Catalytic Degradation of Ciprofloxacin in Aqueous Solution by Peroxymonosulfate Activated with a Magnetic CuFe2O4@Biochar Composite

A magnetic copper ferrite and biochar composite (CuFe2O4@BC) catalyst was prepared by an improved sol-gel calcination method and initially used for the removal of antibiotics ciprofloxacin (CIP) by activated peroxymonosulfate (PMS). Using CuFe2O4@BC as the activator, 97.8% CIP removal efficiency could be achieved in 30 min. After a continuous degradation cycle, CuFe2O4@BC catalyst still exhibited great stability and repeatability and could also be quickly recovered by an external magnetic field. Meanwhile, the CuFe2O4@BC/PMS system presented good stability for metal ion leaching, which was far less than the leaching of metal ions in the CuFe2O4/PMS system. Moreover, the effects of various influencing factors, such as initial solution pH, activator loading, PMS dosage, reaction temperature, humic acid (HA), and the inorganic anions were explored. The quenching experiments and the electron paramagnetic resonance (EPR) analysis manifested that hydroxyl radical (•OH), sulfate radical (SO4•−), superoxide radical (O2•−), and singlet oxygen (1O2) were generated in the CuFe2O4@BC/PMS system, while 1O2 and O2•− are mainly involved in the degradation process. The synergistic effect between CuFe2O4 and BC enhanced the structural stability and electrical conductivity of the material, which promoted the bonding between the catalyst and PMS, resulting in the enhanced catalytic activity of CuFe2O4@BC. This indicates that CuFe2O4@BC activating PMS is a promising remediation technique for CIP-contaminated water.


Introduction
Recently, water contamination caused by the excessive use and improper treatment of antibiotics has obtained increasing attention [1,2], and owing to their poor biodegradability and high toxicity, the majority of antibiotics are hardly eliminated by traditional methods such as biotechnology [3,4]. As a typical and widely used antibiotic, ciprofloxacin (CIP) increases antibiotic resistance of bacteria in wastewater and poses a serious threat to humans and aquatic organisms [5]. Therefore, it is imperative to develop an efficient technology to remove residual antibiotics from contaminated water environments.
A variety of methods have been explored to treat antibiotic pollutants such as biotransformation, adsorption, electrocoagulation method [6,7], and advanced oxidation processes (AOPs). Additionally, different types of highly effective AOPs have been widely studied due to their high oxidation ability to various pollutants, such as Fenton processes [8], UV-photolysis driven processes [9], ozonation [10], and sulfate radical-based AOPs (SR-AOPs) [11][12][13]. Notably, the SR-AOPs have received increasing concerns due to the generation of sulfate radical (SO 4 •− ) with high reactivity [14,15]. Nonetheless, when in direct contact with contaminants, the unactivated peroxymonosulfate (PMS) results in a poor degradation efficiency, which restricts its practical application.
for organic pollutants degradation and the CIP was selected as the target contaminant. Moreover, various characterization methods were used to examine the morphology, crystal structure, and structural characteristics of the produced catalysts. The effects of initial pH, catalyst dosage, PMS concentration, temperature, inorganic anions, and humic acid (HA) in the CuFe 2 O 4 @BC/PMS system were also investigated. Additionally, the potential applications of catalysts were evaluated in terms of magnetism, reusability, and stability. Finally, the degradation mechanism of the free radical and non-free radical processes was investigated by the XPS analysis, free radical quenching experiment, and EPR detection. Accordingly, this research is dedicated to exploiting biochar and developing a novel magnetic catalyst to achieve the effective removal of pollutants.

Catalysts Characterization
To determine whether the loading of CuFe 2 O 4 on the BC was successful, SEM images were characterized with prepared samples (Figure 1a-c). According to the comparison of the CuFe 2 O 4 @BC catalyst ( Figure 1a) and the BC catalyst (Figure 1b), it could be seen that mass particles adhered to the surface of the BC, which were aggregated by the magnetic attraction of the ferrite according to previous reports [28,29].Meanwhile, the diameters of copper ferrite particles ranged from 50 to 200 nm, according to the TEM images of copper ferrite particles presented in Figure S1. EDS was employed to clarify the composition of the CuFe 2 O 4 @BC catalyst ( Figure S2) and the mapping profiles of C, O, Cu, and Fe elements (Figure 1d-g), confirming that four elements were evenly distributed in the material. ferrite (CuFe2O4) by an improved sol-gel combustion reaction to obtain an advanced composite catalyst (CuFe2O4@BC). The prepared material was applied to activate PMS for organic pollutants degradation and the CIP was selected as the target contaminant. Moreover, various characterization methods were used to examine the morphology, crystal structure, and structural characteristics of the produced catalysts. The effects of initial pH, catalyst dosage, PMS concentration, temperature, inorganic anions, and humic acid (HA) in the CuFe2O4@BC/PMS system were also investigated. Additionally, the potential applications of catalysts were evaluated in terms of magnetism, reusability, and stability. Finally, the degradation mechanism of the free radical and non-free radical processes was investigated by the XPS analysis, free radical quenching experiment, and EPR detection. Accordingly, this research is dedicated to exploiting biochar and developing a novel magnetic catalyst to achieve the effective removal of pollutants.

