Aminomethylphosphonic acid and glyphosate adsorption onto goethite: a comparative study

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.