Preparation of Graphite Phase g-C3N4 Supported Metal Oxide Activator and Its Performance in Activating Peroxodisulfate Degradation of Methyl Orange

In order to improve the catalytic activity and recycling performance of semiconductor activators, and improve the activation pathway of persulfate, graphitic carbon nitride (g-C3N4) was prepared by calcining melamine, and a composite activator Ag2O/g-C3N4 based on g-C3N4 supported metal oxide was prepared using a precipitation method. The morphology, structure, and basic properties of the composites were characterized using SEM, XRD, FT-IR and XPS. The activation efficiency of the Ag2O/g-C3N4 composite activator on peroxodisulfate (PDS) was explored. The results showed that Ag2O in the composite activator was highly dispersed on the surface of g-C3N4 and did not change the molecular structure of g-C3N4 significantly. Under different activation systems, the degradation process of MO was best fitted under the pseudo-second-order reaction kinetic model, compared to the separate g-C3N4 or Ag2O activated PDS systems; the activation of the PDS system with Ag2O/g-C3N4 had the best effect on MO degradation; and the composite activator Ag2O/g-C3N4 showed better activation performance. Under the conditions that the mass combined ratio of Ag2O in the activator was 12%, the initial concentration of PDS was 4 mmol/L, the initial concentration of the activator was 1.25 g/L, and the initial pH was 3, the degradation degree of MO reached 99.4% after 40 min reaction. The free radical quenching experiment proved that the active substances that could degrade MO in the system were SO4−· and ·OH, and the effect of SO4−· was greater than that of ·OH. The degradation degree of MO in the reaction system remained above 80% after four cycles of use, and the crystal structure of Ag2O/g-C3N4 did not change significantly before and after the reaction. The above results show that Ag2O/g-C3N4 is an efficient and stable composite activator with good application potential in the treatment of dye wastewater by activating PDS.


Introduction
Water is one of the natural resources that people depend on for survival. In recent years, due to the rapid development of industry, many refractory organic pollutants have emerged, among which dye is one of the refractory organic pollutants. Dye wastewater has the characteristics of high chroma, poor biodegradability, and substantial toxicity [1]. Untreated direct emissions could seriously damage the ecological environment and threaten human health [2]. Methyl orange (MO) is a typical azo dye. At present, there are still many deficiencies in the research on the degradation of MO, for example, photocatalysis requires additional running devices, which increases processing costs [3]. The catalytic wet oxidation method requires high reaction conditions [4], while the biological process requires a long reaction time and the treatment effect is unstable [5]. However, the advanced oxidation of persulfate has attracted wide attention due to its advantages of strong oxidation capacity, high removal efficiency, and low cost.
2.2. Preparation of Activator Ag 2 O/g-C 3 N 4 2.2.1. Preparation of g-C 3 N 4 A total of 10.0 g of melamine was accurately weighed and evenly placed in the corundum ark of the quartz tube of the tube furnace, the mixture gas (79% N 2 + 21% O 2 ) was slowly passed into the ark, and the temperature was increased to 550 • C at a rate of 5 • C/min and held for 4 h. The resulting yellow solid was thoroughly pulverized in an agate mortar to obtain g-C 3 N 4 solid powders.

Preparation of Ag 2 O
Without adding g-C 3 N 4 , a certain amount of NaOH solids was weighed in 50 mL of deionized water for magnetic stirring. After 30 min, a certain amount of AgNO 3 solids was added in the dark, and the brown precipitate was obtained by continuous stirring for 30 min. Then, the reaction solution was centrifuged, washed, and dried. The dried black solids were collected to obtain Ag 2 O particles and stored away from light.

Preparation of Ag 2 O/g-C 3 N 4
A certain amount of g-C 3 N 4 and NaOH solids was weighed in 50 mL of deionized water, stirred for 30 min at room temperature, and then a certain amount of AgNO 3 solids was added away from light. The mixture was stirred continuously for 30 min, and the brown precipitate was obtained by centrifugation, separated, and washed with deionized water. The solid product was dried at 60 • C and collected, which was the compound activator. It was recorded as x% Ag 2 O/g-C 3 N 4 (x represents the mass ratio of Ag 2 O in the solid activator, %), and stored away from light.