Catalysts Characterization
To determine whether the loading of CuFe2O4 on the BC was successful, SEM images were characterized with prepared samples (Figure 1a-c). According to the comparison of the CuFe2O4@BC catalyst ( Figure 1a) and the BC catalyst (Figure 1b), it could be seen that mass particles adhered to the surface of the BC, which were aggregated by the magnetic attraction of the ferrite according to previous reports [28,29].Meanwhile, the diameters of copper ferrite particles ranged from 50 to 200 nm, according to the TEM images of copper ferrite particles presented in Figure S1. EDS was employed to clarify the composition of the CuFe2O4@BC catalyst ( Figure S2) and the mapping profiles of C, O, Cu, and Fe elements (Figure 1d-g), confirming that four elements were evenly distributed in the material. The crystal structures of BC, CuFe2O4, as well as the prepared CuFe2O4@BC catalysts, were obtained by XRD analyses, as displayed in Figure 2a. The great crystallinity of the catalysts could be seen in the diffraction diagram. For CuFe2O4, the characteristic peaks of   [30] was also identified in CuFe 2 O 4 @BC materials, representing that the structure of BC was not ruined during the formation process. Nevertheless, this peak came to be less evident, which might be owing to the diffraction intensity of BC being much weaker than the resulting CuFe 2 O 4 @BC catalysts [31]. Additionally, compared with the fresh CuFe 2 O 4 @BC composite, no extra peak was detected in the XRD spectra of the used CuFe 2 O 4 @BC composite, revealing that the composition and structure of the catalyst remained essentially unchanged after degradation. bands at 480 cm −1 and 570 cm −1 were due to the stretching vibration of the Cu-O bond [32] and verified the formation of the Fe-O bond with tetrahedral geometry [33], which illustrated the formation of Fe(III) and Cu(II) in tetrahedral and octahedral coordination environments, respectively. On the other hand, the C=C, C=O, and O-H stretching vibration peaks corresponding to the absorption peaks at 1500 cm −1 , 1700 cm −1 , and 3470 cm −1 could be observed in pure BC; the existence of rich oxygen functional groups on BC was confirmed, which was essential for the catalyst to maintain hydrophilicity and high dispersity [31]. Additionally, all characteristic peaks of BC and CuFe2O4 can be observed in the FT-IR spectra of fresh and used CuFe2O4@BC materials, demonstrating the successful synthesis of the designed materials and the basic structure of the material after the reaction was almost unchanged. After 30 min of reaction, the leaching rates of active components Fe and Cu are 0.087 mg/L and 0.214 mg/L, respectively (Table S1), which can be ignored, indicating that there was a strong interaction between CuFe2O4 and BC, and the CuFe2O4@BC material could still reach an 87.89% degradation rate of CIP after four cycles, revealing that it had strong stability.  To explore the role of material surface components and several transition metals in the PMS activation process, the surface characteristics of CuFe2O4@BC material before and after the reaction were further studied by XPS. As shown in Figure 3, all binding energies were corrected with reference to C1s at 284.8 eV. The XPS spectroscopy wide scan of fresh and used CuFe2O4@BC material (Figure 3a) confirmed the coexistence of Cu, Fe, O, and C elements, which was consistent with the EDS spectrum results. Furthermore, Figure 3b demonstrated that the peak at 934.1 eV of fresh CuFe2O4@BC was assigned to the characteristic peak of Cu2p3/2, suggesting that Cu(II) was the main valence state of Cu species before catalysis. For the catalyst after reaction, a peak of 932.1 eV in the XPS