Adsorption and photo-induced 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) concentration and the system acidity. 4-Chlorophenol (4CP) has no interference on Cr(VI) adsorption. Cr(VI) can easily be photocatalytically reduced under UV irradiation with TiO2 as catalyst. The reduction is in accord with the Langmuir-Hinshelwood (L-H) kinetic equation. Under the UV irradiation, photo-induced Cr(VI) reduction is observed in either Cr(VI)-4CP or Cr(VI)-4CP-TiO2 system. Light intensity and system acidity are two main roles influencing both reactions. The linear decrease in Cr(VI) concentration reveals the zero order of Cr(VI) reduction rate in both systems. Two different reactions are involved in Cr(VI)-4CP-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 excluding homogeneous reactions between Cr(VI) and 4CP, the promotive effect of 4CP on photocatalytic reduction of Cr(VI) in Cr(VI)-4CP-TiO2 system is distinguished and confirmed. Under the sunlight irradiation, no reaction is observed in Cr(VI)-4CP system. Photocatalytic reduction efficiency of Cr(VI) ion on TiO2 in Cr(VI)-4CP-TiO2 system under the sunlight irradiation is much larger than that in Cr(VI)-TiO2 system. (C) 1998 Elsevier Science S.A. All rights reserved.