Experiment Methods
A MO solution was used as a mock dye effluent with an initial concentration of 20 mg/L and a solution volume of 50 mL. The pH required for the reaction was adjusted with 0.1 mol/L of dilute sulfuric acid solution and 0.1 mol/L of sodium hydroxide solution. Unless otherwise specified, the initial concentration of PDS was 4 mmol/L, the activator concentration was 1.0 g/L, and the reaction time was 60 min.
A certain amount of the activator was weighed and added into the beaker containing the MO solution, and the adsorption-desorption equilibrium was reached by magnetic stirring for 30 min. Then, a certain amount of PDS was weighed and added into the reaction system for the activation degradation experiment of MO. The sample was taken when the reaction was carried out until a specific time and filtered by a 0.45 µm aqueous filter head, and the absorbance of the sample was measured at 465 nm wavelength. In order to exclude the influence of external light on the experimental results, the experiment was conducted in a dark environment, and the ambient temperature was room temperature. The experiments were conducted three times in parallel.

Analysis Methods
The morphology of Ag 2 O/g-C 3 N 4 was analyzed using a MIRA scanning electron microscope (SEM) from TESCAN, Czech Republic. The working voltage was 1 kV and the magnification was 1000~30,000 times. The crystal structure was analyzed using an X-ray diffractometer (XRD) of the Smart Lab type from Rigaku, Japan, with a scan range from 5 • to 80 • , a scan speed of 10 • /min, a Cu target Kβ line, a tube voltage of 40 kV, and a tube current of 100 mA. The molecular structure was analyzed using an IRAffinity-1s Fourier infrared spectrometer (FT-IR) from Shimadzu, Japan, and the spectral measurement range was 400~4000 cm −1 . The element valence state was analyzed using an X-ray photoelectron spectrometer (XPS) from Thermo Flyer, Waltham, MA, USA, and the binding energy range was 0~1400 eV. The concentration of MO was determined using a DR3900 ultraviolet spectrophotometer, produced by Hach with a wavelength scanning range of 320~700 nm.

SEM and XRD Analysis
The microscopic morphology of the g-C 3 N 4 , Ag 2 O, and Ag 2 O/g-C 3 N 4 activators were compared using SEM, and the SEM results are shown in Figure 1. As seen in Figure 1a, g-C 3 N 4 is a blocky structure composed of irregular lamellar aggregates, with a size of 4~8 µm. As seen in Figure 1b, Ag 2 O is formed by the tight aggregation of circular lamellae, with a diameter of 0.5~1 µm. In Figure 1c, Ag 2 O/g-C 3 N 4 has the lamellar structure of g-C 3 N 4 , and the surface is scattered with fine Ag 2 O particles. These particles do not show the circular lamellar morphology as shown in Figure 1b, due to the low Ag 2 O content in the sample and the high dispersion on the surface of g-C 3 N 4 .

Analysis Methods
The morphology of Ag2O/g-C3N4 was analyzed using a MIRA scanning electron croscope (SEM) from TESCAN, Czech Republic. The working voltage was 1 kV an magnification was 1000~30,000 times. The crystal structure was analyzed using an X diffractometer (XRD) of the Smart Lab type from Rigaku, Japan, with a scan range 5° to 80°, a scan speed of 10°/min, a Cu target Kβ line, a tube voltage of 40 kV, and a current of 100 mA. The molecular structure was analyzed using an IRAffinity-1s Fo infrared spectrometer (FT-IR) from Shimadzu, Japan, and the spectral measurement r was 400 ~4000 cm −1 . The element valence state was analyzed using an X-ray photoele spectrometer (XPS) from Thermo Flyer, USA, and the binding energy range was 0~ eV. The concentration of MO was determined using a DR3900 ultraviolet spectropho eter, produced by Hach with a wavelength scanning range of 320~700 nm.