Cu2p3/2 spectrum can be attributed to Cu(I) and its relative content was 27.06%, indicating that Cu(II) was partly reduced to Cu(I) after degradation. In Figure 3c, the Fe2p spectrum of fresh The FT-IR spectrum analysis of pure BC, CuFe 2 O 4 , as the prepared and used CuFe 2 O 4 @BC catalyst, were shown in Figure 2b. For pristine CuFe 2 O 4 @BC, the absorption bands at 480 cm −1 and 570 cm −1 were due to the stretching vibration of the Cu-O bond [32] and verified the formation of the Fe-O bond with tetrahedral geometry [33], which illustrated the formation of Fe(III) and Cu(II) in tetrahedral and octahedral coordination environments, respectively. On the other hand, the C=C, C=O, and O-H stretching vibration peaks corresponding to the absorption peaks at 1500 cm −1 , 1700 cm −1 , and 3470 cm −1 could be observed in pure BC; the existence of rich oxygen functional groups on BC was confirmed, which was essential for the catalyst to maintain hydrophilicity and high dispersity [31]. Additionally, all characteristic peaks of BC and CuFe 2 O 4 can be observed in the FT-IR spectra of fresh and used CuFe 2 O 4 @BC materials, demonstrating the successful synthesis of the designed materials and the basic structure of the material after the reaction was almost unchanged. After 30 min of reaction, the leaching rates of active components Fe and Cu are 0.087 mg/L and 0.214 mg/L, respectively (Table S1), which can be ignored, indicating that there was a strong interaction between CuFe 2 O 4 and BC, and the CuFe 2 O 4 @BC material could still reach an 87.89% degradation rate of CIP after four cycles, revealing that it had strong stability.
To explore the role of material surface components and several transition metals in the PMS activation process, the surface characteristics of CuFe 2 O 4 @BC material before and after the reaction were further studied by XPS. As shown in Figure 3, all binding energies were corrected with reference to C1s at 284.8 eV. The XPS spectroscopy wide scan of fresh and used CuFe 2 O 4 @BC material (Figure 3a) confirmed the coexistence of Cu, Fe, O, and C elements, which was consistent with the EDS spectrum results. Furthermore, Figure 3b demonstrated that the peak at 934.1 eV of fresh CuFe 2 O 4 @BC was assigned to the characteristic peak of Cu2p 3/2 , suggesting that Cu(II) was the main valence state of Cu species before catalysis. For the catalyst after reaction, a peak of 932.1 eV in the XPS Cu2p 3/2 spectrum can be attributed to Cu(I) and its relative content was 27.06%, indicating that Cu(II) was partly reduced to Cu(I) after degradation. In Figure 3c, the Fe2p spectrum of fresh CuFe 2 O 4 @BC catalyst shows two peaks at 711.0 eV and 724.6 eV, assigning to Fe2p 3/2 and Fe2p 1/2 , respectively, which represents that the Fe species on the CuFe 2 O 4 @BC surface was Fe(III) [34]. Notably, after participating in the degradation, the relative content of Fe(II) on the surface of the CuFe 2 O 4 @BC catalyst increased from 13.24% to 18.17%, whereas the relative content of Fe (III) decreased from 86.76% to 81.83%, indicating that Fe(III) was partially reduced to Fe(II) in the degradation process. Additionally, Figure 3d  CuFe2O4@BC catalyst shows two peaks at 711.0 eV and 724.6 eV, assigning to Fe2p3/2 and Fe2p1/2, respectively, which represents that the Fe species on the CuFe2O4@BC surface was Fe(III) [34]. Notably, after participating in the degradation, the relative content of Fe(II) on the surface of the CuFe2O4@BC catalyst increased from 13.24% to 18.17%, whereas the relative content of Fe (III) decreased from 86.76% to 81.83%, indicating that Fe(III) was partially reduced to Fe(II) in the degradation process. Additionally, Figure 3d

