مهندسی عمران شریف (Feb 2016)
SCAVENGER EFFECTS ON ACCELERATING PHOTOCATALYTIC REMOVAL OF DIRECT BLUE 71 DYE WITH NANO TIO2 IMMOBILIZED ON A CEMENTITIOUS BED
Abstract
Prevention of textile industry wastewater discharge into bodies of water is an important matter to save the environment from carcinogenic aromatic non-biodegradable \ compounds. It is, therefore, essential to utilize a \ new \ cost effective \ technique which \ completely \ mineralizes these organic pollutants. The photocatalytic process, with semiconductors such as $TiO_2$, is a new technology that has been used in recent years. The most important fact is that $TiO_2$ nano-particles are more stable, cost effective, and non-carcinogenic. They also use solar irradiation as an exciting agent. The primary step of photo-oxidation of the dye solution excites an electron from the valence band to the convention band, and so, makes an electron-hole as a charge carrier. An important issue is the fast recombination of this new formed electron-hole, which can emit energy, otherwise scavengers are used. Using scavengers will retard the recombination of the pair electron-hole and will enhance the photocatalytic process efficiency. Scavengers are classified into three groups: 1) electron scavengers such as $O_2$, $S_2O_8^2-$, $Fe^3+$, $IO_4^-$ and $BrO_3^-2$) hole scavengers such as formic acid, salicylic acid, oxalic acid, methanol, ethanol, sucrose, peroxide hydrogen, nitrate, nitrite, chlorite, chlorate, potassium iodide, sulfate, phosphate and EDTA and 3) is radical, like alcohol. In this research, the accelerating photocatalytic removal of DB 71 was investigated with scavengers, including hydrogen peroxide, sodium hypochlorite and EDTA. The process was done with immobilized $TiO_2$ nano-particles on a cementitious bed as a photocatalyst. In order to determine the optimum conditions, scavenger dose, pH, initial dye concentration and intensity of UV-C radiation were investigated. After 20 minutes, 100 mg/L dye was decolorized at pH 8 under 90W irradiation of UV-C, and with 0.006 M $H_2O_2$; 15 times faster than a system without $H_2O_2$. Also, 0.01 M NaOCl was decolorized at 100 mg/L DB 71 at pH 11 under 90W irradiation; 35 times faster, and 75 mg/L dye was decolorized by 0.03 M EDTA at pH 6 under 60W irradiation; 2.5 times faster. For comparison, hydrogen peroxide, as a good scavenger, which is environmental friendly with no undesirable intermediate compounds, was chosen as optimum scavenger.
