Synergistic Effects Of Ultrasonic Degradation Of Methyl Orange And TiO₂ Photocatalytic Reduction Of Cr (VI)

With the development of industry production, there have been a great deal of environmental pollutants produced, which are danger to environment and human. Conventional water treatment plants are typically not effective in remediating industrial contaminants. Therefore a large research effort is focused on advanced oxidation processes (AOPs). AOPs are defined as technologies that employ the highly reactive·OH radical as the main oxidative species for the breakdown of organic contaminants and show great promise as part of the water treatment process. The·OH radical can be formed by a number of methods in aqueous systems: O3, Fenton reaction(H2O2/Fe2+), high-energy electrons, electrochemistry, supercritical water oxidation, photocatalysis and ultrasound waves and so on. In these paper, ultrasonic degradation of methyl orange by adding lower concentration of CCl4 and effects of organic specials on photocatalytic reduction of hexavalent chromium over different TiO2 photocatalysts have been explored, the results are as follows:(1) Effects of CCl4 were investigated on the ultrasonic degradation of azo dye methyl orange (MO). The work demonstrated that the rate of MO decolorization was drastically enhanced by the addition of CCl4. The decolorization of MO was observed to behave as a pseudo-first reaction in kinetics under all the conditions tested in the present work. The apparent rate constant of the decolorization was demonstrated to be dependent on CCl4 concentration, MO concentration and the solution pH value. And the rate constant of MO decolorization was able to be increased significantly by increasing initial CCl4 concentration, decreasing MO concentration and lowering solution pH value. This enhancement of MO decolorization by adding of CCl4 was mainly attributed to the oxidizing species such as·Cl radicals and HClO formed out of CCl4 during sonication. Moreover, a linear correlation was observed between the amount of CCl4 and the decolorization rate of MO. Thus, the CCl4 determination was established. Under optimized conditions, Beer's law was obeyed in the range of 0.4-20 mg L-1 of CCl4 (DL = 0.2 mg L-1, R2 = 0.9996). The concentrations of CCl4 in several practical samples have been determined satisfactorily by using this method.(2) Effects of organic specials on photocatalytic reduction of Cr(VI) over anatase-type TiO2 calcined at different temperature have been explored, with a comparison to the process over Degussa P25, an excellent commercial TiO2 photocatalyst. As the calcination temperature was increased, the crystalline structure changes from a somewhat amorphous state to a perfect anatase phase structure, and then to a rutile phase structure at 900°C. However, the BET specific surface area of TiO2 became smaller as the calcination temperature was increased. Under all the tested conditions, the photocatalytic reduction of Cr(VI) behaved as a pseudo-first-order reaction in kinetics. In the absence of any organic species, as the calcination temperature of anatase-type TiO2 was increased, the kCr of photocatalytic reduction of Cr(VI) increases initially, passing a maximum at 500°C, then decreased. The photocatalytic reduction of Cr(VI) was accelerated by adding some amount of organic compounds into the Cr(VI) solution. Especially for the catalyst A000 without the calcinating treatment, the addition of formic acid significantly increases the value of kCr by a factor of about 6.6 times, which was almost 5 times compared to that of Degussa P25. In contrast, the value of kCr was observed to be decreased as the calcination temperature of anatase-type TiO2 was increased when some organic specials were added into the solution. These results demonstrated that a marked synergistic effect was found in the photocatalytic degradation of the mixture of Cr(VI) and organic specials, which was strongly dependent on the nature of the photocatalyst, especially its specific surface area. The larger of the BET specific surface area of the photocatalyst, the stronger of synergistic effect of photocatalytic degradation of the mixture of Cr(VI) and organic specials.