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, mai nly 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 t he mobilities, especially in the acidic branch, and, at higher concentratio ns, to shifts in the isoelectric point (pH(piep)) to lower values, as expec ted for anionic chemisorption. A two-mode adsorption model accounts qualita tively for the results. Under light, pH(piep) shifts in the opposite direct ion as the Cr(VI) concentration increases. A maximum value is attained at [ Cr(VI)] approximate to 10(-3) mol dm(-3), pH(piep) 8.2, which coincides wit h values reported for hydrous chromium(III) oxides. At higher concentration s, Cr(VI) adsorption defines a shift of pH(piep) to lower values. It is con cluded that light absorption by titanium dioxide promotes the reduction of Cr(VI) and the surface precipitation of the hydrous Cr(III) oxide. The valu es of pH(piep) for partially covered surfaces are well described by a simpl e model of surface ionization derived earlier. FTIR/ATR analysis of the sur faces supports this interpretation, and further suggests that one of the mo des of Cr(VI) adsorption implies surface dimerization to yield adsorbed dic hromate. (C) 2000 Academic Press.