The Kinetic Simulation of Persulfate Activation by Nano-Ferrosoferric Oxide

: Nano-ferrosoferric-oxide (nFe 3 O 4 )-activated persulfate (PS) technology was used to remove pollutant bisphenol A (BPA) in water. The effects of nFe 3 O 4 concentration, PS concentration, BPA concentration, temperature, and pH were investigated in terms of the degradation effect of BPA. The results showed that more PS dosage and lower BPA concentration could improve the degradation rate of BPA. When other conditions were constant, the degradation rate of BPA increased with the increase of temperature. When pH was 5, the degradation rate of BPA was the highest. When the initial PS concentration and pH were changed, the degradation rate of BPA was consistent with the pseudo-secondary kinetic model. Under other conditions, the degradation rate of BPA was consistent with the pseudo-ﬁrst-order kinetic model. Sulfate radical (SO 4 •− ) produced by nFe 3 O 4 /PS system was mainly responsible for the degradation of BPA.


Introduction
In the decades, due to the increasing use of synthetic chemicals, people have paid great attention to environmental pollution caused by emerging pollutants [1], especially water pollution. Emerging pollutants are toxic and persistent. They are widely detected in water bodies [2,3], and are difficult to remove using traditional methods [4,5]. In the advanced oxidation process, the application of sulfate radicals (SO 4 •− ) to the oxidative degradation of emerging pollutants has shown a good ability [6]. Nisreen Sabti [7] used Fe 3 O 4 /SiO 2 to adsorb low initial concentration of Methyl Bromide (MB) and reached better MB removal rate. In addition to being an adsorbent, Fe 3 O 4 can also be used as an activator of persulfate (PS) to produce SO 4 •− in water treatment due to its advantages such as easy recovery, large surface area, and high catalytic activity. In this paper, nano-ferrosoferricoxide (nFe 3 O 4 ) was used as an activator to catalyze PS to degrade bisphenol A (BPA). The effects of SO 4 •− on the degradation of BPA were studied by adjusting the five influencing factors: nano-ferrosoferric-oxide dosage, persulfate dosage, initial substrate concentration (BPA), temperature, and initial pH. The degradation effect of BPA was simulated and analyzed from the point of view of kinetics.

NFe 3 O 4 Dosage
It was to take 500 mL of 5 mg/L BPA in 6 beakers at a temperature of 20 • C and add 0.   It can be seen from Figure 1 that, without nFe3O4, the removal rate of BPA can reach 19.68% after 60 min of reaction. The reason for this is that temperature and light can activate PS to produce SO4 •− , which can remove BPA. It was significant to add nFe3O4 to the system for increasing the removal rate of BPA; this was due to the formation of Fe 2+ on the surface of nFe3O4 activated PS to produce SO4 •− , which led to the removal of BPA.
It is shown from Table 1 that the dosage of nFe3O4 has a significant impact on the BPA degradation rate. When nFe3O4 is not in the system, the BPA degradation rate is only 0.00359 min −1 . With the increase in nFe3O4 dosage, the degradation rate of BPA is accelerated firstly, and then slowly lowered. The analysis is due to the increase in nFe3O4 dosage, which leads to the increase in Fe 2+ , and the generation of more SO4 •− , which leads the accelerated degradation rate of BPA. On the one hand, the excessive Fe 2+ can result in a quenching reaction with SO4 •− . On the other hand, the excessive SO4 •− also leads to a mutual quenching reaction between SO4 •− and other free radicals, which will reduce SO4 •− in the system, thus slowing down the degradation rate of BPA.

