Effect of Light on the Electrokinetic Behavior of TiO2 Particles in Contact with Cr(VI) Aqueous Solutions

The electrokinetic behavior of titanium dioxide particles (Degussa P25, mainly composed of anatase) put in contact with chromium(VI) aqueous solutions is highly sensitive to light exposure under normal laboratory conditions. In the dark, adsorption of Cr(VI) gives rise to substantial decrements in the mobilities, especially in the acidic branch, and, at higher concentrations, to shifts in the isoelectric point (pHpiep) to lower values, as expected for anionic chemisorption. A two-mode adsorption model accounts qualitatively for the results. Under light, pHpiep shifts in the opposite direction as the Cr(VI) concentration increases. A maximum value is attained at [Cr(VI)]≈10−3 mol dm−3, pHpiep 8.2, which coincides with values reported for hydrous chromium(III) oxides. At higher concentrations, Cr(VI) adsorption defines a shift of pHpiep to lower values. It is concluded that light absorption by titanium dioxide promotes the reduction of Cr(VI) and the surface precipitation of the hydrous Cr(III) oxide. The values of pHpiep for partially covered surfaces are well described by a simple model of surface ionization derived earlier. FTIR/ATR analysis of the surfaces supports this interpretation, and further suggests that one of the modes of Cr(VI) adsorption implies surface dimerization to yield adsorbed dichromate.