Investigation into the Synergistic Effect of the Zinc Peroxide/Peroxymonosulfate Double-Oxidation System for the Efficient Degradation of Tetracycline

The increasingly severe antibiotic pollution has become one of the most critical issues. In this study, a zinc peroxide/peroxymonosulfate (ZnO2/PMS) double-oxidation system was developed for tetracycline (TC) degradation. A small amount of ZnO2 (10 mg) and PMS (30 mg) could effectively degrade 82.8% of TC (100 mL, 50 mg/L), and the degradation process could be well described by the pseudo-second-order kinetic model. Meanwhile, the ZnO2/PMS double-oxidation system showed high adaptability in terms of reaction temperature (2–40 °C), initial pH value (4–12), common inorganic anions (Cl−, NO3−, SO42− and HCO3−), natural water source and organic pollutant type. The quenching experiment and electron paramagnetic resonance (EPR) characterization results confirmed that the main reactive oxygen species (ROS) was singlet oxygen (1O2). Moreover, three possible pathways of TC degradation were deduced according to the analyses of intermediates. On the basis of comparative characterization and experiment results, a synergistic activation mechanism was further proposed for the ZnO2/PMS double-oxidation system, accounting for the superior degradation performance. The released OH− and H2O2 from ZnO2 could activate PMS to produce major 1O2 and minor superoxide radicals (•O2−), respectively.


Introduction
Antibiotics have been widely applied in the fields of clinical treatment, animal husbandry and crop growth [1][2][3][4][5][6].However, the reckless use of antibiotics results in their frequent detection in water bodies [7][8][9] since the majority of antibiotics are poorly absorbed by organisms or degraded in nature [10].Residual antibiotics in the environment easily trigger serious environmental problems, like fostering antibiotic resistance [11], inducing physiological toxicity [12] and threatening human health [13].It has been reported that the concentration of antibiotics in some pig farm wastewater could even reach up to 25 mg/L [14], posing a great need for the efficient treatment of antibioticpolluted wastewater.

Synthesis of ZnO 2 Particles
Briefly, 1.5 mL of H 2 O 2 was added to 20 mL of Zn(NO 3 ) 2 •6H 2 O solution (0.5 mol/L).Then, 1.6 mL of NH 3 •H 2 O was slowly dropped into the above solution under vigorous agitation.In order to investigate the effect of agitation time on the product, a series of preparation experiments were conducted under different agitation times (10 min, 30 min, 60 min and 240 min).After the reaction, the generated white precipitates were separated by centrifugation and washed several times.Finally, the products were reserved after being dried in an oven at 60 • C for 12 h.

Characterizations
The phase structure was determined using an X-ray diffractometer (XRD, x'pert3 powder).The functional group was clarified via a Fourier transform infrared spectrometer (FTIR, Spectrum 3, PerkinElmer, Waltham, MA, USA).The thermal behavior was investigated using a thermogravimetry analyzer (TGA 5500, Milford, MA, USA).The microstructure was characterized by scanning electron microscopy (SEM, Nova NanoSEM 450, FEI, Eindhoven, Netherlands) and transmission electron microscopy (TEM, Tecnai G2 20, FEI, Eindhoven, The Netherlands).The chemical oxygen demand (COD) was tested using a multi-parameter water quality analyzer (LH-T640, Lianhua Technology, Beijing, China), and total organic carbon (TOC) concentration was measured via a TOC analyzer (TOC-L CPH, Shimadzu, Kyoto, Japan).The generated ROS were characterized using an electron paramagnetic resonance (EPR) spectrometer (EMXmicro-6/1, Bruker, Karlsruhe, Germany).The intermediates generated during the TC degradation process were determined using high-resolution liquid chromatography-mass spectrometry (HR-LC-MS, Thermo Fisher Orbitrap Q Exactive, Thermo Fisher Waltham, MA, USA).The specific analysis conditions and parameters are described in Text S1 and Table S1, which are provided in the Supplementary Materials.
where C o and C t (mg/L) were TC concentrations at initial and certain times (t), respectively.

