KINETICS AND MECHANISM OF SURFACE REACTION OF SALICYLATE ON ALUMINA IN COLLOIDAL AQUEOUS SUSPENSION

Abstract

The reaction kinetics of salicylate with Al(III) in aqueous solution and at the colloidal alumina-water interface was studied by stopped-flow laser fluorescence spectroscopy. Temporal evolution of the fluorescence spectra suggests that formation of a carboxylate monodentate complex was the reaction intermediate that occurs transiently at the beginning of the reaction in aqueous salicylate-Al(III) solution. However, by lowering the pH to 2.0, the formation of such an intermediate can be directly observed as it is the only species formed. The reaction of salicylate with aqueous Al3+ is completed within 10 min at pH 3.3 but is significantly slower at pH 2.0. At both pH the aqueous reaction follows a single pseudo-first order rate law. In alumina suspension the reaction was initially fast but slowed down after ~30 s. Completion of the reaction took up to 12 h, depending on pH and ionic strength. The formation of a carboxylate monodentate surface complex as a transient species is clearly observed in alumina suspensions at near neutral pH. The initial rapid reaction (<30 s), accounting for ~70% of the total reaction, can be best described by elovich rate equation and slower ~30% obeys pseudo-first order kinetics. these results are consistent with a sorption reaction mechanism that is controlled leaving group lability at surface sites (al-oh2+ and Al-OH). The pseudo-first order rate constant varies little with initial salicylate concentration, ionic strength, or pH > 4, suggesting that the slow reaction pathway involves ligand substitution reactions between salicylate and the hydroxyl groups for which the Al-O binding and activation energy are affected by site heterogeneity or site density to a lesser degree than Al-OH2+ sites.