Photocatalytic Degradation of Ethylene Dichloride in Water Using Nano TiO 2 Supported on Clinoptilolite as a Photocatalyst

In this article one of the advanced oxidation processes (AOP) combined methods, photocatalyst /H2O2, is utilized in order to study photodegradation of ethylene dichloride (EDC) in water. Nano Titanium (IV) Oxide, supported on Clinoptilolite (CP) (Iranian natural zeolite) using solid-state dispersion (SSD) method for improvement of its photocatalytic properties. The results show that the TiO2/Clinoptilolite (SSD) is an active photocatalyst. The effects of five important photocatalytic reaction parameters including the initial concentration of ethylene dichloride, the ratio of TiO2/Clinoptilolite, the catalyst concentration, H2O2 concentration and pH in photodegradation of ethylene dichloride were examined. In this experiments, the design and also the optimum parameters were obtained by Taguchi Method, using Design Expert8® software. Taguchi's L27 (5^3) orthogonal array design was employed for the experimental plan. Four parameters were found to be significant whereas, pH was found to be an insignificant parameter after conducting experiments. A first order reaction with K = 0.007 min-1 was observed for the photocatalytic degradation reaction.


Introduction
Effects of several different pollutions such as phenol compounds, alcohols, organic acids, hydro-carbonic sulfur compounds, pesticides and insecticides compounds, dyes, output wastewater from various industries and etc. using photocatalytic oxidation has been investigated on sewage treatment.All of these experiments show high efficiency in degradation and removal of these pollutions from water and sewage by this method [1,2].Usual biological treatment methods for hazardous compounds such as chlorinated hydrocarbons are not efficient, because of high toxicity of these compounds which results in destroying microorganisms.TiO 2 is one of the most effective photocatalysts due to its biological and chemical inertness and photo stability in near-UV band energy gap, and can be used as a fine powder or crystals dispersed in water and wastewater treatment applications.However, the need to filter TiO 2 particles after reaction makes such a process troublesome and costly.Thus, in order to solve this problem, many researchers have examined several methods for fixing TiO 2 on supporting materials including glass beads [3][4][5], fiber glass [6][7][8], silica [9,10], and zeolite [11,12].When using zeolite as TiO 2 support, care should be taken that TiO 2 does not lose its photo activity and the adsorption properties of zeolite are not affected.Matthews [4] showed that the photo efficiency of TiO 2 is suppressed when TiO 2 is in interaction with the zeolite.
In this work TiO 2 was supported on a zeolite without losing photo efficiency and affecting the adsorption properties of zeolite using the exact method suggested by Nikazar et.al. [13] for supporting TiO 2 on Clinoptilolite.This mixture was used for photodegradation of aqueous EDC.

Materials
Degussa P-25 titanium dioxide with a crystallographic mode of 80% anatase and 20% rutile, a 50 m 2 g -1 BET surface area and an average particle size of 30 nm (according to the manufacturer's specifications) and the raw material was an Iranian commercial Clinoptilolite (CP) (Afrand Tuska, Iran) from deposits in the region of Semnan.According to the supplier's specifications, it contains about 90 wt% CP (based on XRD internal standard quantitative analysis) and the Si/Al molar ratio is 5.78.The concentration of Fe 2 O 3 , TiO 2 , MnO and P 2 O 5 impurities has been reported to be 1.30, 0.30, 0.04 and 0.01 wt% respectively, and were used for preparation of the photocatalyst.Merck H 2 O 2 with 30% purity, and Ethylene dichloride (EDC) produced by Bandar Imam Petrochemical Comlex, with 96% purity for making reacting solution.

Preparation of TiO 2 -supported on CP Catalysts
The Solid State Dispersion (SSD) method was applied for supporting photocatalyst on zeolite.In this method, nano titanium peroxide was mixed with CP using ethanol as a solvent and mixture was grinded for 3 hours.Ethanol was then removed by evaporation.Samples were dried at 110°C in the oven and calcined at 450°C in the furnace for 5 hours to obtain TiO 2 -supported zeolite photocatalysts [13].

Apparatus
Photocatalytic reaction was performed in a batch Pyrex double wall reactor of 1.5 L in volume with two 8-W UV-C mercury lamps located in quartz tubes inside the reactor.The tubes were made from quartz because UV-C light cannot pass through glass and Pyrex.The photo reactor used in this experiment is shown in Figure 1.Circulator has been used for temperature adjustment and GC VARIAN CP-3800 was used for EDC concentration measurement.