SEM and XRD Analysis
The microscopic morphology of the g-C3N4, Ag2O, and Ag2O/g-C3N4 activators compared using SEM, and the SEM results are shown in Figure 1. As seen in Figure 1 C3N4 is a blocky structure composed of irregular lamellar aggregates, with a size of μm. As seen in Figure 1b, Ag2O is formed by the tight aggregation of circular lam with a diameter of 0.5~1 μm. In Figure 1c, Ag2O/g-C3N4 has the lamellar structure C3N4, and the surface is scattered with fine Ag2O particles. These particles do not s the circular lamellar morphology as shown in Figure 1b, due to the low Ag2O conte the sample and the high dispersion on the surface of g-C3N4.     [20], respectively. In Figure 2b, the characteristic peaks at 13.38 • and 27.55 • correspond to the (100) crystal plane and (002) crystal plane of g-C 3 N 4 (JCPDS87-1526) [21], respectively.
As seen in Figure 2c, two characteristic peaks of g-C 3 N 4 appear at 13.38 • and 27.55 • for Ag 2 O/g-C 3 N 4 , and at 32.76 • for Ag 2 O, indicating that the Ag 2 O/g-C 3 N 4 complex had been successfully synthesized by the precipitation method. The characteristic peak of Ag 2 O is weaker than that of g-C 3 N 4 , and three weak characteristic peaks of Ag 2 O at 38.04 • , 54.99 • and 65.58 • are not observed. The reason is that the content of Ag 2 O in the complex is low, so these peaks are not detected. Figure 2 shows the XRD patterns of the Ag2O, g-C3N4, and Ag2O/g-C3N4 activators. It can be seen from Figure 2a that there is a strong characteristic peak at 32.76°, corresponding to the (111) crystal plane of Ag2O. The characteristic peaks at 38.04°, 54.99°, and 65.58° correspond to the (200), (220) and (311) crystal planes of Ag2O (JCPDS41-1004) [20], respectively. In Figure 2b, the characteristic peaks at 13.38° and 27.55° correspond to the (100) crystal plane and (002) crystal plane of g-C3N4 (JCPDS87-1526) [21], respectively. As seen in Figure 2c, two characteristic peaks of g-C3N4 appear at 13.38° and 27.55° for Ag2O/g-C3N4, and at 32.76° for Ag2O, indicating that the Ag2O/g-C3N4 complex had been successfully synthesized by the precipitation method. The characteristic peak of Ag2O is weaker than that of g-C3N4, and three weak characteristic peaks of Ag2O at 38.04°, 54.99° and 65.58° are not observed. The reason is that the content of Ag2O in the complex is low, so these peaks are not detected.  Figure 3 shows the FT-IR characterization results of g-C3N4 and Ag2O/g-C3N4. In Figure 3a, the peak at 800 cm −1 is attributed to the stretching vibration of the g-C3N4 triazine ring unit, the peak at 1243 cm −1~1 641 cm −1 is attributed to the stretching vibration of C-N and C=N structure, and the wide peak at 3078 cm −1 is attributed to the stretching vibration of N-H bond, indicating the presence of incomplete melamine in the sample. Figure 3b shows the FT-IR spectrum of Ag2O/g-C3N4. The peak type and peak position of the spectrum are basically consistent with those of Figure 3a, because the content of Ag2O in the sample is relatively low, and they also indicate that the loading of Ag2O by precipitation does not affect the molecular structure of g-C3N4.  Figure 3 shows the FT-IR characterization results of g-C 3 N 4 and Ag 2 O/g-C 3 N 4 . In Figure 3a, the peak at 800 cm −1 is attributed to the stretching vibration of the g-C 3 N 4 triazine ring unit, the peak at 1243 cm −1~1 641 cm −1 is attributed to the stretching vibration of C-N and C=N structure, and the wide peak at 3078 cm −1 is attributed to the stretching vibration of N-H bond, indicating the presence of incomplete melamine in the sample. Figure 3b shows the FT-IR spectrum of Ag 2 O/g-C 3 N 4 . The peak type and peak position of the spectrum are basically consistent with those of Figure 3a, because the content of Ag 2 O in the sample is relatively low, and they also indicate that the loading of Ag 2 O by precipitation does not affect the molecular structure of g-C 3 N 4 .

XPS Analysis
The valence states of each element in the Ag2O/g-C3N4 sample were characterized using XPS, and the results are shown in Figure 4. As seen in Figure 4a, the total spectrum includes the binding energy peaks of four elements, C 1s, N 1s, O 1s, and Ag 3d, indicating that the sample contains four elements, C, N, O, and Ag. In Figure 4b, two binding energy peaks at 284.48 eV and 287.88 eV correspond to C-C in Ag2O/g-C3N4 and sp2 carbon atoms in the structure of nitrogen-containing aromatic ring (n-C = N), respectively. Figure 4c is obtained by fitting and it can be seen that the two binding energy peaks at 398.48 eV and

XPS Analysis
The valence states of each element in the Ag 2 O/g-C 3 N 4 sample were characterized using XPS, and the results are shown in Figure 4. As seen in Figure 4a, the total spectrum includes the binding energy peaks of four elements, C 1s, N 1s, O 1s, and Ag 3d, indicating that the sample contains four elements, C, N, O, and Ag. In Figure 4b, two binding energy peaks at 284.48 eV and 287.88 eV correspond to C-C in Ag 2 O/g-C 3 N 4 and sp2 carbon atoms in the structure of nitrogen-containing aromatic ring (n-C = N), respectively. Figure 4c is obtained by fitting and it can be seen that the two binding energy peaks at 398.48 eV and 399.98 eV correspond to the N atom in the triazine structural unit (C = N-C) and the N-(C)3 structure in g-C 3 N 4 , respectively. In Figure 4d, the binding energy peak at 531.48 eV corresponds to lattice oxygen in Ag 2 O. In Figure 4e, the two binding energy peaks at 367.78 eV and 373.78 eV correspond to Ag 5/2 and Ag 3/2 [22] in Ag 2 O, respectively. The results of XPS indicate the successful recombination of Ag 2 O and g-C 3 N 4 in the sample

