ADSORPTION AND PHOTOINDUCED REDUCTION OF CR(VI) ION IN CR(VI)-4CP(4-CHLOROPHENOL) AQUEOUS SYSTEM IN THE PRESENCE OF TIO2 AS PHOTOCATALYST

Dark adsorption of Cr(VI) ion on TiO2 is mainly dependent on Cr( VI) c oncentration and the system acidity. 4-Chlorophenol (4CP) has no inter ference on Cr(VI) adsorption. Cr(VI) can easily be photocatalytically reduced under UV irradiation with TiO2 as catalyst. The reduction is i n accord with the Langmuir-Hinshelwood (L-H) kinetic equation. Under t he UV irradiation, photo-induced Cr(VI) reduction is observed in eithe r Cr(VI)-4CP or Cr(VI)-4CP-TiO2 system. Light intensity and system aci dity are two main roles influencing both reactions. The linear decreas e in Cr(VI) concentration reveals the zero order of Cr(VI) reduction r ate in both systems. Two different reactions are involved in Cr(VI)-4C P-TiO2 system under the experimental UV irradiation: the photo-induced homogeneous reaction between Cr( VI) and 4CP, and the photocatalytic reaction on TiO2 semiconductor photocatalyst. In the condition of excl uding homogeneous reactions between Cr(VI) and 4CP, the promotive effe ct of 4CP on photocatalytic reduction of Cr(VI) in Cr(VI)-4CP-TiO2 sys tem is distinguished and confirmed. Under the sunlight irradiation, no reaction is observed in Cr(VI)-4CP system. Photocatalytic reduction e fficiency of Cr(VI) ion on TiO2 in Cr(VI)-4CP-TiO2 system under the su nlight irradiation is much larger than that in Cr(VI)-TiO2 system. (C) 1998 Elsevier Science S.A. All rights reserved.