Chromium oxides in sediments: comparison of surface characterization by electrophoretic mobilities and Fourier transform IR–attenuated total eflection measurements

Abstract

Adsorption of oxalate and chromate onto hydrous chromium(III) oxide and onto α-Cr2O3 was studied by spectroscopic methods (Fourier transform IR) and by measurement of electrophoretic mobilities. The Fourier transform IR spectra demonstrate the formation of surface complexes, and also the surface dimerization of chromate to give surface-bound dichromate. The mobility profiles are interpreted in terms of two successive surface complexation equilibria. The first stage yields uncharged species, and is responsible for a sharp decrease in mobilities in the acidic range. The second stage gives rise to negatively charged species, and is responsible for the charge reversal observed at higher concentrations. The postulated equilibria are given by equations (6)–(10). The dependence of the isoelectric point (pH piep ) on the total oxalate concentration is modeled on the assumption that the surface can be described as a mixture of oxalated and non-oxalated oxide, with two distinct extreme pH piep values. The linear dependence of pH piep on log[Cr(VI)], with slope 1.5, was explained in terms of the contribution of the stoichiometry of adsorption by the two complexation modes.