XPS Analysis
The valence states of each element in the Ag2O/g-C3N4 sample were charact using XPS, and the results are shown in Figure 4. As seen in Figure 4a, the total spe includes the binding energy peaks of four elements, C 1s, N 1s, O 1s, and Ag 3d, indi that the sample contains four elements, C, N, O, and Ag. In Figure 4b, two binding e peaks at 284.48 eV and 287.88 eV correspond to C-C in Ag2O/g-C3N4 and sp2 carbon in the structure of nitrogen-containing aromatic ring (n-C = N), respectively. Figur obtained by fitting and it can be seen that the two binding energy peaks at 398.48 e 399.98 eV correspond to the N atom in the triazine structural unit (C = N-C) and t (C)3 structure in g-C3N4, respectively. In Figure 4d, the binding energy peak at 531 corresponds to lattice oxygen in Ag2O. In Figure 4e, the two binding energy pe 367.78 eV and 373.78 eV correspond to Ag5/2 and Ag3/2 [22] in Ag2O, respectively. T sults of XPS indicate the successful recombination of Ag2O and g-C3N4 in the s Ag2O/g-C3N4.   Figure 5 shows the degradation of MO under different activation systems. Under the conditions of g-C3N4, Ag2O, or Ag2O/g-C3N4 alone, the concentration of MO does not change after the adsorption-desorption equilibrium, indicating that these three materials have no noticeable degradation effect on MO. Under the condition of PDS alone, the degradation degree of MO for 60 min is 60.7%, indicating that the presence of PDS produces some free radicals in the system and has a certain degradation effect on MO. Under the co-existence condition of PDS and g-C3N4, the degradation degree of MO for 60 min is 65.3%; the degradation degree is only 4.6% higher than that of PDS alone, and there is  Figure 5 shows the degradation of MO under different activation systems. Under the conditions of g-C 3 N 4 , Ag 2 O, or Ag 2 O/g-C 3 N 4 alone, the concentration of MO does not change after the adsorption-desorption equilibrium, indicating that these three materials have no noticeable degradation effect on MO. Under the condition of PDS alone, the degradation degree of MO for 60 min is 60.7%, indicating that the presence of PDS produces some free radicals in the system and has a certain degradation effect on MO. Under the co-existence condition of PDS and g-C 3 N 4 , the degradation degree of MO for 60 min is 65.3%; the degradation degree is only 4.6% higher than that of PDS alone, and there is almost no change in the reaction rate, indicating that g-C 3 N 4 has little activation effect on PDS. Under the co-existence condition of PDS and Ag 2 O, the degradation degree of MO for 60 min is 86.2%, which is 20.9% higher than that of the g-C 3 N 4 system co-existing with PDS, indicating that Ag 2 O can activate PDS to produce SO 4 − · and improve the degradation degree of MO. However, under the co-existence condition of PDS and 8% Ag 2 O/g-C 3 N 4 , the degradation degree of MO reaches 95.7% after 60 min reaction, which is 9.5% higher than that of the co-existence of the Ag 2 O system with PDS; moreover, it is higher than the degradation degree of MO in the system of the Ag/AgCl/p-g-C 3 N 4 nanocomposite after 60 min reaction under visible light irradiation [23]. Meanwhile, it greatly saves the reaction time compared with the degradation degree of MO reaches 99.3% after 4 h of reaction [24].