Characterizations of ZnO 2
The XRD patterns of as-synthesized ZnO 2 particles under different agitation times are displayed in Figure 1a.Evidently, these four characteristic diffraction peaks centered at 31.9 • , 36.9 • , 53.4 • and 63.3 • could be well assigned to the (111), (200), ( 220) and (311) planes of ZnO 2 with the standard cubic phase (JCPDS 76−1364), respectively.Furthermore, the intensity of the diffraction peaks was enhanced with the agitation time, which indicated that extending the agitation time could be well conducive to the crystallinity.According to the Scherrer equation, the crystallite size of ZnO 2 under agitation for 240 min was calculated to be 10.72 nm (Table S2), which was consistent with an earlier reported study [36].As for the FTIR spectra of ZnO 2 (Figure 1b), the absorption peaks around 3450 and 1572 cm −1 were attributed to -OH vibration [37].In addition, the two adsorption peaks located at 1388 and 849 cm −1 were assigned to the characteristic O-O vibration [9].It could be clearly observed that the intensity of the adsorption peak at 1388 improved with the agitation time.This point confirmed that extending the agitation time contributed to the formation of the O-O bond, which is consistent with the XRD results.
Figure 1c displays the thermal behavior of ZnO 2 under agitation for 240 min.It could be observed that around 4.5% of the mass was lost from 30 to 116 • C, which is attributed to the desorption of adsorbed water.The main mass loss (15.81%) rapidly occurred between 116 and 300 • C, which was attributed to the thermal decomposition of ZnO 2 .Thus, the purity could be calculated to be around 96.4% in view of the theoretical value (16.4%).Meanwhile, the mass losses of ZnO 2 samples under agitation for 10 min, 30 min and 60 min at this stage were 20.25, 18.81 and 18.38%, respectively (Figure 1d).They were all higher than the theoretical value, which indicated the generation of amorphous impurities due to incomplete oxidation.Accordingly, the ZnO 2 sample prepared after agitation for 240 min was chosen for subsequent characterizations and degradation experiments.
was calculated to be 10.72 nm (Table S2), which was consistent with an earlier reported study [36].As for the FTIR spectra of ZnO2 (Figure 1b), the absorption peaks around 3450 and 1572 cm −1 were attributed to -OH vibration [37].In addition, the two adsorption peaks located at 1388 and 849 cm −1 were assigned to the characteristic O-O vibration [9].It could be clearly observed that the intensity of the adsorption peak at 1388 improved with the agitation time.This point confirmed that extending the agitation time contributed to the formation of the O-O bond, which is consistent with the XRD results.
Figure 1c displays the thermal behavior of ZnO2 under agitation for 240 min.It could be observed that around 4.5% of the mass was lost from 30 to 116 °C, which is attributed to the desorption of adsorbed water.The main mass loss (15.81%) rapidly occurred between 116 and 300 °C, which was attributed to the thermal decomposition of ZnO2.Thus, the purity could be calculated to be around 96.4% in view of the theoretical value (16.4%).Meanwhile, the mass losses of ZnO2 samples under agitation for 10 min, 30 min and 60 min at this stage were 20.25, 18.81 and 18.38%, respectively (Figure 1d).They were all higher than the theoretical value, which indicated the generation of amorphous impurities due to incomplete oxidation.Accordingly, the ZnO2 sample prepared after agitation for 240 min was chosen for subsequent characterizations and degradation experiments.The microscopic morphology of as-prepared ZnO 2 was characterized by SEM and TEM techniques.As shown in Figure 1e, the as-prepared ZnO 2 was mainly composed of multiple dispersive spherules with a particle size of around 100 nm.As seen from the TEM image (Figure 1f), the spherules were indeed assembled from much smaller primary particles, which was verified by the crystallite size calculation results.Undoubtedly, the highly dispersed particles would tend to expose active sites, thus contributing to the catalytic activity.In order to investigate the effect of PMS dosage on the performance of the ZnO 2 /PMS double-oxidation system, the ZnO 2 dosage was fixed at 10 mg.As shown in Figure 2a, only around 5% of TC was removed without PMS addition, which can be attributed to the physical adsorption of ZnO 2 .After PMS addition, the degradation rate of TC increased with the PMS dosage.After 60 min, the degradation rate of TC rose significantly from 49.3% to 80.2% when the dosage of PMS was increased from 10 mg to 30 mg.However, the degradation rate was slightly enhanced as the PMS dosage was further increased to 40 mg.In consideration of the reagent cost and degradation efficiency, 30 mg of PMS was selected for the ZnO 2 /PMS double-oxidation system in subsequent degradation experiments.
The influence of the ZnO 2 dosage is displayed in Figure 2b.It can be clearly observed that nearly 31% of TC was degraded within 90 min by PMS alone.After the addition of a small dosage of ZnO 2 (5 mg), the degradation rate improved remarkably to 55.7% after 90 min, indicating the significant role of ZnO 2 in the degradation system.It was noted that a slight enhancement of the degradation rate (only 3.2%) was realized after increasing the ZnO 2 dosage from 10 to 15 mg.Hence, 10 mg of ZnO 2 was deemed suitable for the ZnO 2 /PMS double-oxidation system in subsequent degradation experiments.Meanwhile, pseudo-zero-order, pseudo-first-order and pseudo-second-order kinetic models (Equations (S1)-(S3)) were employed to explore the kinetic characteristics of TC degradation (Text S2).The results (Figure S1) indicate that the pseudo-second-order kinetic model was more suitable to describe TC degradation on the basis of linear correlation coefficients, as listed in Table S3.This point illustrated that the degradation rate of TC was positively correlated with the square of dominant ROS concentrations generated in the ZnO 2 /PMS double-oxidation system.
ticles, which was verified by the crystallite size calculation results.Undoubtedl highly dispersed particles would tend to expose active sites, thus contributing to th alytic activity.

