Visible light Cr(VI) reduction and organic chemical oxidation by TiO2 photocatalysis

Here we report the simultaneous Cr(VI) reduction and 4-chlorophenol (4-CP) oxidation in water under visible light (wavelength > 400 nm) using commercial Degussa P25 TiO(2). This remarkable observation was attributed to a synergistic effect among TiO(2), Cr(VI), and 4-CP. It is well known that TiO(2) alone cannot remove either 4-CP or Cr(VI) efficiently undervisible light. Moreover,the interaction between Cr(VI) and 4-CP is minimal if not negligible. However, we found that the combination of TiO(2), Cr(VI), and 4-CP together can enable efficient Cr(VI) reduction and 4-CP oxidation under visible light. The specific roles of the three ingredients in the synergistic system were studied parametrically. It was found that optimal concentrations of Cr(VI) and TiO(2) exist for the Cr(VI) reduction and 4-CP oxidation. Cr(VI) was compared experimentally with other metals such as Cu(II), Fe(III), Mn(IV), Ce(IV), and V(V). Among all these metal ions, only Cr(VI) promotes the photocatalytic oxidation of 4-CP. The amount of 4-CP removed was directly related to the initial concentration of Cr(VI). The system was also tested with four other chemicals (aniline, salicylic acid, formic acid, and diethyl phosphoramidate). We found that the same phenomenon occurred for organics containing acid and/or phenolic groups. Cr(VI)was reduced at the same time as the organic chemicals being oxidized during photoreaction under visible light. The synergistic effect was also found with pure anatase TiO(2) and rutile TiO(2). This study demonstrates a possible economical way for environmental cleanup under visible light.