Different Activation Systems
The results indicate that g-C 3 N 4 loaded with Ag 2 O effectively increases the specific surface area of g-C 3 N 4 and provides more active sites on the surface of the carrier. At the same time, the dispersion of Ag 2 O on the surface of the carrier is significantly improved and hinders the production of elemental Ag. The activator with 8% Ag 2 O/g-C 3 N 4 can promote the production of more SO 4 − · by PDS and significantly improve the degradation degree of MO.  Figure 6 shows the fitting results of the degradation rate constants of MO under different activation systems. When comparing the fitting results of the different systems under the zero-order kinetic reaction model (Figure 6a), the pseudo-first-order kinetic reaction model (Figure 6b), and the pseudo-second-order kinetic reaction model (Figure 6c), it shows that the fitting results of the degradation process of MO in different systems are better under the pseudo-first-order and pseudo-second-order kinetic reaction models. Figure 6d shows that the correlation coefficients of the different systems under the pseudo-second-order kinetic reaction model are better than those under the pseudo-firstorder kinetic reaction model. Table 1 shows the kinetic parameters of the pseudo-firstorder and pseudo-second-order reactions of MO under different systems. Table 1b shows that the reaction rate constants of PDS and PDS co-existing systems with g-C3N4 are 0.0013 L/(mg•min) and 0.0016 L/(mg•min), respectively, indicating that the addition of g-C3N4 had little effect on the degradation rate of MO. The reaction rate constant of the PDS and Ag2O co-existing system is 0.00503 L/(mg•min), which is 2.87 times higher than that of the PDS alone system, indicating that Ag2O can activate PDS to promote the degradation of MO. The reaction rate constant increases to 0.01773 L/(mg•min) when PDS and 8% Ag2O/g-C3N4 co-exist, which is 2.52 times higher than that in the co-existence system of PDS and Ag2O. It is 0.63 times higher than that of the system of PDS coexisting with 8% Ag2O/g-C3N4 relative to the system of PDS coexisting with Ag2O under the pseudo-firstorder kinetic reaction model. Thus, 8% Ag2O/g-C3N4 can better activate PDS and accelerate the degradation of MO. The results show that, under the Ag2O/g-C3N4-activated PDS  Figure 6 shows the fitting results of the degradation rate constants of MO under different activation systems. When comparing the fitting results of the different systems under the zero-order kinetic reaction model (Figure 6a), the pseudo-first-order kinetic reaction model (Figure 6b), and the pseudo-second-order kinetic reaction model (Figure 6c), it shows that the fitting results of the degradation process of MO in different systems are better under the pseudo-first-order and pseudo-second-order kinetic reaction models. Figure 6d shows that the correlation coefficients of the different systems under the pseudo-secondorder kinetic reaction model are better than those under the pseudo-first-order kinetic reaction model. Table 1 shows the kinetic parameters of the pseudo-first-order and pseudosecond-order reactions of MO under different systems. Table 1b shows that the reaction rate constants of PDS and PDS co-existing systems with g-C 3 N 4 are 0.0013 L/(mg·min) and 0.0016 L/(mg·min), respectively, indicating that the addition of g-C 3 N 4 had little effect on the degradation rate of MO. The reaction rate constant of the PDS and Ag 2 O co-existing system is 0.00503 L/(mg·min), which is 2.87 times higher than that of the PDS alone system, indicating that Ag 2 O can activate PDS to promote the degradation of MO. The reaction rate constant increases to 0.01773 L/(mg·min) when PDS and 8% Ag 2 O/g-C 3 N 4 co-exist, which is 2.52 times higher than that in the co-existence system of PDS and Ag 2 O. It is 0.63 times higher than that of the system of PDS coexisting with 8% Ag 2 O/g-C 3 N 4 relative to the system of PDS coexisting with Ag 2 O under the pseudo-first-order kinetic reaction model. Thus, 8% Ag 2 O/g-C 3 N 4 can better activate PDS and accelerate the degradation of MO. The results show that, under the Ag 2 O/g-C 3 N 4 -activated PDS system, the reaction rate constant is the largest and the degradation rate of MO is the fastest, which are consistent with the previous experimental results.   Figure 7 shows the degradation of MO by Ag2O/g-C3N4 under different Ag2O mass composite ratios. The results indicate that the degradation degree increases with an increase in the Ag2O mass recombination ratio in the activator. The degradation degree of MO by the activator are 95.7%, 99.1%, and 99.4% within 60 min at 8%, 10%, and 12%Ag2O composite ratio, respectively. The degradation degree of MO is the highest when the mass recombination ratio of Ag2O is 12%. Thus, 12% Ag2O/g-C3N4 was selected for the subsequent experiment.   Figure 7 shows the degradation of MO by Ag 2 O/g-C 3 N 4 under different Ag 2 O mass composite ratios. The results indicate that the degradation degree increases with an increase in the Ag 2 O mass recombination ratio in the activator. The degradation degree of MO by the activator are 95.7%, 99.1%, and 99.4% within 60 min at 8%, 10%, and 12%Ag 2 O composite ratio, respectively. The degradation degree of MO is the highest when the mass recombination ratio of Ag 2 O is 12%. Thus, 12% Ag 2 O/g-C 3 N 4 was selected for the subsequent experiment.  Figure 8 shows the degradation of MO by Ag2O/g-C3N4 at different initial concentrations of PDS. The results indicate that the degradation degree increases from 90.1% to 99.4% when the initial concentration of PDS increases from 1 mmol/L to 4 mmol/L but decreases from 99.4% to 96.7% when the initial concentration of PDS increases from 4 to 5 mmol/L. The reason why the degradation degree increases first and then decreases with the increase of the initial concentration of PDS is that the increase of the concentration of PDS can produce more active substances in the system and promote the faster degradation of MO. However, when the concentration of PDS is too high, the excessive SO4 − · produced in the system leads to the quenching reaction between free radicals. As a result, the concentration of SO4 − · decreases (Equations (1) and (2)) [25]. The initial concentration of PDS was 4 mmol/L for the subsequent experiment. In addition, when the initial concentration of PDS was 4 mmol/L, there was almost no change in the degradation degree of MO after 40 min reaction, so the time of the subsequent experiment was shortened to 40 min.   Figure 8 shows the degradation of MO by Ag 2 O/g-C 3 N 4 at different initial concentrations of PDS. The results indicate that the degradation degree increases from 90.1% to 99.4% when the initial concentration of PDS increases from 1 mmol/L to 4 mmol/L but decreases from 99.4% to 96.7% when the initial concentration of PDS increases from 4 to 5 mmol/L. The reason why the degradation degree increases first and then decreases with the increase of the initial concentration of PDS is that the increase of the concentration of PDS can produce more active substances in the system and promote the faster degradation of MO. However, when the concentration of PDS is too high, the excessive SO 4 − · produced in the system leads to the quenching reaction between free radicals. As a result, the concentration of SO 4 − · decreases (Equations (1) and (2)) [25]. The initial concentration of PDS was 4 mmol/L for the subsequent experiment. In addition, when the initial concentration of PDS was 4 mmol/L, there was almost no change in the degradation degree of MO after 40 min reaction, so the time of the subsequent experiment was shortened to 40 min. (1)   Figure 8 shows the degradation of MO by Ag2O/g-C3N4 at different initial concentrations of PDS. The results indicate that the degradation degree increases from 90.1% to 99.4% when the initial concentration of PDS increases from 1 mmol/L to 4 mmol/L but decreases from 99.4% to 96.7% when the initial concentration of PDS increases from 4 to 5 mmol/L. The reason why the degradation degree increases first and then decreases with the increase of the initial concentration of PDS is that the increase of the concentration of PDS can produce more active substances in the system and promote the faster degradation of MO. However, when the concentration of PDS is too high, the excessive SO4 − · produced in the system leads to the quenching reaction between free radicals. As a result, the concentration of SO4 − · decreases (Equations (1) and (2)) [25]. The initial concentration of PDS was 4 mmol/L for the subsequent experiment. In addition, when the initial concentration of PDS was 4 mmol/L, there was almost no change in the degradation degree of MO after 40 min reaction, so the time of the subsequent experiment was shortened to 40 min. Figure 8. Effect of initial peroxydisulfate concentration on the degradation of MO. Figure 8. Effect of initial peroxydisulfate concentration on the degradation of MO. Figure 9 shows the degradation of MO with different initial activator concentrations. As can be seen from the figure, when the initial concentration of the activator increases from 0.75 g/L to 1.25 g/L, the degradation degree of MO at 10 min shows an increasing trend, and the change is more obvious. The degradation degree of MO at 40 min increases from 95.3% to 99.4%. The results show that the increase of the initial concentration of activator increases more active sites for the activation of PDS, and more free radicals are generated in the system, thus improving the degradation efficiency of MO. When the initial concentration of the activator increases from 1.25 g/L to 1.50 g/L, the degradation degree of MO at 40 min only increases by 0.1% to 99.5%. Considering the production cost of the activator, the initial concentration of the activator was 1.25 g/L for the subsequent experiment.