Effects of PMS and ZnO2 Dosages
In order to investigate the effect of PMS dosage on the performance of the ZnO2 double-oxidation system, the ZnO2 dosage was fixed at 10 mg.As shown in Figu only around 5% of TC was removed without PMS addition, which can be attributed physical adsorption of ZnO2.After PMS addition, the degradation rate of TC incr with the PMS dosage.After 60 min, the degradation rate of TC rose significantly 49.3% to 80.2% when the dosage of PMS was increased from 10 mg to 30 mg.How the degradation rate was slightly enhanced as the PMS dosage was further increased mg.In consideration of the reagent cost and degradation efficiency, 30 mg of PM selected for the ZnO2/PMS double-oxidation system in subsequent degradation e ments.
The influence of the ZnO2 dosage is displayed in Figure 2b.It can be clearly obs that nearly 31% of TC was degraded within 90 min by PMS alone.After the additio small dosage of ZnO2 (5 mg), the degradation rate improved remarkably to 55.7% af min, indicating the significant role of ZnO2 in the degradation system.It was noted slight enhancement of the degradation rate (only 3.2%) was realized after increasin ZnO2 dosage from 10 to 15 mg.Hence, 10 mg of ZnO2 was deemed suitable fo ZnO2/PMS double-oxidation system in subsequent degradation experiments.Meanw pseudo-zero-order, pseudo-first-order and pseudo-second-order kinetic models ( tions (S1)-( S3)) were employed to explore the kinetic characteristics of TC degrad (Text S2).The results (Figure S1) indicate that the pseudo-second-order kinetic mode more suitable to describe TC degradation on the basis of linear correlation coefficien listed in Table S3.This point illustrated that the degradation rate of TC was posi correlated with the square of dominant ROS concentrations generated in the ZnO2 double-oxidation system.