Procedures
A solution containing known concentration of EDC was prepared; subsequently 800 cc of this solution was poured into the reactor.The solution pH value was adjusted at desired level using dilute NaOH and H 2 SO 4 .Then certain amount of prepared photocatalyst and H 2 O 2 was added to the solution.Photocatalytic reaction took place under the radiation of mercury lamps while agitation and aeration was maintained to keep the suspension homogeneous and oxygenized.Sampling was performed at specified times and concentration of EDC was determined using GC.

Design of Experiments
Effects of five parameters that influence the efficiency of photocatalytic reaction have been studied in these experiments.Initial concentration of pollutant (EDC), H 2 O 2 concentration, catalyst amount, TiO 2 % and pH, each of them in three levels, are shown in Table 1.
Because of numerous studying parameters, each at 3 different levels, Taguchi method for design of experiments using Design Expert 8.0.5 ® was employed to decrease the number of experiments to 27 for obtaining optimum terms.Temperature is one of the effective parameters on photocatalytic reactions that are usually set at ambient temperature, but due to high volatility of ethylene dichloride in the ambient temperature and aeration during process, large amount of EDC would be vaporized from the solution.Therefore, reaction's temperature was set at 5°C using circulator.
Temperature is one of the effective parameters on photocatalytic reactions that are usually set at ambient temperature, but due to high volatility of ethylene dichloride in the ambient temperature and aeration during process, large amount of EDC would be vaporized from the solution.Therefore, reaction's temperature was set at 5°C using circulator.

Taguchi Method
ANOVA analysis is shown in the Table 2. SUM of Squares: sum the squared differences between the average values for the blocks and the overall mean.
DF: degrees of freedom attributed to the blocks, generally equal to one less than the number of blocks.
Mean square: estimate of the block variance, calculated by the bock sum of squares divided by block degrees of freedom.
The F-value of 33.10 implies the model is significant Values of "Prob > F" less than 0.0500 indicate model terms are significant.In this case A, B, C, E are significant model terms.
In the Figure 2 we can see a graph of the predicted response  values versus the actual response values.It is clear that all of the values are predicted by the model.Responses should be assigned as "larger is better" for enhancing optimized parameters as showed below:

Kinetics of Photocatalytic Degradation of EDC
Several experimental results indicated that the degradation rates of photocatalytic oxidation over illuminated TiO 2 fitted by the first-order kinetic model [14][15][16].Figure 3 shows the plot of ln([EDC] 0 /[EDC]) vs. irradiation time for EDC.The linearity of plot suggests that the photodegradation reaction approximately follows the pseudo-first order kinetics with K = 0.007 min -1 .

Effects of UV Irradiation and Photocatalyst Ingredient
In

Conclusion
1. SSD method is an effective method for supporting TiO 2 on Clinoptololite.
2. The following optimum terms obtained with Taguchi method : Initial concentration of EDC 200 ppm, catalyst concentration 0.25 g/L, H 2 O 2 concentration 50 ppm, TiO 2 % 15 and effect of pH and two parameters interactions were not significant enough.
3. Initial concentration of EDC, Catalyst concentration, TiO 2 % and H 2 O 2 concentration were effective in reaction efficiency, respectively.
4. Maximum efficiency of 74% for photocatalytic degradation of EDC was obtained with optimized parameters.
5. The kinetic of photocatalytic degradation of EDC is of the pseudo-first order with K = 0.007 min -1 .

Figure 4
the comparison of four experiments is shown.First column is degradation efficiency of EDC using only UV light without photocatalyst, this column shows the importance of photocatalyst because eliminating photocatalyst from reaction caused decrease in efficiency about 47%.Second column is about degradation efficiency of EDC employing 15% wt TiO 2 photocatalyst without UV irradiation, this column shows influence of UV light in activating photocatalyst, reaction efficiency with elimination of UV light cause 45% efficiency reduction.Third column is shown degradation efficiency of EDC using pure TiO 2 (degussa P25 without zeolite) catalyst with UV irradiation, supporting catalyst on zeolite increase reaction efficiency about 37%.In last column degradation efficiency of EDC with optimum parameters has been brought for comparison.All of the other parameters are the same.