Catalytic Activity
To systematically assess the catalytic activity of CuFe2O4@BC, a series of comparison tests with different catalysts were carried out to remove CIP. As shown in Figure 4a, the adsorption of CIP on the CuFe2O4@BC and BC surface was not obvious, and the CIP degradation rate was less than 20% during PMS oxidation alone. In addition, compared with the CuFe2O4/PMS and BC/PMS systems, the CuFe2O4@BC/PMS system had higher degradation activity for CIP, reaching a 98.72% degradation rate within 30 min. The CuFe2O4@BC exhibited higher catalytic activity for PMS than the traditional heterogeneous catalyst (Table S2). Previously reported that the activation of PMS was initiated by the

Catalytic Activity
To systematically assess the catalytic activity of CuFe 2 O 4 @BC, a series of comparison tests with different catalysts were carried out to remove CIP. As shown in Figure 4a, the adsorption of CIP on the CuFe 2 O 4 @BC and BC surface was not obvious, and the CIP degradation rate was less than 20% during PMS oxidation alone. In addition, compared with the CuFe 2 O 4 /PMS and BC/PMS systems, the CuFe 2 O 4 @BC/PMS system had higher degrada-tion activity for CIP, reaching a 98.72% degradation rate within 30 min. The CuFe 2 O 4 @BC exhibited higher catalytic activity for PMS than the traditional heterogeneous catalyst (Table S2). Previously reported that the activation of PMS was initiated by the electron transfer between PMS and the catalyst to produce reactive oxygen species (ROS) [37]. As a matrix rich in defect structure [25], BC could introduce a large number of oxygen vacancies (OVs) after compounding with CuFe 2 O 4 , which could be used as defect sites to promote the adsorption and bonding with PMS, indicating that the participation of Fe(III)/Fe(II) and Cu(II)/Cu(I) redox pairs [37] and the rich oxygen-containing functional groups on BC had synergistic effects on the activation of PMS to generate free radicals [26].
, 24, x FOR PEER REVIEW 6 of 16 Fe(III)/Fe(II) and Cu(II)/Cu(I) redox pairs [37] and the rich oxygen-containing functional groups on BC had synergistic effects on the activation of PMS to generate free radicals [26]. Furthermore, the leached metal ions in the two reaction systems were collected to evaluate the stability of the catalyst structure. As displayed in Table S1, in the CuFe2O4/PMS system (pH = 7), the dissolution concentrations of iron ions and copper ions were 0.455 and 1.279 mg/L at 30 min, respectively. However, the leaching of iron ions and copper ions reached the maximum concentration of 0.099 and 0.214 mg/L after 30 min in the CuFe2O4@BC/PMS system (pH = 7), which was far less than the leaching of metal ions in the CuFe2O4/PMS system. In contrast, the CuFe2O4@BC/PMS system had good stability for metal ion leaching, which might be attributed to the strong binding of CuFe2O4 to the BC matrix. According to SEM images in Figure 1a-c, CuFe2O4 particles were uniformly inserted in the BC basis. The unique structure decreased the leaching of metal ions, thereby, enhancing the synergistic effect and structural stability of CuFe2O4@BC.

Influences of Initial Solution pH Value
The initial pH value of the reaction solution was a significant factor closely related to the catalytic performance of heterogeneous catalysts. Thus, we evaluated the effect of pH (3-10) on CIP removal. As presented in Figure 4b, the degradation efficiency increased from 54.3% to 100.0% as the solution pH value increased from 3.0 to 10.0. The efficient removal of CIP was achieved under neutral and weak alkali conditions, while the efficiency decreased dramatically in acidic conditions, which was consistent with previous studies [38,39]. Additionally, the zero-charge point of the material was 5.72 measured by the zetasizer, representing that the material surface carried a positive charge at pH < 5.72. Meanwhile, according to the pKa1 and pKa2 of PMS which are less than 0 and 9.4, respectively, PMS mainly exists in the form of HSO5 -under acidic conditions. However, previous studies have manifested that plenty of H + would interact with the peroxide bond (-O-O-) of PMS to form hydrogen bonds, which inhibited its interaction with the positively charged CuFe2O4@BC surface by attaching the positive charge to HSO5 − . Additionally, free Furthermore, the leached metal ions in the two reaction systems were collected to evaluate the stability of the catalyst structure. As displayed in Table S1, in the CuFe 2 O 4 /PMS system (pH = 7), the dissolution concentrations of iron ions and copper ions were 0.455 and 1.279 mg/L at 30 min, respectively. However, the leaching of iron ions and copper ions reached the maximum concentration of 0.099 and 0.214 mg/L after 30 min in the CuFe 2 O 4 @BC/PMS system (pH = 7), which was far less than the leaching of metal ions in the CuFe 2 O 4 /PMS system. In contrast, the CuFe 2 O 4 @BC/PMS system had good stability for metal ion leaching, which might be attributed to the strong binding of CuFe 2 O 4 to the BC matrix. According to SEM images in Figure 1a-c, CuFe 2 O 4 particles were uniformly inserted in the BC basis. The unique structure decreased the leaching of metal ions, thereby, enhancing the synergistic effect and structural stability of CuFe 2 O 4 @BC.