Effects of Temperature and Initial Solution pH
The performance of the ZnO 2 /PMS double-oxidation system for TC degradation was examined under different temperatures (2-40 • C).The results (Figure 3a) exhibit that over 75% of TC could be degraded even at a low temperature (2 • C), revealing the terrific degradation performance of the ZnO 2 /PMS double-oxidation system.The degradation rate was continuously enhanced with the rise in temperature, and the degradation rate reached up to 80% within 60 min at 20 • C. Considering the extent of improvement became quite weak as the temperature exceeded 20 The performance of the ZnO2/PMS double-oxidation system for TC degradation examined under different temperatures (2-40 °C).The results (Figure 3a) exhibit that o 75% of TC could be degraded even at a low temperature (2 °C), revealing the terrific d radation performance of the ZnO2/PMS double-oxidation system.The degradation was continuously enhanced with the rise in temperature, and the degradation reached up to 80% within 60 min at 20 °C.Considering the extent of improvement bec quite weak as the temperature exceeded 20 °C, the subsequent degradation experim were conducted under 20 °C.To explore the effect of initial pH, the degradation experiments were performed w different initial pH values (2)(3)(4)(5)(6)(7)(8)(9)(10)(11)(12).As seen from Figure 3b, under strong acid condit (pH = 2), only 7.8% of TC was degraded after 90 min.This was due to the fact that Z had been completely decomposed by the large numbers of hydrogen ions (H + ) in the lution.The generation of ROS via the self-decomposition of PMS was greatly inhib under strong acid conditions [38], resulting in the low degradation rate of TC.Nota the degradation rate was significantly improved with the increase in the initial solu pH, and around 77% of TC could be effectively degraded when the initial solution pH adjusted to 4. It could be found that the ZnO2/PMS double-oxidation system showed cellent performance for TC degradation in a wide initial pH range (4-12).To explore the effect of initial pH, the degradation experiments were performed with different initial pH values (2)(3)(4)(5)(6)(7)(8)(9)(10)(11)(12).As seen from Figure 3b, under strong acid conditions (pH = 2), only 7.8% of TC was degraded after 90 min.This was due to the fact that ZnO 2 had been completely decomposed by the large numbers of hydrogen ions (H + ) in the solution.The generation of ROS via the self-decomposition of PMS was greatly inhibited under strong acid conditions [38], resulting in the low degradation rate of TC.Notably, the degradation rate was significantly improved with the increase in the initial solution pH, and around 77% of TC could be effectively degraded when the initial solution pH was adjusted to 4. It could be found that the ZnO 2 /PMS double-oxidation system showed excellent performance for TC degradation in a wide initial pH range (4)(5)(6)(7)(8)(9)(10)(11)(12).
Moreover, the Arrhenius equation (Equation (S4)) was applied to calculate the reaction activation energy (Text S3).As illustrated in Figure S2, the positive activation energy (32.3 KJ/mol) indicated that the TC degradation process was endothermic and in agreement with the contribution of increasing temperature to the degradation performance of the ZnO 2 /PMS double-oxidation system.Figure 3c exhibits the change in COD during the TC degradation process.It can be observed that the COD value significantly dropped from an initial 82.16 to 39.03 mg/L after 90 min, and the COD removal efficiency was over 52%.The TOC removal results (Figure 3d) show that the TOC concentration declined from 25.29 to 10.60 mg/L.Over 58% of TOC was removed after 180 min, suggesting a high mineralization efficiency of the ZnO 2 /PMS double-oxidation system in TC degradation.

Adaptability of ZnO 2 /PMS Double-Oxidation System
Firstly, the adaptability of the ZnO 2 /PMS double-oxidation system was evaluated by four common matrix species (NaCl, NaNO 3 , Na 2 SO 4 and NaHCO 3 ) with different concentrations (10-100 mM).As shown in Figure 4a-c, NaCl, NaNO 3 and Na 2 SO 4 could rarely affect TC degradation.Surprisingly, the existence of NaHCO 3 remarkably enhanced TC degradation (Figure 4d), and this promotion effect could be attributed to the improvement of the solution pH value after NaHCO 3 's addition (Figure S3a), which was consistent with the effect of the initial solution pH on TC degradation (Figure 3b). the ZnO2/PMS double-oxidation system.Figure 3c exhibits the change in COD during the TC degradation process.It can be observed that the COD value significantly dropped from an initial 82.16 to 39.03 mg/L after 90 min, and the COD removal efficiency was over 52%.
The TOC removal results (Figure 3d) show that the TOC concentration declined from 25.29 to 10.60 mg/L.Over 58% of TOC was removed after 180 min, suggesting a high mineralization efficiency of the ZnO2/PMS double-oxidation system in TC degradation.

