[Photocatalytic reductive degradation of direct red 4BE by phosphotungstic acid]

The photoreductive degradation of Azo-dye Direct Red 4BE (4BE) in aqueous solution was studied in a batch photoreactor, with phosphatotungstic acid (H3PW12O40, PW12) as the homogeneous catalyst and isopropanol as the electron donor. The parameters such as concentrations of PW12, isopropanol and 4BE, ionic strength were carefully evaluated. The results showed that 4BE could be reductively decolorized by heteropoly blue, which was produced by phosphatotungstic acid in the presence of isopropanol under UV irradiation. The decolorization rate reached 90.39% within 50 min at a pH value of 2.0, a 4BE initial concentration of 50 mg x L(-1), a PW12 and IS concentration of 600 mg x L(-1) and 0.13 mol x L(-), respectively. The decolonization rate of 4BE increased with the increase of PW12 and isopropanol concentrations until reaching a constant value. However, the first-order rate constants k for the degradation of 4BE decreased with the increase of the 4BE initial concentration. Mutual effects were found between the concentration of isopropanol and PW12 on the photocatalytic degradation of 4BE. Moreover, the concentration of salt showed a negative effect on the photoreductive degradation of 4BE. It was assumed that the charge-transfer occurred within the complex formed by heteropoly bule and 4BE, which led to the reduction of 4BE and regeneration of heteropoly bule. This study indicates that PW12/isopropanol/UV system could be used for the reductive degradation of azo dyes.