Influences of Initial Solution pH Value
The initial pH value of the reaction solution was a significant factor closely related to the catalytic performance of heterogeneous catalysts. Thus, we evaluated the effect of pH (3-10) on CIP removal. As presented in Figure 4b, the degradation efficiency increased from 54.3% to 100.0% as the solution pH value increased from 3.0 to 10.0. The efficient removal of CIP was achieved under neutral and weak alkali conditions, while the efficiency decreased dramatically in acidic conditions, which was consistent with previous studies [38,39]. Additionally, the zero-charge point of the material was 5.72 measured by the zetasizer, representing that the material surface carried a positive charge at pH < 5.72. Meanwhile, according to the pK a1 and pK a2 of PMS which are less than 0 and 9.4, respectively, PMS mainly exists in the form of HSO 5 − under acidic conditions. However, previous studies have manifested that plenty of H + would interact with the peroxide bond (-O-O-) of PMS to form hydrogen bonds, which inhibited its interaction with the positively charged CuFe 2 O 4 @BC surface by attaching the positive charge to HSO 5 − . Additionally, free radicals •OH, and SO 4 •− can be eliminated by H + (Equations (1) and (2)), which was also the course for the decrease in the CIP removal rate under acidic conditions [40].
On the other hand, the remarkable CIP degradation efficiency under neutral and alkaline conditions was achieved in the CuFe 2 O 4 @BC/PMS system. Previous studies verified that the surface hydroxyl group was an important active species on the surface of heterogeneous catalysts, which could promote the chemical bonding between materials and PMS, thus, accelerating electron transfer [35]. Moreover, the increase in pH was conducive to the formation of surface hydroxyl groups in heterogeneous catalysts [41]. According to the strong coordination between phosphate and surface transition metal, which affects the formation of surface hydroxyl of the material [42], the experiment of adding 10 mM phosphate in the CuFe 2 O 4 @BC/PMS system was carried out to evaluate the influence of hydroxyl. As presented in Figure S3, with the addition of phosphate, the CIP degradation rate decreased from 100.0% to 22.05%, indicating that the surface hydroxyl plays a key role in the activation of PMS [41]. In addition, PMS could also directly generate superoxide radicals (O 2 •− ) and singlet oxygen ( 1 O 2 ) [43]. For these reasons, the CuFe 2 O 4 @BC/PMS system realized the prominent degradation efficiency of CIP in both neutral and alkaline conditions.

Influences of Activator Loading and PMS Dosage
To preferably identify the activation efficiency of CuFe 2 O 4 @BC, the influences of activator loading and PMS dosage in the CuFe 2 O 4 @BC/PMS system were explored. As shown in Figure 5a, the CIP degradation rate was 92.22% within 60 min as the loading amount of the activator was 0.05 g/L. Additionally, when the loading increased from 0.1 g/L to 0.3 g/L, CIP could be degraded completely on CuFe 2 O 4 @BC within 60 min. A reasonable explanation might be that in the CuFe 2 O 4 @BC/PMS system when the amount of catalyst was too low, the active sites of PMS activation and CIP degradation tended to be insufficient [44]. Analogously, CIP degradation between 1 mM and 2.5 mM was positively dependent on PMS concentration in Figure 5b, which substantiated that the removal efficiency of CIP was limited on account of deficient PMS.

Influences of Reaction Temperature
In the CuFe 2 O 4 @BC/PMS system, the influence of reaction temperature was also investigated. As shown in Figure 5c, the degradation rate of CIP increases with an increasing reaction temperature. The initial reaction rate constants at 10, 20, 30, and 40 • C were calculated to be 0.3645 min −1 , 0.4714 min −1 , 0.6744 min −1 , and 0.8052 min −1 , respectively. Which was consistent with the previous reports that PMS could accelerate the generation of ROS under thermal activation [45]. Meanwhile, according to the Arrhenius equation (Equation (3)) based on the first-order kinetics at four different temperatures, the activation energy for the catalytic degradation of CIP by CuFe 2 O 4 @BC was calculated to be 16.2 kJ/mol (as illustrated in the inset of Figure 5c), which was much less than that of other studies (Table S3). 10 6 M −1 s −1 and 3.6 × 10 6 M −1 s −1 [49]. The oxidation activities of the generated CO3 •− was lower than that of •OH and SO4 •− , which demonstrated that HCO3 − had an obvious inhibitory effect [25]. In addition, the addition of 5 mM humic acid (HA), which represented natural organic matter in the aquatic system, decreased the degradation efficiency to 86.89% with a marginal inhibition of the catalytic activity. This might be due to the competitive effect of CIP and HA on ROS [49].

