## 3 Results and Discussion
### Effects of Various Factors on the Removal of Et-PABA  
The concentration of oxidant dosage directly affects the amounts of free radicals produced; thus, it is necessary to study the effect of PDS dosage on the degradation of Et-PABA. As shown in Figure 3A, the degradation rate of Et-PABA increased as the concentrations of PDS increased from 1.0 to 4.0 mM within 15 min, which indicated that the removal of pollutants increased rapidly with the increase of oxidant dosage within a certain range. The main reason may be that as the concentration of PDS increases, the production of SO\({}_{4}\)\({}^{\bullet-}\) and \({}^{\bullet}\)OH free radicals also increases, which promotes the degradation efficiency of Et-PABA in a short time [45; 46]. The remaining concentration of PDS (when adding 2.0 mM PDS) was determined using the iodometric titration method. The remaining rate was about 27%, indicating that increasing the oxidant dosage is effective but not economical. Figure 2: Removal efficiencies of (**A**) Et-PABA and (**B**) PABA using different oxidation processes; (**C**) removal efficiency of Et-PABA using persulfate (PDS) activated by heat energy. Conditions: [Et-PABA] = [PABA]= 0.05 mM, [PDS] = 2 mM, and without pH adjustment. Theoretically, 23.5 Mol of PDS is required to completely mineralize 1 Mol of Et-PABA. Thus, 2 mM of PDS (40:1) was chosen in this experiment, considering the actual excess consumption of oxidants. The acidity may affect the existence of free radicals in the UV/PDS process. Thus, the effect of pH on the Et-PABA removal was examined in this experiment. The initial pH values of the solution were adjusted from 3 to 11 with 0.1 M H2 SO4 or 0.1 M NaOH, respectively, to investigate the impact of pH on the removal efficiency. As Figure 3B shows, the removal efficiency of Et-PABA was slightly better under acidic conditions, while it was slightly worse under the alkaline conditions in a short time of less than 20 min. It may be due to the existence of a large amount of H+ in the reaction system which reacts with PDS to produce more SO4\({}^{\bullet-}\), the predominate radicals in the UV/PDS system. SO4\({}^{\bullet-}\) has a higher redox potential under acidic conditions, thereby accelerating the reaction rate [47]. [PERSON] and coworkers found that acid-catalyzed decomposition of persulfate anion under acidic pH gives a higher concentration of SO4\({}^{\bullet-}\)[48]. The slight decrease in the removal efficiency at pH 11 may be due to the fewer radicals generated as a result of the alkaline condition. However, as the reaction progressed, the final removal efficiency was greater than 97%, indicating that a wide range of pH value was applicable for the Et-PABA degradation using the UV/PDS process. From these results, the UV/PDS process is expected to be a prospective treatment in the remediation of emerging contaminants in wastewater. In the case of scavenger reactions between the radicals generated during the UV/PDS process and the large number of anions in the natural water body such as Cl-, NO3-, SO4\({}^{2-}\), HCO3-, H2 PO4-, it is of great significance to investigate the effects of anions in water on the degradation of Et-PABA in the UV/PDS process [49]. To compare the effects of different anions on the degradation of Et-PABA, sodium chloride, sodium nitrate, sodium sulfate, sodium hydrogen carbonate, and sodium dihydrogen phosphate were added to the reaction system at a concentration of 5 mM [31], respectively. As shown in Figure 4A, Cl- showed no inhibition effect on the decomposition, which may be due to the fact that Cl- can react with sulfate radicals and produce relatively weak free radicals such as Cl\({}^{\bullet}\), Cl2- and ClHO\({}^{\bullet}\)[32]. HCO3- anions were thought to be scavengers of hydroxyl radicals that can react with sulfate radicals to produce bicarbonate radicals in the solution. Previous studies have shown that Cl- and HCO3- at small concentrations can promote the degradation of organic compounds by the UV/PDS process [50].