Initial steps of ligand-promoted dissolution of gibbsite

The initial steps of dissolution of gibbsite by salicylate were examined using adsorption isotherms, electrophoresis, infrared spectroscopy, and controlled rate thermal analysis. A contact time of 24 h was chosen to avoid alteration of the gibbsite platelets. The results clearly show that adsorption of salicylate takes place only on the edge faces of the gibbsite platelets, where the OH groups are monocoordinated to Al atoms and bear a half charge. Monodentate and binuclear complexes are formed. The consequence is charge reversal on the solid surface. The dissolution rate is linearly correlated to the surface density of salicylate. These observations demonstrate that dissolution occurs from the edge faces of the gibbsite platelets. Normalized dissolution rates should therefore be calculated using the surface area of the edges of gibbsite and not the total surface area. (C) 2000 Elsevier Science B.V. All rights reserved.