Adaptability of ZnO2/PMS Double-Oxidation System
Firstly, the adaptability of the ZnO2/PMS double-oxidation system was evaluated by four common matrix species (NaCl, NaNO3, Na2SO4 and NaHCO3) with different concentrations (10-100 mM).As shown in Figure 4a-c, NaCl, NaNO3 and Na2SO4 could rarely affect TC degradation.Surprisingly, the existence of NaHCO3 remarkably enhanced TC degradation (Figure 4d), and this promotion effect could be attributed to the improvement of the solution pH value after NaHCO3's addition (Figure S3a), which was consistent with the effect of the initial solution pH on TC degradation (Figure 3b).Then, several natural water resources (Yangtze River water, Han River water, East Lake water, Yujia Lake water and tap water) were studied to explore the adaptability of the ZnO2/PMS double-oxidation system.The specific water quality parameters of these several natural water resources (Yangtze River water, Han River water, East Lake water, Yujia Lake water and tap water) were studied to explore the adaptability of the ZnO 2 /PMS double-oxidation system.The specific water quality parameters of these natural water resources are listed in Table S4.Unexpectedly, the results (Figure 5a) show that TC degradation was enhanced in these natural water resources.This could be due to the higher initial pH values of the TC solution when prepared using natural water resources (Figure S3b).
Additionally, diverse kinds of organic pollutants (DOXH, CR and CTCH) were used to investigate the adaptability of the ZnO 2 /PMS double-oxidation system.As displayed in Figure 5b, the degradation rates of DOXH, CR and CTCH could reach 81.2%, 80.1% and 84.1% after 90 min, respectively.On the basis of the above results, it was confirmed that the ZnO 2 /PMS double-oxidation system possessed high adaptability to common matrix species, water resources and organic pollutant types.les 2024, 29, x FOR PEER REVIEW 8 Figure 5b, the degradation rates of DOXH, CR and CTCH could reach 81.2%, 80.1% 84.1% after 90 min, respectively.On the basis of the above results, it was confirmed the ZnO2/PMS double-oxidation system possessed high adaptability to common ma species, water resources and organic pollutant types.

Quenching Experiments
Quenching experiments were conducted to explore the generated ROS in ZnO2/PMS double-oxidation system.Specifically, MeOH, IPA and FFA were selecte quench the possible SO4• − , •OH and 1 O2 generated in the system, respectively.Due to fact that p-BQ can activate PMS to produce non-radical 1 O2 [39], TEMPOL was utilize quench generated •O2 − radicals as a substitute for p-BQ [40].As displayed in Figure 6 the addition of MeOH and IPA with different concentrations (10-100 mM) had little e on TC degradation, which indicated that the ZnO2/PMS double-oxidation system co hardly produce SO4• − and •OH radicals.
As seen from Figure 6c, there was a slight inhibition effect, and the degradation only decreased by 5% after 30 mM TEMPOL was added into the system, suggesting existence of a small amount of •O2 − radicals.As for FFA (Figure 6d), it could significa inhibit TC degradation, and the degradation rate dropped from 82.8% to 55.1% after addition of 50 mM FFA.As the concentration of FFA reached 500 mM, TC degrada was completely inhibited in the system.Accordingly, the above results indicated tha main ROS was 1 O2, accompanied by a small amount of •O2 − .
Further, EPR tests were carried out to identify the ROS generated in the ZnO2/P double-oxidation system.It was noted that SO4• − , •OH and •O2 − radicals could react w DMPO to form characteristic signals.As shown in Figure 7a, no obvious signals coul

