Ligand exchange reactions at the metal oxide-water interface. Equilibrium and dynamic conditions

MARCELO AVENA

Lleida

Congreso; Interfaces Against Pollution; 2016

Universidad de Lleida

Ligand exchange (or ligand substitution)reactions are key environmental reactions. They involve the exchange of oneligand for another in a metal complex, with no change in oxidation state at themetal center. These reactions are well-known by inorganic chemists that studythe properties of metal complexes in solution, and by surface chemists thatinvestigate the surface properties of metal oxides in aqueous media. A typicalexample of a ligand exchange reaction at the surface of a metal (hydr)oxide isthe exchange of the herbicide glyphosate by phosphate at the surface ofgoethite (α-FeOOH). Another example is the desorption of glyphosate caused by theadsorption of humic acid (HA) on goethite.Ligand exchange reactions result in theformation of inner-sphere complexes at the mineral surface, and many of thesecomplexes can be experimentally detected by spectroscopies as infrared, Raman,synchrotron based spectroscopies, etc. With these techniques, the adsorption ofthe entering ligand and the desorption of the outgoing ligand can besimultaneously monitored as the reaction proceeds giving important kineticinformation on the studied system. In addition, since the shape of the spectrais also evaluated as a function of time, the surface complexes and reactionintermediates can be sometimes identified, giving information on the reactionmechanism. Together with adsorption isotherms and surface complexation models,which explore the system under equilibrium conditions, a good understanding ofenvironmentally relevant processes occurring at interfaces can be achieved.This presentation focusses onadsorption-desorption processes that take place by ligand exchange at thegoethite-water interface. Examples are given for the pairs of ligandsglyphosate-HA and glyphosate-phosphate. Depending on the system, ATR-FTIRspectroscopy data, adsorption isotherms with single- and double-ligand systemsand/or surface complexation models are used to obtain desorption rate constants,rate laws and to gain insights into the surface speciation and competition forsurface sites.