Magnetism and Reusability of the Catalyst
As presented in Figure 6a, CIP degradation efficiency could still achieve 88.01% after five circulations. The decrease in the CIP removal efficiency might be assigned to the loss of active ingredients. Meanwhile, the copper-iron ion leaching experiment showed that 0.087 mg/L iron ion and 0.214 mg/L copper ion were detected at 30 min, which was insignificant for the overall degradation process. The interaction between graphitized biochar and CuFe2O4 particles enhances the stability of the overall structure. Furthermore, the measured saturation magnetization value of the material 35.78 emu/g was displayed in Figure 6b, suggesting that CuFe2O4@BC had noteworthy magnetism and the CuFe2O4@BC material was easily recovered from the reaction solution according to the illustration in Figure 6b. Simply implement an external magnetic field without a cumbersome separation process.
where k is the rate constant, R is the molar gas constant, T is the thermodynamic temperature, Ea is the apparent activation energy, and A is the pre-exponential factor.

Influences of Chloride, Bicarbonate, Nitrate Ions, and Humic Acid (HA)
A variety of inorganic anions and natural organics such as humic acid are ubiquitous in actual wastewater, which can quickly eliminate ROS [46], thus, inhibiting the oxidation process. Accordingly, the effects of Cl − , HCO 3 − , NO 3 − , and HA on CIP removal efficiency were analyzed, as presented in Figure 5d. The degradation rate of CIP decreased to 81.52% with the presence of 5 mM Cl − and the cause might be that Cl − reacted with free radicals (•OH and SO 4 •− ) to form chlorine-containing substances with high oxidation potential such as Cl • , Cl 2 •− , and ClOH •− (k Cl − + SO4 [25,47]. When the same amount of NO 3 − was added, the inhibitory effect on CIP removal was less obvious, which was attributed to the fact that the NO 3 − could scavenge SO 4 •− with an extremely small reaction rate (5.0 × 10 4 M −1 s −1 ) [48]. Whereas, when 5 mM HCO 3 − was added, the removal efficiency of the reaction system decreased to 73.87% after 60 min. According to previous reports, HCO 3 − could react with •OH and SO 4 •− at the rate of 8.5 × 10 6 M −1 s −1 and 3.6 × 10 6 M −1 s −1 [49]. The oxidation activities of the generated CO 3 •− was lower than that of •OH and SO 4 •− , which demonstrated that HCO 3 − had an obvious inhibitory effect [25]. In addition, the addition of 5 mM humic acid (HA), which represented natural organic matter in the aquatic system, decreased the degradation efficiency to 86.89% with a marginal inhibition of the catalytic activity. This might be due to the competitive effect of CIP and HA on ROS [49].

Magnetism and Reusability of the Catalyst
As presented in Figure 6a, CIP degradation efficiency could still achieve 88.01% after five circulations. The decrease in the CIP removal efficiency might be assigned to the loss of active ingredients. Meanwhile, the copper-iron ion leaching experiment showed that 0.087 mg/L iron ion and 0.214 mg/L copper ion were detected at 30 min, which was insignificant for the overall degradation process. The interaction between graphitized biochar and CuFe 2 O 4 particles enhances the stability of the overall structure. Furthermore, the measured saturation magnetization value of the material 35.78 emu/g was displayed in Figure 6b, suggesting that CuFe 2 O 4 @BC had noteworthy magnetism and the CuFe 2 O 4 @BC material was easily recovered from the reaction solution according to the illustration in Figure 6b. Simply implement an external magnetic field without a cumbersome separation process.

