Kinetics and mechanism of surface reaction of salicylate on alumina in colloidal aqueous suspension

The reaction kinetics of salicylate with Al(III) in aqueous solution and at the colloidal aluminawater 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 similar to 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 similar to 70% of the total reaction, can be best described by the Elovich rate equation and the slower reaction, accounting for similar to 30% of the total reaction, obeys pseudo-first order kinetics. These results are consistent with a sorption reaction mechanism that is controlled by the leaving group lability at the 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. Copyright (C) 2000 Elsevier Science Ltd.