Mechanism Analysis 3.3.1. Quenching Experiments
Quenching experiments were conducted to explore the generated ROS in the ZnO 2 /PMS double-oxidation system.Specifically, MeOH, IPA and FFA were selected to quench the possible SO 4 • − , •OH and 1 O 2 generated in the system, respectively.Due to the fact that p-BQ can activate PMS to produce non-radical 1 O 2 [39], TEMPOL was utilized to quench generated •O 2 − radicals as a substitute for p-BQ [40].As displayed in Figure 6a,b, the addition of MeOH and IPA with different concentrations (10-100 mM) had little effect on TC degradation, which indicated that the ZnO 2 /PMS double-oxidation system could hardly produce SO 4 • − and •OH radicals.
As seen from Figure 6c, there was a slight inhibition effect, and the degradation rate only decreased by 5% after 30 mM TEMPOL was added into the system, suggesting the existence of a small amount of •O 2 − radicals.As for FFA (Figure 6d), it could significantly inhibit TC degradation, and the degradation rate dropped from 82.8% to 55.1% after the addition of 50 mM FFA.As the concentration of FFA reached 500 mM, TC degradation was completely inhibited in the system.Accordingly, the above results indicated that the main ROS was 1   7a, no obvious signals could be detected in the ZnO 2 + PMS system.This point demonstrated that the 2 + PMS system could not produce SO 4 • − and •OH radicals, which is consistent with the quenching experiment results (Figure 6a,b).The characteristic signals related to the DMPO-•O 2 − adduct were not detected either, which might be attributed to the low number of •O 2 − radicals.The EPR results using TEMP as a capturing agent are exhibited in Figure 7b.Clearly, the characteristic triplet signal of the TEMP-1 O 2 adduct with an intensity ratio of 1:1:1 could be observed in the PMS system because PMS could generate 1 O 2 through a self-decomposition reaction [41].As for the ZnO 2 + PMS system, the signal intensity of TEMP-1 O 2 was much stronger than that in the sole PMS system.This point confirmed that ZnO 2 can activate PMS to produce more 1 O 2 , thereby contributing to the superior performance for TC degradation compared with other PMS-based systems (Table 1).

Intermediate Identification and TC Degradation Pathway Investigation
To deduce the possible pathways of TC degradation in the ZnO2/PMS double-o tion system, the HR-LC-MS technique was employed to determine the generated inte diates.As shown in the ESI-MS spectra (Figure S4), TC (P445) and nine main intermed (P493, P449, P461, P477, P431, P417, P267, P163 and P114) were found at different reten times and the detailed information on this is supplied in Table S5.Based on the dete intermediates, three possible degradation pathways are proposed in Figure 8.In path A, a TC molecule first lost a methyl group around C4 to generate P431 [48] and for P417 through further demethylation.In pathway B, the product of P461 was first ge ated through dipolar cycloaddition and the rearrangement of the hydroxyl group at C C12 in the TC molecule.P461 progressively transformed into P477 via hydroxylation As for pathway C, the TC molecule opened the ring at C7 to produce P493 [50].Then, was produced via the decarboxylation of P493 at C8. Subsequently, the smaller inte diates, including P163, P267 and P114, were produced by further oxidation [51].Fin these intermediates were degraded and mineralized into CO2, H2O and NH4 + .

Intermediate Identification and TC Degradation Pathway Investigation
To deduce the possible pathways of TC degradation in the ZnO 2 /PMS doubleoxidation system, the HR-LC-MS technique was employed to determine the generated intermediates.As shown in the ESI-MS spectra (Figure S4), TC (P445) and nine main intermediates (P493, P449, P461, P477, P431, P417, P267, P163 and P114) were found at different retention times and the detailed information on this is supplied in Table S5.Based on the detected intermediates, three possible degradation pathways are proposed in Figure 8.In pathway A, a TC molecule first lost a methyl group around C4 to generate P431 [48] and formed P417 through further demethylation.In pathway B, the product of P461 was first generated through dipolar cycloaddition and the rearrangement of the hydroxyl group at C11a-C12 in the TC molecule.P461 progressively transformed into P477 via hydroxylation [49].As for pathway C, the TC molecule opened the ring at C7 to produce P493 [50].Then, P447 was produced via the decarboxylation of P493 at C8. Subsequently, the smaller intermediates, including P163, P267 and P114, were produced by further oxidation [51].Finally, these intermediates were degraded and mineralized into CO 2 , H 2 O and NH 4 + .
Figure 8. Possible pathways TC degradation in ZnO2/PMS double-oxidation system.

