ADSORPTION OF CR6+ OXO ANIONS ON PURE AND PEROXIDE-MODIFIED TIO2 (ANATASE)

The adsorption of Cr6+ from ammonium chromate solution onto anatase has been studied at room temperature and at different pH values. It is established that the adsorption is a slow process (saturation of the active sites is reached after more than 24 h). Measurable adsorption proceeds below pH 8. The decrease in pH leads to an increase in the adsorbed amount. The adsorption isotherm (pH 5) has a complex character and allows three different parts to be distinguished: (i) irreversible adsorption, when all the added amount of Cr6+ is strongly adsorbed; (ii) a concentration interval in which the adsorbed amount of Cr6+ depends weakly on the concentration; (iii) a Langmuir-type adsorption of Cr2O72-. A small amount of hydrogen peroxide added to the Cr6+ solution leads to an increase in the adsorption capacity of titania, whereas a higher H2O2 concentration causes the reverse effect. However, when anatase is pretreated with hydrogen peroxide, an increase in its adsorption capacity towards Cr6+ is observed in all cases. The adsorption kinetic and adsorption isotherms are similar to those observed with pure anatase. An adsorption mechanism is proposed and the observed phenomena are discussed on the basis of this mechanism.