Investigations on the Photo-Catalytic Degradation Reaction of Direct Red 16 in Water Using a Circulating Fixed Catalyst
Bed Photo-Reactor
J. Saien, M. Asgari and A. R. Soleymani
Department of applied chemistry, Bu-ali Sina Uuniversity, Hamedan, Iran E-mail: [email protected]
Introduction:
In recent years, there has been increasing interest in the use of photocatalytic methods for the treatment of wastewaters [2]. An important advantage of photo-catalytic oxidation is to oxidize organic pollutant into CO2 and water to avoid a problem of contaminants shifting from one phase to another. Also, the operation at room temperature and atmospheric pressure prevents volatilization and discharge of unreacted wastes, and the reaction can be simply terminated in seconds by cutting off the power [3]. A problem that should be solved in operating of photo-catalytic process is releasing catalyst particles in wastewater. In this order we designed a specific catalytic reactor in order to photocatalytic degradation of a target azo dye named as Direct Red 16. This azo dye is highly used in textile industries.
Experimetnal
In order to photodegrade the dye, a TiO2 coated on quartz packed reactor was designed.
The main body was in conic shape and was irradiated by direct UV-C light form a lamp (250 W), positioned centrally in the reactor. The reactor has the advantage of no need to recover the used catalyst particles. Solutions containing 30 mg/L of dye were prepared and after adjusting temperature and pH to desired values, transferred to the reactor. After which the UV lamp was switched on to initiate the process. During the experiment, circulation of reactor content was maintained to keep concentration and temperature homogeneous, also the temperature of solution was controlled. Samples (4 ml) were obtained after an appropriate elimination time. The concentration of dye in each taken sample was determined by a spectrophotometer (UV-VIS spectrophotometer Perkin-Elmer) at Zmax=526.57 nm using several calibration curves, obtained at different pH values.
Using this method the conversion of DR16 can be obtained at different times during
0
the degradation. The photodegradation conversion (X) is given by:
X = C0 − C C0
Where C0 is the initial concentration of DR16 and C is its concentration at time t.
Result and discussion
Effect of UV irradiation and TiO2 particle: Figure 1 demonstrates the effect of UV irradiation and TiO2 particles on photodegradation of DR16. The figure shows that in the presence of both TiO2 and light (photocatalysis), a conversion of about 100% of dye (initial concentration of 30 mg/L) can be obtained after an irradiation time of about 90 min, under its natural pH value of 6.75 and temperature of 25 ºC.
1.0
0.8
0.6
0.4
0.2
0.0
PACKED ONLY UV
0 30 60 90 120
Time (min)
Figure 1. Effect of UV light and TiO2, [DR16] =30mg/L , pH=neutral value (6.75) and T=25 ºC.
Effect of pH value: As Fig. 2 demonstrates, during an irradiation time of about 2 h, at pH 4 and 7, the efficiency of degradation can reach to level of nearby 100%, but in alkaline media this level could only reach to 85% and 91% at pH 8 and 9 respectively.
The natural pH (6.75) shows promising results, compared with other pH values; therefore this pH can be the candidate as optimum.
1.0
0.9
0.8
0.7
0.6
0.5
0.4
3 4 5 6 7 8 9 10 11
pH
Figure 2. Effect of pH on degradation process. [DR16]0 =30mg/L and T=25 ºC.
30 min 60 min 90 min 120 min
XX
0
Effect of Temperature: the appropriate results are presented in Fig. 3. In the range of 25-45 ºC the experiments were done and observed that the enhancing effect of temperature will be decreased as the temperature increases. It is obvious that the total effect of temperature is not significant which also has been reported by other investigators [4].
Effect of hydroxyl radicals: For studying in details by using of ethanol as a strong radical scavenger in medium, the role of these active oxidizing species in the photocatalytic oxidation process were investigated. The results are compared in Fig. 4.
The two plots of conversions due to OH radical produced in the UV+TiO2 process and due to the only UV photolysis have been presented in this figure.
1.0
0.8
0.6
0.4
T=25 35 45
0.2
0.0
0 20 40 60 80 100 120
Time (min)
Figure 3. Effect of temperature, [DR16]0 =30mg/L , pH=neutral value (6.75).
The difference between these two values indicates the conversion due to that part of OH which corresponds to the presence of catalyst. The plot of overall conversion (including electron-hole effect and OH radical activities, obtained from the difference between two plots in Figure 1) are also given in this figure. The close agreement between these two late mentioned results indicate that the main role in the photocatalytic branch of degradation process is due to the activity of hydroxyl radicals and direct electron-hole dye oxidation and reduction is negligible.
X for OH radical from UV/TiO2 X for phot X for OH radical only from TiO2 X for OH
0.50 0.40 0.30 0.20 0.10
0.00
0 30 60 90 120
Time (min)
Figure 4. Effect of hydroxyl radicals. [DR16] =30mg/L , pH=neutral value (6.75) and T=25 ºC
X X
References
1. J. Saein, A.R. Soleymani, J. Hazard. Mater, 144 (2007) 506-512.
2. Y. Deng, J.D. Englehardt, Waste Manag, 27 (2007) 380-388.
3. N. Daneshvar, M. Rabani, N. Modirshahla, M.A. Behnajady. J.
Photochem. Photobiol. A: Chemistry 168 (2004) 39.
