at high pH and low Γ the prevailing structure was that of a bridging bidentate complex. The change in denticity of the surface MPA under different solution conditions may explain the charge reversals shown in the electrophoretic mobility curves.

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