Degradation Reaction Conditions
Int. J. Environ. Res. Public Health 2022, 19, x FOR PEER REVIEW 12 of 16 Figure 9 shows the degradation of MO with different initial activator concentrations. As can be seen from the figure, when the initial concentration of the activator increases from 0.75 g/L to 1.25 g/L, the degradation degree of MO at 10 min shows an increasing trend, and the change is more obvious. The degradation degree of MO at 40 min increases from 95.3% to 99.4%. The results show that the increase of the initial concentration of activator increases more active sites for the activation of PDS, and more free radicals are generated in the system, thus improving the degradation efficiency of MO. When the initial concentration of the activator increases from 1.25 g/L to 1.50 g/L, the degradation degree of MO at 40 min only increases by 0.1% to 99.5%. Considering the production cost of the activator, the initial concentration of the activator was 1.25 g/L for the subsequent experiment.  Figure 10 shows the effect of different initial pH values on the degradation of MO by Ag2O/g-C3N4. As can be seen from the figure, with the initial pH value increasing from 3 to 10, the degradation degree decreases. When the initial pH is less than 7, the degradation degree of MO remains above 99% after 40 min reaction. When the initial pH is greater than 7, the degradation degree of MO is low, and after 40 min, the degradation degree of MO decreases from 96.6% at an initial pH of 8 to 83.9% at an initial pH of 10. The results show that the Ag2O/g-C3N4-activated PDS system has a good removal effect on MO under acidic or neutral conditions. The initial pH value determines the existence form of the active components in the system [26]. Under acidic or neutral conditions, SO4 − · in the system participates in the degradation of MO as the primary active substance, whereas under alkaline conditions, SO4 − · in the system reacts with OHto generate ·OH with a redox potential lower than that of SO4 − · (Equation (3)). Therefore, the degradation effect of MO under alkaline conditions is worse than that under acidic or neutral conditions [27].
Zhong et al. [28] used g-C3N4 loaded Fe3O4 to activate PDS to degrade Rhodamine B. Their experimental results showed that the degradation effect of Rhodamine B was the best under the condition of pH 2.1, and Rhodamine B could be completely degraded within 120 min. The Ag2O/g-C3N4 activator prepared in this study, after optimizing the conditions, activated PDS and could degrade MO with a degradation degree of more than 99% within 30 min or 40 min, significantly shortening the reaction time.
SO4 − •+OH -→SO4 2− +•OH (3) Figure 9. Effect of initial activation concentration on the degradation of MO. Figure 10 shows the effect of different initial pH values on the degradation of MO by Ag 2 O/g-C 3 N 4 . As can be seen from the figure, with the initial pH value increasing from 3 to 10, the degradation degree decreases. When the initial pH is less than 7, the degradation degree of MO remains above 99% after 40 min reaction. When the initial pH is greater than 7, the degradation degree of MO is low, and after 40 min, the degradation degree of MO decreases from 96.6% at an initial pH of 8 to 83.9% at an initial pH of 10. The results show that the Ag 2 O/g-C 3 N 4 -activated PDS system has a good removal effect on MO under acidic or neutral conditions. The initial pH value determines the existence form of the active components in the system [26]. Under acidic or neutral conditions, SO 4 − · in the system participates in the degradation of MO as the primary active substance, whereas under alkaline conditions, SO 4 − · in the system reacts with OHto generate ·OH with a redox potential lower than that of SO 4 − · (Equation (3)). Therefore, the degradation effect of MO under alkaline conditions is worse than that under acidic or neutral conditions [27].

