Electrokinetic Behaviour and Interaction with Oxalic Acid of Different Hydrous Chromium(III) Oxides

Abstract

Three samples of hydrous chromium(III) oxide have been prepared by different procedures, and their electrokinetic mobilities have been measured. In mineral media, isoelectric points were found to be 8.45 ±0.15. This value is shifted from the solution isoelectric point by –0.35 ±0.15, due to the contribution of solvation energy to the energy of ionic adsorption. On the basis of a simple model, it is suggested that the two successive surface acidity constants of hydrous chromium(III) oxide are pKS a1 = 7.17 and pKS a2 = 9.72. The influence of oxalic acidity on mobility may be described in terms of two successive adsorption equilibria, the first one conducive to the neutralization of positive protonic charge, and the second one originating a charge reversal. It is shown that the apparent affinity for this latter mode is lower in one of the samples, suggesting that surface complexation constants may in fact be sensitive to the history of hydrous chromium(III) oxide particles. The three samples show similar dissolution behaviour but the specific rates differ, in one case by two orders of magnitude. This difference is explained in terms of the influence of crosslinking on the reactivity