Investigation of Reactive Species in CuFe2O4@BC/PMS System
To testify the types of ROS generated in the CuFe2O4@BC/PMS system and their contribution to the catalytic reaction, free radical quenching experiments were performed. Accordingly, the reaction rate constants of EtOH, TBA, BQ, and FFA with different ROS are summarized in Table S4. With the high reaction rate constants, EtOH was adopted as a quencher of both SO4 •− and •OH. In contrast, TBA mainly quenched •OH and shows a weak ability to react with SO4 •− . Thus, TBA acted as an •OH scavenger. Additionally, BQ can react with O2 •− at a high reaction rate, which was selected as an effective scavenger of O2 •− to ascertain the role of O2 •− in the CuFe2O4@BC/PMS system. Owing to both TBA and FFA exhibiting the same quenching ability •OH and FFA showed an effective scavenging effect on 1 O2, thus, FFA was chosen as a scavenger to explore the effect of 1 O2 in the CuFe2O4@BC/PMS system. As displayed in Figure 7a, after adding 500 mM EtOH and 500 mM TBA, the CIP degradation rate decreased slightly from 100.0% to 60.2% and 64.1%, respectively. In contrast, the CIP degradation rate decreased significantly from 100.0% to 36.3% with the addition of 5 mM BQ. It is noteworthy that BQ can not only quench O2 •− efficiently but also react with •OH and SO4 •− , compared with the decrease rate of the CIP degradation rate after adding EtOH. The addition of 5 m M BQ observably inhibited the degradation of CIP, ∼63.8% inhibition of CIP degradation efficiency could be obtained. Moreover, the addition of 5 mM FFA prominently reduced the performance of the composite and the CIP degradation rate decreased from 100.0% to 27.4%, which manifested a more remarkable ability to capture free radicals compared with the addition of 500 mM TBA. Considering the high inhibition efficiency of O2 •− and 1 O2, different concentrations

Investigation of Reactive Species in CuFe 2 O 4 @BC/PMS System
To testify the types of ROS generated in the CuFe 2 O 4 @BC/PMS system and their contribution to the catalytic reaction, free radical quenching experiments were performed. Accordingly, the reaction rate constants of EtOH, TBA, BQ, and FFA with different ROS are summarized in Table S4. With the high reaction rate constants, EtOH was adopted as a quencher of both SO 4 •− and •OH. In contrast, TBA mainly quenched •OH and shows a weak ability to react with SO 4 •− . Thus, TBA acted as an •OH scavenger. Additionally, BQ can react with O 2 •− at a high reaction rate, which was selected as an effective scavenger of O 2 •− to ascertain the role of O 2 •− in the CuFe 2 O 4 @BC/PMS system. Owing to both TBA and FFA exhibiting the same quenching ability •OH and FFA showed an effective scavenging effect on 1 O 2 , thus, FFA was chosen as a scavenger to explore the effect of 1 O 2 in the CuFe 2 O 4 @BC/PMS system. As displayed in Figure 7a, after adding 500 mM EtOH and 500 mM TBA, the CIP degradation rate decreased slightly from 100.0% to 60.2% and 64.1%, respectively. In contrast, the CIP degradation rate decreased significantly from 100.0% to 36.3% with the addition of 5 mM BQ. It is noteworthy that BQ can not only quench O 2 •− efficiently but also react with •OH and SO 4 •− , compared with the decrease rate of the CIP degradation rate after adding EtOH. The addition of 5 m M BQ observably inhibited the degradation of CIP, ∼63.8% inhibition of CIP degradation efficiency could be obtained. Moreover, the addition of 5 mM FFA prominently reduced the performance of the composite and the CIP degradation rate decreased from 100.0% to 27.4%, which manifested a more remarkable ability to capture free radicals compared with the addition of 500 mM TBA. Considering the high inhibition efficiency of O 2 •− and 1 O 2 , different concentrations of BQ and FFA were added to the reaction solution to verify the exact effect of O 2 •− and 1 O 2 ( Figure S4). The results manifested that the CIP degradation efficiency was remarkably inhibited with increasing BQ concentration from 1 to 5 mM, and the degradation efficiency of CIP also decreased with the addition of higher concentrations of FFA, indicating the important role of O 2 •− and 1 O 2 in the CuFe 2 O 4 @BC/PMS system.