Detection of Active Species
In order to explore the main active substances produced during the activation of PDS by the activator Ag2O/g-C3N4, MeOH and TBA were, respectively, used as the quenchers − Figure 10. Effect of initial pH value on the degradation of MO.
Zhong et al. [28] used g-C 3 N 4 loaded Fe 3 O 4 to activate PDS to degrade Rhodamine B. Their experimental results showed that the degradation effect of Rhodamine B was the best under the condition of pH 2.1, and Rhodamine B could be completely degraded within 120 min. The Ag 2 O/g-C 3 N 4 activator prepared in this study, after optimizing the conditions, activated PDS and could degrade MO with a degradation degree of more than 99% within 30 min or 40 min, significantly shortening the reaction time.

Detection of Active Species
In order to explore the main active substances produced during the activation of PDS by the activator Ag 2 O/g-C 3 N 4 , MeOH and TBA were, respectively, used as the quenchers of SO 4 − ·and ·OH in the activation system [29]. The concentrations of MeOH and TBA in the control system were 0.8, 1.0, and 1.2 mol/L, respectively. As shown in Figure 11, after TBA has been added, the degradation degree of MO is significantly inhibited, and the inhibition effect becomes more evident with an increase in TBA concentration. After 40 min of reaction, the degradation degree is 83.6%, 78.1%, and 74.2%, respectively, indicating that ·OH is produced in the reaction system and participates in the degradation of MO. After the addition of MeOH, the degradation degree of MO drops sharply, and after 40 min reaction, the degradation degree is 64.1%, 51.6%, and 45.8%, respectively, indicating that SO 4 − · is involved in the system of Ag 2 O/g-C 3 N 4 activated PDS to degrade MO, and the effect of SO 4 − · is greater than that of ·OH. The quenching experiments showed that SO 4 − · and ·OH are the main active substances in the process of Ag 2 O/g-C 3 N 4 activation of PDS, and both participate in the degradation of MO, in which SO 4 − · plays a more significant role.

Detection of Active Species
In order to explore the main active substances produced during the activation of PDS by the activator Ag2O/g-C3N4, MeOH and TBA were, respectively, used as the quenchers of SO4 − ·and ·OH in the activation system [29]. The concentrations of MeOH and TBA in the control system were 0.8, 1.0, and 1.2 mol/L, respectively. As shown in Figure 11, after TBA has been added, the degradation degree of MO is significantly inhibited, and the inhibition effect becomes more evident with an increase in TBA concentration. After 40 min of reaction, the degradation degree is 83.6%, 78.1%, and 74.2%, respectively, indicating that ·OH is produced in the reaction system and participates in the degradation of MO. After the addition of MeOH, the degradation degree of MO drops sharply, and after 40 min reaction, the degradation degree is 64.1%, 51.6%, and 45.8%, respectively, indicating that SO4 − · is involved in the system of Ag2O/g-C3N4 activated PDS to degrade MO, and the effect of SO4 − · is greater than that of ·OH. The quenching experiments showed that SO4 − · and ·OH are the main active substances in the process of Ag2O/g-C3N4 activation of PDS, and both participate in the degradation of MO, in which SO4 − · plays a more significant role.  Figure 11. Effect of the quencher on the degradation of MO.