Dosage of Sodium Persulfate
Six parts of nFe3O4 ( each 0.1 g/L) were added to the six beakers (each 500 mL) containing BPA (5 mg/L), then different concentrations of PS (0 mM, 0.1 mM, 0.2 mM, 0.3 mM, 0.4 mM, 0.5 mM) were put respectively into the six beakers to initiate chemical reactions. The test results are shown in Figure 2a and the fitted curve is shown in Figure 2b. The fitted curve equations and parameters are shown in Table 2. It can be seen from Table 2 that the amount of PS has a significant impact on the degradation rate of BPA. When the amount of PS increased from 0.1 mM to 0.3 mM, the degradation rate of BPA was accelerated. The degradation of BPA was the slowest when the PS dosage was 0.1 mm. The results showed that sodium persulfate could accelerate the degradation rate of BPA. Because it can produce more SO4 •− , it accelerates the degradation of BPA. However, exces-  It can be seen from Figure 1 that, without nFe 3 O 4 , the removal rate of BPA can reach 19.68% after 60 min of reaction. The reason for this is that temperature and light can activate PS to produce SO 4 •− , which can remove BPA. It was significant to add nFe 3 O 4 to the system for increasing the removal rate of BPA; this was due to the formation of Fe 2+ on the surface of nFe 3 O 4 activated PS to produce SO 4 •− , which led to the removal of BPA. It is shown from Table 1 that the dosage of nFe 3 O 4 has a significant impact on the BPA degradation rate. When nFe 3 O 4 is not in the system, the BPA degradation rate is only 0.00359 min −1 . With the increase in nFe 3 O 4 dosage, the degradation rate of BPA is accelerated firstly, and then slowly lowered. The analysis is due to the increase in nFe 3 O 4 dosage, which leads to the increase in Fe 2+ , and the generation of more SO 4 •− , which leads the accelerated degradation rate of BPA. On the one hand, the excessive Fe 2+ can result in a quenching reaction with SO 4 •− . On the other hand, the excessive SO 4 •− also leads to a mutual quenching reaction between SO 4 •− and other free radicals, which will reduce SO 4 •− in the system, thus slowing down the degradation rate of BPA.

Dosage of Sodium Persulfate
Six parts of nFe 3 O 4 (each 0.1 g/L) were added to the six beakers (each 500 mL) containing BPA (5 mg/L), then different concentrations of PS (0 mM, 0.1 mM, 0.2 mM, 0.3 mM, 0.4 mM, 0.5 mM) were put respectively into the six beakers to initiate chemical reactions. The test results are shown in Figure 2a and the fitted curve is shown in Figure 2b. The fitted curve equations and parameters are shown in Table 2. It can be seen from Table 2 that the amount of PS has a significant impact on the degradation rate of BPA. When the amount of PS increased from 0.1 mM to 0.3 mM, the degradation rate of BPA was accelerated. The degradation of BPA was the slowest when the PS dosage was 0.1 mm. The results showed that sodium persulfate could accelerate the degradation rate of BPA. Because it can produce more SO 4 •− , it accelerates the degradation of BPA. However, excessive sodium persulfate can cause the degradation rate of BPA to slow down, because excessive sodium persulfate can also become the quenching agent of SO 4 •− [8,9]. This indicates that sodium persulfate has an appropriate concentration, and so the optimal dosage of sodium persulfate is 0.3 mM.
Catalysts 2022, 12, x FOR PEER REVIEW 3 of 7 sive sodium persulfate can cause the degradation rate of BPA to slow down, because excessive sodium persulfate can also become the quenching agent of SO4 •− [8,9]. This indicates that sodium persulfate has an appropriate concentration, and so the optimal dosage of sodium persulfate is 0.3 mM.

Initial Substrate Concentration
The experiment required 500 mL of 1 mg/L, 2 mg/L, 3 mg/L, 4 mg/L, 5 mg/L BPA solution in 6 beakers, at a temperature 20 °C, with the addition of 0.1 g/L nFe3O4, 0.2 mM sodium persulfate. The test results are shown in Figure 3a and the fitted curve is shown in Figure 3b. The fitted curve equations and parameters are shown in Table 3. According to Figure 3, with the increase in initial BPA concentration, the amount of BPA removal increased, but the removal rate of BPA decreased. In this analysis, the amount of BPA removed raise because the initial BPA concentration increased the amount of BPA per unit volume, which in turn increased the probability of collision between them. However, as the concentration of nFe3O4 and sodium persulfate remained unchanged and only a certain amount of SO4 •− was able to be produced, so the removal rate of BPA was reduced [10,11]. It can be seen from Table 3 that the initial BPA concentration has a significant impact on the degradation rate of BPA. With the increase in the initial BPA concentration, the degradation rate of BPA gradually slows down. When the initial concentration of BPA was 5 mg/L, the degradation rate of BPA was 0.00942 min −1 , and the reaction was the slowest.

Initial Substrate Concentration
The experiment required 500 mL of 1 mg/L, 2 mg/L, 3 mg/L, 4 mg/L, 5 mg/L BPA solution in 6 beakers, at a temperature 20 • C, with the addition of 0.1 g/L nFe 3 O 4 , 0.2 mM sodium persulfate. The test results are shown in Figure 3a and the fitted curve is shown in Figure 3b. The fitted curve equations and parameters are shown in Table 3. According to Figure 3, with the increase in initial BPA concentration, the amount of BPA removal increased, but the removal rate of BPA decreased. In this analysis, the amount of BPA removed raise because the initial BPA concentration increased the amount of BPA per unit volume, which in turn increased the probability of collision between them. However, as the concentration of nFe 3 O 4 and sodium persulfate remained unchanged and only a certain amount of SO 4 •− was able to be produced, so the removal rate of BPA was reduced [10,11]. It can be seen from Table 3 that the initial BPA concentration has a significant impact on the degradation rate of BPA. With the increase in the initial BPA concentration, the degradation rate of BPA gradually slows down. When the initial concentration of BPA was 5 mg/L, the degradation rate of BPA was 0.00942 min −1 , and the reaction was the slowest.

