Photooxidative Removal of Phenazopyridine by UV / H 2 O 2 Process in a Batch Re-circulated Annular Photoreactor : Influence of Operational Parameters

In this work, efficiency of UV/H2O2 process in the removal of Phenazopyridine (PhP) as a model contaminant from pharmaceutical compounds was investigated. For this purpose, a suitable batch re-circulated annular photoreactor with a replaceable UV-C lamp in the center of photoreactor was used. Effect of operational parameters such as initial PhP concentration, H2O2 concentration, UV-light source power and pH of the solution was investigated. Results indicate UV/H2O2 process is a powerful method for removal of PhP from aqueous solutions so that a considerable removal of this contaminant has been obtained in the short irradiation time.


INTRODUCTION
Development of ecologically clean and safe chemical technologies, processes and materials is vital for maintaining and controlling the environmental pollution 1 .Such techniques are getting significant attention in academic and industrial researches due to the shortage in clean and natural resources.Beside many other polluters of natural water resources, waste pharmaceutical compounds as a result of conventional disposal of these chemicals through sewer or incineration are approaches needed to be invented 4,5 .Advanced oxidation processes (AOPs) are introduced as destructive techniques for water treatment by removal of organic contaminants through oxidative degradation procedures.In general, AOPs involve oxidation of chemicals through ultraviolet radiation in the presence of H 2 O 2 , O 3 , Fenton, or TiO 2 which results in an efficient production of hydroxyl radicals [6][7][8] .These radicals are highly reactive oxidant with a short life cycle and non-selectively attack to contaminants [8][9][10] .Application of hydrogen peroxide in AOPs seems to be more promising among other approaches due to some of its advantages over others such as: it's complete miscibility with water as well as it's commercially availability.Moreover, the process can be done in ambient conditions and a whole mineralization of organic carbon into CO 2 may take place.In this process, hydroxyl radicals are generated by photolysis of H 2 O 2 through direct absorption of UV light by H 2 O 2 in a photoreactor.The maximum hydroxyl radical yields are obtained by using shortwave ultraviolet radiations (200-280 nm) 11 .
In this work, the efficiency of UV/H 2 O 2 process in the removal of a Phenazopyridine (PhP) as a model contaminant from pharmaceutical compounds was investigated in a batch recirculated annular photoreactor.The effect of operational parameters such as initial concentration of PhP, concentration of H 2 O 2 , power of light source and pH were studied.

Materials
Hydrogen peroxide (37%, w/w) was purchased from Merck (Germany).Phenazopyridine (PhP) with structural formula shown in Fig. 1, is a commonly used drug, and is chosen here as a model drug contaminant.PhP was prepared from Tehran pharmaceutical company (Iran).The presence of the azo group makes PhP as a refractory pollutant 12,13 .
In each experiment 500 mL of the PhP and H 2 O 2 solution was transferred to the photoreactor.Then the lamp was switched on to initiate the reaction.During irradiation, the solution was agitated in a constant rate and was transferred with peristaltic pump through annular photoreactor with volumetric flow rate of 140 mL min -1 .At certain reaction intervals, 5 mL of sample was withdrawn and the concentration of PhP was determined by means of a spectrophotometer (Shimadzu UV mini-1240).

Effect of the initial concentration of PhP
The effect of initial PhP concentration ranging between 20 -50 mg L -1 on the removal of PhP was investigated.Figure 3 illustrates the result of the experiment with different initial PhP concentrations and fixed amount of hydrogen peroxide.Based on these results, efficiency of PhP removal decreases with increasing the initial PhP concentration.This may be due to the fact that in the higher PhP concentration a stronger inner filter effect makes the solution more impermeable to UV radiation.Hence, efficiency of photooxidative removal of PhP decreases 14 .

Effect of the initial concentration of H 2 O 2
Figure 4 illustrates the effect of initial concentration of H 2 O 2 ranging between 50 -600 mg L -1 on the removal of PhP.In general, increasing the initial H 2 O 2 concentration increases the efficiency of the UV/H 2 O 2 process.A significant increase in the removal is observed in the range of 50 -500 mg L -1

Effect of light source power
As shown in Fig. 5 the removal percent of PhP increases with increasing power of light source from 4 to 12 W.This is justified with higher production rate of • OH at higher UV-light power and thus, an increase in reaction efficiency.According to Eq. (1) higher • OH produces when stronger light source uses in the photoreactor.

Effect of pH
The effect of pH ranging from 3-9.5 on the photooxidative removal of PhP was investigated as shown in Fig. 6.It was observed that the efficiency of the UV/ H 2 O 2 process in the removal of PhP was lower in the pH = 9.5.This can be due to dissociation of H 2 O 2 to the HO 2 -and H + at pH = 9.5 according to the Eq. 5. Hence, a decline in the process is rational in the alkaline pH 16 .

CONCLUSIONS
In this work, efficiency of the UV/ H 2 O 2 process using a batch photoreactor has been studied in the removal of PhP from aqueous solution.The effects of operational parameters such as, initial PhP concentration, H 2 O 2 concentration, UV-light source power and pH have been investigated on the efficiency of the process.According to the results, H 2 O 2 concentration has a critical effect in the removal of PhP and highest removal percent can be obtained by 500 mg L -1 from H 2 O 2 concentration.Also, efficiency of the process in the removal of PhP is higher in the range of 3 -8 for pH.UV-light source power has also positive effect on the efficiency of reactor and stronger light source causes higher removal percent.