Photocatalytic reduction of environmental pollutant Cr(VI) over some semiconductors under UV/visible light illumination

A novel technique based on photocatalysis to eliminate Cr(VI) ions, a toxic pollutant in the environment, was applied. The reduction in aqueous suspensions of ZnO (12 g per dm(3) of solution) under air-equilibration and irradiation by a medium pressure mercury lamp (UV/visible) was investigated. The photoreduction to the less harmful Cr(III) on the surface of the semiconductor particles was studied as a function of pH of the suspension, initial Cr(VI) concentration, mass of the semiconductor in suspension, and different semiconductors as photocatalysts (ZnO, Hombikat UV100, Degussa P25, WO3). An increase in the Cr(VI) photoreduced with decreasing pH values was noticed, suggesting an acid-catalysed behaviour. First-order kinetics were observed from the results at different initial concentrations of Cr(VI). A limiting value of the mass of ZnO of 12 g per dm3 of solution was attained, where a maximum light absorption by ZnO was ascertained. The effect of oxygen in solution of the photoreduction process was studied by experiments performed on either air-equilibrated or nitrogen-purged suspensions, and accordingly the results were compared. A tentative scheme of the possible catalytic reactions for the photoreduction of Cr(VI) over ZnO is given. (C) 1998 Elsevier Science B.V. All rights reserved.