Photodegradation Of Sulfamethoxazole In Aqueous Solution By Advanced Oxidation Processes

Drug consumption and industrial development are pervasively linked with the disposal of huge amount of toxic pharmaceutical pollutants into the environment. So far, scientific works conducted worldwide have reported widespread occurrence of these pollutants in various environmental matrixes including drinking water, surface water, groundwater, sewage treatment plant (STP) effluent and soil. Some of these pollutants are not readily biodegradable or treatable via conventional wastewater treatment processes. Therefore, advance treatment technologies often refer to as advance oxidation processes (AOPs) are used to mineralize these pollutants in aquatic environments. AOPs are based on the combination of oxidizing agents in the presence of a catalyst or light (UV or sunlight) and the generation of powerful oxidation species (OH) that oxidize the organic pollutants.Sulfamethoxazole (SMX) is a sulfonamide type antibiotic used in human and animal medicine and have been occurred widely throughout the aquatic environment up to the level ofμg L-1. In this work, the degradation of SMX in aqueous media by photolysis, UV/H2O2, VUV/H2O2, TiO2 and ZnO-assisted Photodegradation, O3 and O3/H2O2 are reported. The degradation process was conducted at a laboratory scale using pure water and the effluent from a real STP. The investigated factors included SMX concentration, solution pH, the presence of inorganic anions, humic acid and radical scavengers, temperature, catalyst concentration and H2O2 concentration, etc.Ozonation was the most efficient process with regard to removal efficiency and time consumption (10 min are required to completely remove 100 mg L-1 of SMX from pure water with ozone dose between 26.5 to 36.9 mg L-1) and the lower the concentration, the faster the removal. In general the efficiency of the studied processes for SMX removal in pure and/or in STP effluent water (pH not buffered and T= 20℃) followed the order of O3/H2O2> O3> VUV/H2O2> VUV> UV/H2O2> UV> solar/TiO2> solar/ZnO. The solution pH was an important factor that controlled the removal mechanism in O3 and O3/H2O2 process, which can be due to direct oxidation by O3 molecules and/or OH radicals. Water constituents such as anions or HA influenced the rate of ozonation treatment either positively or negatively depending on their nature and their concentration.UV254 photolysis showed that 10 mg L-1 of SMX was removed within 30 min of irradiation in pure water. The removal of SMX fitted the pseudo-first-order kinetic model with the rate constants in the range of 0.170 to 0.932 min-1 for initial SMX concentrations of 1.0 to 10 mg L-1. Solution pH of 2.0-5.5 was more favourable for SMX degradation compared with caustic condition. Slight improvements in SMX degradation in water matrixes with Cl-, SO42- and NO3- anions at 1.0 mM concentration and HA (when used at 5 mg L-1) were observed. However, HCO3- (used at 1.0 mM) and HA (when used at concentrations of 20,50 and 100 mg L-1) slowed down the SMX degradation rate. The complete degradation of SMX was almost achieved in STP effluent spiked with 10 mg L-1 of SMX after 60 min of irradiation. Total organic carbon was hardly changed even after 8 h of irradiation.Indirect oxidation of SMX by -OH generation was the main degradation mechanism in VUV/H2O2, VUV, UV/H2O2 while direct photolysis predominated in UV processes. The quenching tests showed that some other reactive species along with OH radicals were also responsible to the SMX degradation under VUV process. The addition of 20 mg L-] HA significantly inhibited SMX degradation, whereas, the inhibitive effects of both NO3- and HCO3- (0.1 mol L-1) were observed as well in all processes except in UV irradiation with NO3- existence. The removal rate decreased 1.7-3.6 times when applying these processes to STP effluent due to the complex constituents.The investigation of the effect of three temperatures (T1 (9.5-11.5℃), T2 (19.5-21.0℃) and T3 (29.0-30.0℃)) on SMX degradation in UV/H2O2 process showed that the removal rates were proportional to the temperature. T3 led to high degradation rate (0.241 and 0.588 min-1 at pH 7.0 and 5.5, respectively) after 30 min and 10 min of irradiation of 10 mg L-1 of SMX solution in pure water. HPLC chromatograms with similar retention time observed for both processes implied that the degradation pathway was identical and was not altered by the presence of H2O2. Sulfanilic, maleic, fumeric acids (observed by direct comparison of HPLC chromatograms of pure compound to those of treated solutions) and hydroxylated SMX were identified among intermediates. TOC hardly changed suggesting no mineralization. In STP effluent, over 99% of SMX removal were achieved under T3 after 30 min, while 90% and 80% were reached under T2 and T1 respectively, after 40 min of irradiation.In TiO2 and ZnO-assisted photodegradation of SMX, both solar radiation and catalyst were needed for an effective SMX degradation. At all catalyst dosage (0.2,0.5,1.0 and 2.0 g/L),10 mg/L of SMX was removed in 120 min when treated with TiO2 and in 180 min with ZnO. The addition of either HA (20 mg/L), H2O2(10,50 and 100 mg/L) or anions (used at 1 mM and 0.1 M) slowed down the degradation rate. When these processes were applied in STP effluent, the degradation rates decreased by 2.8 and 2.2 folds respectively with TiO2 and ZnO catalysts because of complex constituents.