Role of Electron Scavengers in Direct Blue 71 Removal by Nano-TiO2 Immobilized on Cementitious Bed

1- Introduction
The presence of synthetic dyes in water and wastewater causes toxicity to aquatic life and bacteria and changes the brightness, color and clarity of the water. Common treatment methods for these compounds are not capable of complete mineralization. Therefore, new methods are needed to maximize the efficiency. Among them, Advanced Oxygen Processes (AOPs), especially Photocatalytic oxidation with nano particles such as TiO2 are efficient and appropriate. According to investigations, very reactive species such as hydroxyl radicals are produced while this process to oxidize organic dyes in wastewater efficiently. According to mechanism equations 1 to 6, anatase and rutile as two phases of TiO2 are activated with appropriate wavelength of 390 and 415 nm and Reactive Oxygen Species (ROSs) are made. Among them O2•-, HO2• and OH• are known as the most active radicals.
(1) TiO2+ hν→TiO2 (eCB-+hVB+)
(2) OH-+hVB+→OH•
(3) O2+eCB-→O2•-
(4) O2•-+H+→HO2•
(5) 2HO2•→O2+H2O2
(6) H2O2+O2•-→OH-+OH•+O2
This method has some problems such as long irradiation time required to decompose hard degradable compounds such as azoic dye structures and rehabilitation time and expense. To reduce these problems, some methods are proposed including scavenger application and stabilization of nano particles on an inert cementitious bed to simulate real wastewater treatment plant situation that these are mentioned in our investigation.
Application of scavenger compounds are necessary to elongate the immediate recombination of electron-hole pairs which are exited on irradiated TiO2 surfaces with UV-C light. The scavengers are compounds that will scavenge the electron or hole to retard the recombination of electron-hole and or bulk medium radicals to reduce interferences to make reactions faster be done, if they be correctly chosen and with initiating the photocatalytic chain reactions will enhance the efficiency of dye removal. With direct participating in redox reactions of colored pollutant and with elimination of ROS interaction effects in medium will also enhance the removal efficiency.
Therefore, in this research for enhancing the removal of Direct Blue 71 dye as a complex triazoic compound, the application of accelerating electron scavengers H2O2 and NaOCl in immobilized photocatalytic process was investigated. Immobilization of nano TiO2 particles slurry on inert cementations beds did also allow easy and more frequent use of them without the need of rehabilitation.

2- Materials & Methods
The procedure included of pilot preparation, making cementitious bed with Portland cement and water and immobilization of nano aqueous TiO2 dispersion on cementitious bed with epoxy concrete adhesive. The main material included Direct Blue 71 dye was obtained from Alvan Sabet Hamedan Co. and used without any purification, nano TiO2 powder (P25) as Photocatalyst (80:20 anatase to rutile, approximate diameter of 21 nm) was purchased from Degussa Co., Hydrogen Peroxide (H2O2) & Sodium Hypochlorite (NaOCl) was obtained from Dr. Mojallali Chemical Labs with 35% & 10-14% purity respectively as electron scavengers, Portland cement and water to make cementitious beds, Epoxy concrete adhesive was obtained from chemistry concrete Best Co., Oil to make pilots greasy and deionized water to make solutions.
Equipments included Spectrophotometer DR4000 Hach Co. to read absorption, ultrasonic cleaner UE-6SFD Fungilab to disperse agglomerated nano particles, digital pH meter metrohm 691 to measure pH, digital balance PLS360-3-Kern to weigh the materials, COD reactor DRB200 Hach Co. to measure chemical oxygen demand of wastewater, 30 W low pressure mercury lamps Lumiaction brand from Taiwan to excite nano particles, magnetic stirrer RH-B2 Ika to stir up nano particles mixture to make them uniform.
To perform experiments, a 2 cm depth cementitious bed was made of 2:1 Portland cement to water in a 23*15*4 cm framework. Afterward it was fixed in the similar pilot to make a surface of nano TiO2 with SSP (Separated Sealer and Photocatalyst) method on it (density=40 gr/m2).
Experiments were done in a UV-C case which was covered with thick aluminum sheets and a 250 cc sample solution of synthetic wastewater with approximate 2 cm depth on pilot surface was irradiated under UV-C radiation in it. Synthetic wastewater was made of dye powder and deionized water mixture and scavenger was injected simultaneously to reach objective concentration.
In the first steps an absorption spectrum of 20 mg/L dye solution was plotted in the range of 200-900 nm of wavelength so that the maximum absorption was obtained in 586 nm as . Afterward a calibration curve was obtained for different dye concentrations in this wavelength with the calibration equation of Abs=0.0241*[C] and R2=0.99 in which Abs (Absorption) is a non-dimensional parameter and [C] is dye concentration based on mg/L.
The experiments included main, blank and the supplementary tests. For this reason the impact of effective parameters on photocatalytic dye removal including scavenger concentration, pH, dye concentration and irradiation of UV-C intensity was investigated as main tests. Control tests were done in optimum conditions which were derived from main tests and included of Scavenger/Dye, TiO2/Dye, TiO2/UV-C/Dye, TiO2/Scavenger/Dye, Scavenger/UV-C/Dye systems. In the end, COD/COD0 trend, absorption curve of dye removal and kinetics of reactions were studied as supplementary tests.

3- Discussion of Results
The results showed that 100 mg/L dye in the presence of 0.006 M H2O2 at pH 6 under 90 W UV-C lamps irradiation in 20 minutes was decolorized. However the time required for bleaching without H2O2 under these conditions was 255 minutes. The concentration of 0.01 M NaOCl could also reduce the completely dye removal time of 100 mg/L dye at pH 11 and a light intensity of 90 W UV-C lamps from 255 to 15 minutes.Also, with an investigation of absorption spectrum while color removal, it was also proved that reduction in peaks of azoic bonds (586 nm) and benzene and naphthalene compounds (310 nm) was an approval of an appropriate decomposition process progress. The rates of color removal in presence of H2O2 and NaOCl via removal kinetics study of TiO2/scavenger/UV-C/dye system were respectively 15 and 35 times faster than kinetics of systems of dye removal without any scavengers which all these results were an approval of accelerating effect of used scavengers for dye removal.

4- Conclusion
According to study of reaction kinetics, dye removal trend was based on first order reaction. Sodium Hypochlorite accelerate the reaction to 35 times faster than the same situation without NaOCl and Hydrogen Peroxide did it 15 times faster in comparison of reaction without H2O2.
Although Sodium Hypochlorite was more effective to accelerate the photocatalytic removal of Direct Blue 71 Dye in the mentioned situation because of higher scavenger power of it compared with Hydrogen Peroxide, But application of Hydrogen Peroxide as an electron scavenger in Photocatalysis, made no by product but CO2 and water which they are safe for the environment. Sodium Hypochlorite combined with intermediate compounds of dye destruction to make some chlorinated products in the environment which need to be more treated.
In industrial scale 1 M Sodium Hypochlorite is equivalent to 10 L and this amount for Hydrogen Peroxide is 6 L for each cubic meter of wastewater, So this amount is more feasible according to price and accelerating rate in comparison to Sodium Hypochlorite. Therefore, between these two scavengers, Hydrogen Peroxide was preferred with acceptable efficiency and good scavenger effect on Photocatalysis of Direct blue 71 as a model dye.