Photocatalytic Degradation Of Malachite Green And P-Nitroaniline Using H₃PW₁₂O₄₀/TiO₂ As Composite Photocatalyst

As a cationic triphenylmethane dye, malachite green, has been extensively used in the textile industry for dyeing; p-Nitroaniline is an important intermediate in the manufacture of azo dyes. Both of them are typical organic pollutants in industrial wastewater and have been listed as one of the major priority contaminants in water for removal by the National Environmental Protection of the People s Republic of China, due to their toxicity, carcinogenic and mutagenic effects. In this paper, a novel catalyst, nanosized TiO2 coupled with phosphotungstic acid, was prepared for photocatalytic degradation of malachite green and p-Nitroaniline.(1) The composite H3PW12O40/TiO2 photocatalyst was prepared by impregnating H3PW12O40 onto the nano size TiO2, which was prepared by sol-gel method with tetrabutyl titanate. The composite photocatalyst was characterized by SEM,TEM,FT-IR,XRD,DRS to investigate its optical, physical and chemical properties. The results showed that the size of most particles in the as-prepared composite H3PW12O40/TiO2 was found to be ca.20-30nm, and still have uniform anatase structure. Meanwhile, there is a chemical interaction, not simple physical adsorption, exists between the Keggin units and the surface of the titania matrix. Compared with original H4PW12O40 or TiO2 matrix, the adsorption band of the H3PW12O40/TiO2 composite had a red shift; and the adsorption threshold onset of the composite extended to the visible region.(2) The effects of different operational parameters including impregnation volume, the dosage of catalyst, initial malachite green concentration, pH value, aeration rate and light intensity on the rate of photocatalytic degradation under ultraviolet and visible light irradiation were investigated. The photocatalytic degradation mechanism of malachite green using H3PW12O40/TiO2 was investigated. The results showed that the photocatalytic degradation of malachite green by H3PW12O40/TiO2 follows pseudo-first order kinetics. Under the optimum conditions, the degradation efficiency of MG using H3PW12O40/TiO2, could reach 54% and 37%, respectively, with UV and visible light irradiation, which was significantly higher than those obtained from using TiO2 within 60min and 4h. The result indicates that there is a synergistic effect between H3PW12O40 and TiO2. Meanwhile, the photocatalyst H3PW12O40/TiO2 particle was easy to be separated from the MG solution, and the recovered photocatalyst still had good reusability to get 86% and 67% degradation of MG, under UV and visible light irradiation, respectively. Based on the intermediates detected by gas chromatography-mass spectrometry (GC/MS) and the analysis from UV-Vis and FT-IR, the possible degradation mechanism of MG included three processes:the N-demethylation process, the cleavage of center carbon process, open-ring reaction.(3) The photocatalytic degradation of p-Nitroaniline using H3PW12O40/TiO2 under 250W UV light irradiation was investigated. Effects of H3PW12O40/TiO2 dosage, pH value, initial p-NA concentration on the photocatalytic degradation of p-NA were investigated in detail. The results showed that the optimal H3PW12O40/TiO2 dosage and pH value for degradation of 10mg/L p-NA were 0.6g/L and 3.0. The degradation rate of p-NA by H3PW12O40/TiO2 process could be fitted pseudo-first-order kinetics. Moreover,66% degradation of p-NA was still observed in the 5th reused experiment. Moreover, seven kinds of intermediates were detected by GG/MS, and the changes of FT-IR data combined with the intermediates identified by GC/MS gave the evidence that the photocatalytic degradation of p-NA proceeded by denitration reaction to generate 4-aminophenol, followed by deamination reaction and open-ring reaction to form carboxylic acid compounds, until eventually mineralized into CO2,H2O and NO3-.