## 3 Results and Discussion
### Control Experiments  
Figure 4 shows the removal of OTC under different experimental conditions. OTC was removed when both PS and HWWC were present. The degradation efficiency of OTC by PS activation was greater than 99% in 24 min, and the estimated pseudo first-order rate constant (k) was 0.21 \(\pm\) 0.03 min\({}^{-1}\). This removal rate was comparable to OTC degradation through the Fenton process using H\({}_{2}\)O\({}_{2}\)/Fe\({}^{2+}\) (\(\mathrm{k_{app}}\) = 0.068-0.213 min\({}^{-1}\)) [33], which indicates that the PS activation process using HWWC can be a promising technique for removing antibiotics from water. The removal rate of OTC in the presence of PS and HWWC in 24 min was low (<6.0%). The degradation mechanism of OTC by PS activation can be expressed by the following equations (Equations (1)-(5)) [34]:  
\[\mathrm{Pollutant+Fe(III)\rightarrowPollutant+Fe(II)} \tag{1}\]  
\[\mathrm{Fe(II)+S_{2}O_{8}}^{2-}\rightarrow\mathrm{Fe(III)+SO_{4}}^{\bullet-}+ \mathrm{SO_{4}}^{2-} \tag{2}\]  
\[\mathrm{Fe(III)+H_{2}O\rightarrowFe(II)+HO^{\bullet}+H^{+}} \tag{3}\]  
\[\mathrm{H_{2}O+SO_{4}}^{\bullet-}\rightarrow\mathrm{H^{+}+HO^{\bullet}+SO_{4} }^{2-} \tag{4}\]  
Figure 3: Vibrating sample magnetometer (VSM) analysis result of HWWC.  
Figure 2: (**a**) Scanning electron microscope image and (**b**) energy dispersive X-ray spectrometer result of the prepared hand-warmer waste catalyst.  
The electrons could be transferred to Fe(III) when the pollutant was adsorbed onto the Fe\({}_{2}\)O\({}_{3}\) surface (Equation (1)). Therefore, a Fenton-like reaction occurred between S\({}_{2}\)O\({}_{8}\)\({}^{2-}\) and Fe(II) at the surface of Fe\({}_{2}\)O\({}_{3}\), therefore generating SO\({}_{4}\)\({}^{\bullet-}\) and reforming Fe(III) (Equation (2)). HO\({}^{\bullet}\) also might have been formed by this reaction and contributed to pollutant degradation (Equations (3)-(5)) [35; 36; 37]. Therefore, the pollutant could be degraded by the generated surface-adsorbed radicals (SO\({}_{4}\)\({}^{-}\) and HO\({}^{\bullet}\)) that might diffuse into the aqueous solution (Equation (5)) [34].  
Figure 5 shows the effect of two radical scavengers, ethanol (EtOH) and _t_-butanol (TBA), on the degradation of OTC. According to equations (Equations (6)-(9)) [38], the reaction rate of EtOH and HO\({}^{\bullet}\) is 50 times faster than that of EtOH and SO\({}_{4}\)\({}^{\bullet-}\), while TBA reacts with HO\({}^{\bullet}\) almost 1000 times faster than that with SO\({}_{4}\)\({}^{\bullet-}\). From the results, the oxidation of OTC was significantly reduced with the addition of TBA and EtOH, suggesting that EtOH can effectively inhibit the oxidation efficiency. In addition, compared with EtOH, the removal rate of OTC was found to be lower in the presence of TBA, which demonstrates that a small amount of SO\({}_{4}\)\({}^{\bullet-}\) remaining in the solution can still decompose OTC. Thus, it can be concluded that SO\({}_{4}\)\({}^{\bullet-}\) is the dominant oxidizing species in the OTC degradation process by persulfate activation using HWWC catalysts.  
\[\text{EtOH}+\text{HO}^{\bullet}\rightarrow\text{Intermediates (k = (1.2-2.8) \times 10^{9} M^{-1}s^{-1})} \tag{6}\]  
\[\text{EtOH}+\text{SO}_{4}\)\({}^{\bullet-}\rightarrow\text{Intermediates (k = (1.6-7.7) \times 10^{7} M^{-1}s^{-1})} \tag{7}\]  
\[\text{TBA}+\text{HO}^{\bullet}\rightarrow\text{Intermediates (k = (3.8-7.6) \times 10^{8} M^{-1}s^{-1})} \tag{8}\]  
\[\text{TBA}+\text{SO}_{4}\)\({}^{\bullet-}\rightarrow\text{Intermediates (k = (4.0-9.1) \times 10^{5} M^{-1}s^{-1})} \tag{9}\]