Temperature
When the temperature was 20 °C, 35 °C, 50 °C, 70 °C and bisphenol A was 5 mg/L, the removal rate of BPA increased from 20.72% to 40.36%. One experiment was to add 0.2 mM sodium persulfate and 0.1 g/L nFe3O4, and the other experiment was to add 0.2 mM sodium persulfate without nFe3O4. The test results are shown in Figure 4a, and the fitted curve is shown in Figure 4b. The kinetic equations of BPA degradation at different initial reaction temperature could be expressed by the pseudo-first-order reaction kinetic equation (Table 4).

Temperature
When the temperature was 20 • C, 35 • C, 50 • C, 70 • C and bisphenol A was 5 mg/L, the removal rate of BPA increased from 20.72% to 40.36%. One experiment was to add 0.2 mM sodium persulfate and 0.1 g/L nFe 3 O 4 , and the other experiment was to add 0.2 mM sodium persulfate without nFe 3 O 4 . The test results are shown in Figure 4a, and the fitted curve is shown in Figure 4b. The kinetic equations of BPA degradation at different initial reaction temperature could be expressed by the pseudo-first-order reaction kinetic equation (Table 4).  When the temperature was 20 °C, the degradation of BPA was the slowest. When the temperature elevated to 70 °C, the degradation rate of BPA increased to 2.4 times faster than the rate at 20 °C. Considering the actual situation of sewage treatment, the optimal temperature is 35 °C from the perspective of energy saving.

pH
When the pH was 3, 5, 7, 9, the method required the addition of 0.2 mM sodium persulfate and 0.1 g/L nFe3O4 at the temperature 20 °C. The BPA concentration was 5 mg/L. The test results are shown in Figure 5a, and the fitted curve is shown in Figure 5b. The fitted curve equations and parameters are shown in Table 5.  When the temperature was 20 • C, the degradation of BPA was the slowest. When the temperature elevated to 70 • C, the degradation rate of BPA increased to 2.4 times faster than the rate at 20 • C. Considering the actual situation of sewage treatment, the optimal temperature is 35 • C from the perspective of energy saving.

pH
When the pH was 3, 5, 7, 9, the method required the addition of 0.2 mM sodium persulfate and 0.1 g/L nFe 3 O 4 at the temperature 20 • C. The BPA concentration was 5 mg/L. The test results are shown in Figure 5a, and the fitted curve is shown in Figure 5b. The fitted curve equations and parameters are shown in Table 5. When the temperature was 20 °C, the degradation of BPA was the slowest. When temperature elevated to 70 °C, the degradation rate of BPA increased to 2.4 times fas than the rate at 20 °C. Considering the actual situation of sewage treatment, the optim temperature is 35 °C from the perspective of energy saving.

pH
When the pH was 3, 5, 7, 9, the method required the addition of 0.2 mM sodiu persulfate and 0.1 g/L nFe3O4 at the temperature 20 °C. The BPA concentration wa mg/L. The test results are shown in Figure 5a, and the fitted curve is shown in Figure  The fitted curve equations and parameters are shown in Table 5.  As shown in Figure 5, with the increase in pH, the removal of BPA increased firs and then decreased. After the reaction for 60 min, the removal rate of BPA was the high at pH 5 and the lowest at pH 9, with the removal rates of 59.16% and 25.5%, respective Table 5 shows that pH has a significant effect on the degradation rate of BPA. W the gradual increase in pH, the reaction speed was accelerated at first and then slow  As shown in Figure 5, with the increase in pH, the removal of BPA increased firstly and then decreased. After the reaction for 60 min, the removal rate of BPA was the highest at pH 5 and the lowest at pH 9, with the removal rates of 59.16% and 25.5%, respectively. Table 5 shows that pH has a significant effect on the degradation rate of BPA. With the gradual increase in pH, the reaction speed was accelerated at first and then slowed down. The degradation rate of BPA under acidic conditions was much higher than that under alkaline conditions. When pH and acidity conditions are more favorable for the formation of SO 4 •− , the degradation rate of BPA is faster. Thus, the optimal pH is 5.