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 aque ous suspensions of ZnO (12 g per dm(3) of solution) under air-equilibr ation and irradiation by a medium pressure mercury lamp (UV/visible) w as investigated. The photoreduction to the less harmful Cr(III) on the surface of the semiconductor particles was studied as a function of p H of the suspension, initial Cr(VI) concentration, mass of the semicon ductor in suspension, and different semiconductors as photocatalysts ( ZnO, Hombikat UV100, Degussa P25, WO3). An increase in the Cr(VI) phot oreduced with decreasing pH values was noticed, suggesting an acid-cat alysed 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 solu tion of the photoreduction process was studied by experiments performe d on either air-equilibrated or nitrogen-purged suspensions, and accor dingly the results were compared. A tentative scheme of the possible c atalytic reactions for the photoreduction of Cr(VI) over ZnO is given. (C) 1998 Elsevier Science B.V. All rights reserved.