Activation Pathway Study
It was well known that hydroxide ions (OH − ) could activate PMS to generate 1 O2 [52,53] and that Zn(OH)2 released from ZnO2 would spontaneously produce OH − in solution via the ionization effect.Thus, it is of great significance to monitor the pH value of the ZnO2/PMS double-oxidation system during TC degradation in order to deeply understand the generation mechanism of ROS in the ZnO2/PMS double-oxidation system.As illustrated in Figure 9a, the pH value of the ZnO2/PMS double-oxidation degradation system (ZnO2 + PMS) increased from the initial 5.6 to 6.4 and gradually stabilized at around 6.6.
Then, two comparative degradation experiments (NaOH + PMS and NaOH + H2O2 + PMS) were conducted under the same pH condition as the ZnO2 + PMS system, and the molar amount of H2O2 equaled the molar amount of ZnO2.The results showed that the degradation rate of TC in the NaOH + PMS system reached 67% within 90 min, proving that OH − could indeed activate PMS to produce 1 O2.As for the NaOH + H2O2 + PMS system, on one hand, TC degradation was apparently enhanced in comparison with the NaOH + PMS system, demonstrating that PMS might be activated by H2O2.It was reported

Activation Pathway Study
It was well known that hydroxide ions (OH − ) could activate PMS to generate 1 O 2 [52,53] and that Zn(OH) 2 released from ZnO 2 would spontaneously produce OH − in solution via the ionization effect.Thus, it is of great significance to monitor the pH value of the ZnO 2 /PMS double-oxidation system during TC degradation in order to deeply understand the generation mechanism of ROS in the ZnO 2 /PMS double-oxidation system.As illustrated in Figure 9a, the pH value of the ZnO 2 /PMS double-oxidation degradation system (ZnO 2 + PMS) increased from the initial 5.6 to 6.4 and gradually stabilized at around 6.6.
Then, two comparative degradation experiments (NaOH + PMS and NaOH + H 2 O 2 + PMS) were conducted under the same pH condition as the ZnO 2 + PMS system, and the molar amount of H 2 O 2 equaled the molar amount of ZnO 2 .The results showed that the degradation rate of TC in the NaOH + PMS system reached 67% within 90 min, proving that OH − could indeed activate PMS to produce 1 O 2 .As for the NaOH + H 2 O 2 + PMS system, on one hand, TC degradation was apparently enhanced in comparison with the NaOH + PMS system, demonstrating that PMS might be activated by H 2 O 2 .It was reported that H 2 O 2 could activate PMS to produce 2 − radicals [54], which was consistent with the TEMPOL quenching experiment results (Figure 6c).On the other hand, the degradation performance of the NaOH + H 2 O 2 + PMS system was still inferior to the ZnO 2 + PMS system, revealing the contribution of ZnO 2 release to the TC degradation in the ZnO 2 /PMS double-oxidation system.
Additionally, equimolar ZnO and Zn(OH) 2 were selected to replace ZnO 2 for PMS activation for TC degradation under the same conditions.The results (Figure 9b) showed that both the ZnO + PMS system and the Zn(OH) 2 + PMS system had an inferior performance in TC degradation.This point confirmed that the released H 2 O 2 from ZnO 2 was indeed able to activate PMS, thereby promoting the degradation performance of the ZnO 2 /PMS double-oxidation degradation system.that H2O2 could activate PMS to produce •O2 − radicals [54], which was consistent with the TEMPOL quenching experiment results (Figure 6c).On the other hand, the degradation performance of the NaOH + H2O2 + PMS system was still inferior to the ZnO2 + PMS system, revealing the contribution of ZnO2 release to the TC degradation in the ZnO2/PMS double-oxidation system.Additionally, equimolar ZnO and Zn(OH)2 were selected to replace ZnO2 for PMS activation for TC degradation under the same conditions.The results (Figure 9b) showed that both the ZnO + PMS system and the Zn(OH)2 + PMS system had an inferior performance in TC degradation.This point confirmed that the released H2O2 from ZnO2 was indeed able to activate PMS, thereby promoting the degradation performance of the ZnO2/PMS double-oxidation degradation system.Based on the above results and analyses, the generation mechanism of ROS in the ZnO2/PMS double-oxidation system could be deduced by the following equations (Equations (2)-( 8)).It was well known that PMS could directly produce 1 O2 through the selfdecomposition reaction (Equation ( 2)), according to previous research [55].In addition, as illustrated in Equation (3), ZnO2 would transform into Zn(OH)2 and H2O2 via hydrolysis in solution [36].On the one hand, the generated Zn(OH)2 could spontaneously release OH − ions (Equation ( 4)).It was reported that OH − could activate PMS to generate 1 O2 via Equation ( 5) [27], thus contributing to TC degradation.On the other hand, the ionization reaction of the produced H2O2 (Equation ( 6)) was greatly promoted under alkaline conditions, which was revealed in the base-activated PMS system [56].Further, the ionization product HO2 − would react with PMS to form •O2 − radicals [33] through Equation (7).Therefore, TC was effectively degraded by the 1 O2 and •O2 − produced in the ZnO2/PMS double-oxidation system (Equation ( 8)).Based on the above results and analyses, the generation mechanism of ROS in the ZnO 2 /PMS double-oxidation system could be deduced by the following equations (Equations ( 2)-( 8)).It was well known that PMS could directly produce 1 O 2 through the self-decomposition reaction (Equation ( 2)), according to previous research [55].In addition, as illustrated in Equation (3), ZnO 2 would transform into Zn(OH) 2 and H 2 O 2 via hydrolysis in solution [36].On the one hand, the generated Zn(OH) 2 could spontaneously release OH − ions (Equation ( 4)).It was reported that OH − could activate PMS to generate 1 O 2 via Equation (5) [27], thus contributing to TC degradation.On the other hand, the ionization reaction of the produced H 2 O 2 (Equation ( 6)) was greatly promoted under alkaline conditions, which was revealed in the base-activated PMS system [56].Further, the ionization product HO 2 − would react with PMS to form •O 2 − radicals [33] through Equation (7).Therefore, TC was effectively degraded by the 1 O 2 and •O 2 − produced in the ZnO 2 /PMS double-oxidation system (Equation ( 8