Activation Stability of Ag 2 O/g-C 3 N 4
In order to verify the stability of the Ag 2 O/g-C 3 N 4 activator, five repeated experiments were conducted on the degradation of the Ag 2 O/g-C 3 N 4 activated PDS, as shown in Figure 12. After four cycles, the degradation degree of MO in the system can still reach 80.7% after 40 min of reaction, and after the fifth cycle experiment, the degradation degree of MO drops to 75.6%. With the increase in the number of experiment cycles, the degradation degree of MO in the reaction system shows a downward trend. The reason may be that, with the progress of the reaction, Ag 2 O in the activator falls off the surface of g-C 3 N 4 , resulting in a decrease in the activation efficiency of the activator to PDS. At the same time, it may also be because, after the end of each cycle experiment, the activator needs to be centrifugated for washing, during which there is an inevitable loss of the activator, which also leads to a decrease in MO degradation degree with an increase in cycle times.
which also leads to a decrease in MO degradation degree with an increase in cycle times. Figure 13 shows the XRD patterns before and after the reaction of the Ag2O/g-C3N4 activated PDS to degrade MO. As can be seen from the figure, there is little difference between the XRD patterns of Ag2O/g-C3N4 before and after the degradation of MO, indicating that the crystal structure of Ag2O/g-C3N4 has not changed significantly. The stability experiment showed that the activator Ag2O/g-C3N4 has considerable stability in the activated PDS system.    Figure 13 shows the XRD patterns before and after the reaction of the Ag 2 O/g-C 3 N 4 activated PDS to degrade MO. As can be seen from the figure, there is little difference between the XRD patterns of Ag 2 O/g-C 3 N 4 before and after the degradation of MO, indicating that the crystal structure of Ag 2 O/g-C 3 N 4 has not changed significantly. The stability experiment showed that the activator Ag 2 O/g-C 3 N 4 has considerable stability in the activated PDS system. of MO drops to 75.6%. With the increase in the number of experiment cycles, the degradation degree of MO in the reaction system shows a downward trend. The reason may be that, with the progress of the reaction, Ag2O in the activator falls off the surface of g-C3N4, resulting in a decrease in the activation efficiency of the activator to PDS. At the same time, it may also be because, after the end of each cycle experiment, the activator needs to be centrifugated for washing, during which there is an inevitable loss of the activator, which also leads to a decrease in MO degradation degree with an increase in cycle times. Figure 13 shows the XRD patterns before and after the reaction of the Ag2O/g-C3N4 activated PDS to degrade MO. As can be seen from the figure, there is little difference between the XRD patterns of Ag2O/g-C3N4 before and after the degradation of MO, indicating that the crystal structure of Ag2O/g-C3N4 has not changed significantly. The stability experiment showed that the activator Ag2O/g-C3N4 has considerable stability in the activated PDS system.

Conclusions
Ag 2 O/g-C 3 N 4 composited with graphite phase g-C 3 N 4 as a metal oxide carrier was prepared using a low-cost and safe method. Using SEM, XRD, FT-IR, and XPS, the Ag 2 O/g-C 3 N 4 composites were successfully synthesized. Ag 2 O in the activator was highly dispersed on the surface of the g-C 3 N 4 layer in the form of fine particles, and the degradation ability of MO by the composites loaded with Ag 2 O was significantly improved. There was a similar synergistic effect between g-C 3 N 4 and Ag 2 O. The pseudo-secondorder kinetic reaction model was the best fit for the degradation process of MO under different systems. The reaction rate constant of the Ag 2 O/g-C 3 N 4 -activated PDS for MO degradation was significantly higher than that of other systems. Free radical quenching experiments showed that SO 4 − · and ·OH were the main active substances in the activation of PDS by Ag 2 O/g-C 3 N 4 , and the effect of SO 4 − · was greater than that of ·OH. The degradation degree of MO by the Ag 2 O/g-C 3 N 4 composites reached 99.4%, which showed stronger catalytic activity and could still maintain more than 80% degradation degree after four cycles of use. The prepared activator Ag 2 O/g-C 3 N 4 had good catalytic activity and recycling performance.