AVENA marcelo Javier
Desorption rate of glyphosate from goethite as affected by different entering ligands. Hints on the desorption mechanism
JEISON MANUEL ARROYAVE; CAROLINA WAIMAN; GRACIELA P. ZANINI; WENFENG TAN; MARCELO J. AVENA
Lugar: Collingwood; Año: 2017
The desorption kinetics of glyphosate (Gly) from goethite was studied in a flow cell using ATR-FTIR spectroscopy. Since Gly forms an inner-sphere surface complex by coordinating Fe atoms at the goethite surface, the desorption process is actually a ligand exchange reaction, where Gly is the leaving ligand and water molecules or dissolved substances are the entering ligands. A series of possible entering ligands that can be found in nature was tested to evaluate their effect on the desorption kinetics of Gly. Contrarily to expectations, the desorption rate was rather independent of the entering ligand concentration. Moreover, the identity of this ligand (phosphate, citrate, sulfate, oxalate, EDTA, thiocyanate, humic acid, water) had only a relatively small effect on the value of the desorption rate constant. By analogy with the reactivity of transition metal complexes in solution, it is concluded that the rate is mainly controlled by the breaking of the Fe-Gly bond, through a dissociative or a dissociative interchange mechanism. The results are relevant to understand and predict the environmental mobility of Gly: irrespective of the identity of the entering ligand, Gly will always desorb from iron (hydr)oxides in nature at nearly the same rate, simplifying enormously calculations and predictions. The importance of studying desorption kinetic using mineral surfaces and environmentally relevant molecules is also highlighted.