Figure 1 .
Figure 1.XRD spectra (a) and FTIR patterns (b) of the products under different conditions; TG plots of ZnO2 samples under agitation for 240 min (c) and other agitation times (d); SEM (e) and TEM (f) pictures of as-synthesized ZnO2 under agitation for 240 min.

Figure 1 .
Figure 1.XRD spectra (a) and FTIR patterns (b) of the products under different conditions; TG plots of ZnO 2 samples under agitation for 240 min (c) and other agitation times (d); SEM (e) and TEM (f) pictures of as-synthesized ZnO 2 under agitation for 240 min.

Figure 2 .
Figure 2. Effects of PMS dosage (a) and ZnO2 dosage (b) on TC degradation.

Figure 2 .
Figure 2. Effects of PMS dosage (a) and ZnO 2 dosage (b) on TC degradation.

Figure 3 .
Figure 3. Effects of temperature (a) and initial solution pH (b) on TC degradation; COD (c) and (d) removal in the ZnO2/PMS double-oxidation system.

Figure 3 .
Figure 3. Effects of temperature (a) and initial solution pH (b) on TC degradation; COD (c) and TOC (d) removal in the ZnO 2 /PMS double-oxidation system.

Figure 5 .
Figure 5. Adaptability experiments of ZnO2 + PMS system to natural water resources (a) and org pollutant types (b).

Figure 5 .
Figure 5. Adaptability experiments of ZnO 2 + PMS system to natural water resources (a) and organic pollutant types (b).
O 2 , accompanied by a small amount of •O 2 − .Further, EPR tests were carried out to identify the ROS generated in the ZnO 2 /PMS double-oxidation system.It was noted that SO 4 • − , •OH and •O 2 − radicals could react with DMPO to form characteristic signals.As shown in Figure

Figure 9 .
Figure 9.Comparison of degradation performance of NaOH + PMS, NaOH + H2O2 + PMS and ZnO2 + PMS systems under the same pH conditions (a), and using equimolar amount of ZnO, Zn(OH)2 and ZnO2 to activate PMS for TC degradation (b).

Figure 9 .
Figure 9.Comparison of degradation performance of NaOH + PMS, NaOH + H 2 O 2 + PMS and ZnO 2 + PMS systems under the same pH conditions (a), and using equimolar amount of ZnO, Zn(OH) 2 and ZnO 2 to activate PMS for TC degradation (b).
• C, the subsequent degradation experiments were conducted under 20 • C.

Table 1 .
Comparison of performances over the reported PMS-based systems.
MS-Based Systems PMS Dosage (g/L)Catalyst Dosage (g/L)Concentration of TC (mg/L)

Table 1 .
Comparison of performances over the PMS-based systems.