Comaring the Capability of Photocatalyst Nano Zinc Oxide Process by Two Slurry and Immobilized Methods in Dye Acid Orange 7 Removal

Comaring the Capability of Photocatalyst Nano Zinc Oxide Process by Two Slurry and Immobilized Methods in Dye Acid Orange 7 Removal



Introduction
Everyday million liters of color wastewater are produced in different industries and cuases environmental problems. The textile industry significantly affects the environment, producing high volume of wastewater from water and chemicals used in processes including sizing, washing, bleaching, dyeing, printing, and finishing. The wastewater contains salts, acid, or alkali, washing solvents, other chemicals, intermediates, and residue dyes. In the presence of toxic colored substances in water light fails to penetrate the waters lower layers and consequently the photosynthesis of water plants and the level of dissolved oxygen decreases, and results in the death of aquatic life. Dyes are stable compounds that are not easily biodegradable; especially Azo dyes which are considered to be carcinogens that pollute the surface and groundwater.
Generally, the physical, chemical and biological methods or a combination of them have been used for treating textile wastewater can be mentioned as membrane filtration, and ultrasonic waves, electrocoagulation, adsorption, Fenton, photo-Fenton, ion exchange, electrolysis, coagulation, conventional, and advanced oxidation and photo-catalytic process.
In recent years, the ability of advanced oxidation processes in treating various hazardous wastes has brought this technology to the forefront of research. Among advanced oxidation processes, application of heterogeneous photo-catalysis by using semiconductors has been proved to be real interest as an efficient tool for degrading both aquatic and atmospheric organic contaminants. Semiconductors are photo-reactive metal oxides for contaminants eradication that refer to photo-catalysts. These methods mineralize and converse pollutants into CO2, H2O and inorganic ions, by the action of hydroxyl radical, which acts as a nonselective and strong oxidant of organics.
Because of the excellent properties and wide applications on nanomaterials, recent years, researchers start to focus on the nano-zinc oxide granule size lies in the 1-100 nms, because of having nanophase structure and char acteristics, it gets the surface effect and bulk effect and gains some special capabilities (on magnetism, light, electric and sensitivity etc.) and many new usages.
Photocatalytic reactors for water and wastewater treatment can be classified to slurry and immobilized systems. In the slurry reactors, the nano particles are freely dispersed in the water phase and consequently the photo-catalyst is fully integrated in the liquid mobile phase. Whereas the immobilized catalyst reactor design features a catalyst anchored to a fixed support and dispersed on the stationary phase.
The mail aim of this sudy was to compare removal efficiency of acid orange 7, one of the high consumed textile dyes, in slurry and immobilized methods using photocatalyst UV/ZnO process.

Methods and materials
In this study, after detrmination of the maximum absorption wavelength of acid orange 7 (Table 1), the initial experiments were done to determine the main role of photocatalyst process (Figure). Then the effect of different amounts of dye concentration and catalyst, pH, irradiation power and energy consumption were investigated and optimized for both slurry and immobilized systems by changing one papramter and keepig the others constant.
The Water Sealer Method (WSM) was used to immobilize the nano ZnO powder on concrete surface. The process began by complete mixing of 100 mL of selected concrete sealer (colorless liquid) with measured amounts of ZnO. To the cured concrete surface of one reactor, 10 mL of 10% (by mass) ZnO sealer solution was prepared and put into ultrasonic cleaner (Fungilab UE-6SFD) for 5 min to ensure the complete separation of nano particles. Then the solution was applied and allowed to be cured for at least 3 d.

Table 1 Characteristic of Acid Orange 7
Structure
Formula C16H11N2NaO4S
Molar mass 350.3243
λmax 487 nm
Solution pH 3


Slurry and immobilized photo-catalyst reactors

According to the Beer Lambert law, by measuring the absorbance of samples at the maximum absorption wavelength of the dye (487 nm) in a spectrophotometer, dye concentration and removal rate (Eq. 1) were calculated, where CR was color removal efficiency (%), C0 represented initial dye concentration, and C indicated instant dye concentration. In order to eliminate flocs errors, samples were filtered and centrifuged prior to placing them in the spectrophotometer.
(1)
The main equipments used in this study include a Kern PLS 360-3 digital scale with 0.001 accuracy and Metrohm 691 pH meter. The amount of dye in solution was measured by using a Hach DR-4000 spectrophotometer at a wavelength of maximum absorption of acid orange 7 (485 nm) and the calibration curve of dye concentrations, respectively. All experiments were performed according to the method of analysis of water and wastewater and repeated at least 3 times.

Results and discussions
In summary, acid orange 7 could be successfully degraded and mineralized by nanophotocatalysis in both slurry and immobilized ZnO nanoparticle photocatalytic reactors.
The experiments in optimum conditions in both slurry and immobilized methods were shown that in pH of 7, catalyst concentration of 80 gr/m2 (10.32 gr/L) and 32 Watt UVc lamps, over 95 and 35 percent of 50 ppm dye was removed in slurry and immobilized methods, respectively. The amount of COD removal was higher in the slurry method (Table 2).

Table 2- Comparison of optimum conditions in the studied systems
Immobilized System Slurry System parameter
80 gr/m2 10.32 gr/L Catalyst Concetration
50 50 Dye Concentration (ppm)
7 7 pH
32 32 Power (Watt)
375 56 Time
4 6 COD Removal after 60 minutes
35 45 COD Removal after 375 minutes

Keywords: COD, radiation intensity, pH, dye concentration, zinc oxide, photocatalyst