Proposed Activating Mechanism of CuFe2O4@BC
Based on the aforementioned analyses, the proposed catalytic degradation mechanism in the CuFe2O4@BC/PMS system was presented in Figure 8. During the activating process, the peroxy bond (-O-O-) of PMS can be broken through the Fe(III)/Fe(II) and Cu(II)/Cu(I) circulation to generate SO4 •− and •OH (Equations (4) and (5)) and the Fe(III)/Cu(II) could react with PMS to produce SO5 •− with activity lower than SO4 •− (Equation (6)). Furthermore, according to the previous quenching tests [50]combined with EPR analysis, 1 O2 and O2 •− play important roles in the system. Based on the previous literature reports and the experiment results obtained in this paper, it could be inferred that there were two generation processes of the O2 •− in the CuFe2O4@BC/PMS system: (i) the defect structures of biochars could facilitate the electron transfer of O2 and then produce O2 •− (Equation (7)); (ii) with the assistance of the abundant OVs in the CuFe2O4 crystal, electrons are transferred to O2 through CuFe2O4 and then reduced to O2 •− (Equation (8)). In addition, O2 •− could not only promote the generation of 1 O2 (Equations (9) and (10)) [51] but also facilitate the decomposition of PMS to generate SO4 •− and •OH (Equations (11) and (12)). Then the O2 reacted during the degradation process and could be supplemented by Equations (11)- (13). Finally, the produced ROS attacked CIP and degraded into intermediates, which might even be mineralized into CO2 and H2O (Equation (14)). Therefore BC could improve the dispersion of CuFe2O4 which enhances the structural stability of the  Figure 7b; therefore, the SO 4 •− and •OH were considered to exist in the CuFe 2 O 4 @BC/PMS system. Analogously, the typical spectral line of DMPO-O 2 •− EPR signals was also clearly identified in Figure 7c, indicating that the O 2 •− was generated in the degradation process. Additionally, as presented in Figure 7d, the typical equal triplet signal attributed to the characteristic TEMP-1 O 2 adducts certify the existence of 1

Proposed Activating Mechanism of CuFe 2 O 4 @BC
Based on the aforementioned analyses, the proposed catalytic degradation mechanism in the CuFe 2 O 4 @BC/PMS system was presented in Figure 8. During the activating process, the peroxy bond (-O-O-) of PMS can be broken through the Fe(III)/Fe(II) and Cu(II)/Cu(I) circulation to generate SO 4 •− and •OH (Equations (4) and (5)) and the Fe(III)/Cu(II) could react with PMS to produce SO 5 •− with activity lower than SO 4 •− (Equation (6)). Furthermore, according to the previous quenching tests [50] combined with EPR analysis, 1

O 2 and O 2
•− play important roles in the system. Based on the previous literature reports and the experiment results obtained in this paper, it could be inferred that there were two generation processes of the O 2 •− in the CuFe 2 O 4 @BC/PMS system: (i) the defect structures of biochars could facilitate the electron transfer of O 2 and then produce O 2 •− (Equation (7)); (ii) with the assistance of the abundant OVs in the CuFe 2 O 4 crystal, electrons are transferred to O 2 through CuFe 2 O 4 and then reduced to O 2 •− (Equation (8)). In addition, O 2 •− could not only promote the generation of 1 O 2 (Equations (9) and (10)) [51] but also facilitate the decomposition of PMS to generate SO 4 •− and •OH (Equations (11) and (12)). Then the O 2 reacted during the degradation process and could be supplemented by Equations (11)- (13). Finally, the produced ROS attacked CIP and degraded into intermediates, which might even be mineralized into CO 2 and H 2 O (Equation (14)). Therefore, BC could improve the dispersion of CuFe 2 O 4 which enhances the structural stability of the catalyst and also facilitates electron conversion due to the excellent electrical conductivity, showing the synergistic effects between CuFe 2 O 4 and BC that could effectively catalyze PMS to degrade CIP.

Preparation of CuFe 2 O 4 @BC Catalyst
After being washed with deionized water several times, the waste mulberry branches were dried at 90 • C, ground into particles, and then pyrolyzed under oxygen-limited conditions for 2 h at 700 • C, at a rate of 10 • C min −1 . After cooling, the obtained biochar was ground and sieved for later use. CuFe 2 O 4 @BC was prepared by the modified sol-gel method as reported in the literature [52]. In brief, 2.020 g Fe(NO 3 ) 3 ·9H 2 O, 0.605 g Cu(NO 3 ) 2 ·3H 2 O, and 0.6 g prepared BC were firstly mixed in 50 mL DI water (the mass ratio of CuFe 2 O 4 /BC was optimized to be 1:1). Then the mixture was stirred in an oil bath for 3 h at 60 • C, added 3.75 g citric acid, and continue to stir for another 3 h to form homogeneous dispersion. After that, subjected the mixture to a vacuum oven to remove water at 100 • C. The resulting black sticky gel was injected into a porcelain crucible and heated in a tube furnace at 700 • C for 2 h at a heating rate of 10 • C/min in an N 2 atmosphere (60 mL/min). Then, the obtained material was washed to neutral with 1 M KOH and 2 M HCl to remove the residual impurities. Finally, the catalyst was dried at 90 • C. For comparison, naked CuFe 2 O 4 was also synthesized under the same conditions except for no addition of BC.

Catalyst Characterization
The structural morphologies and particle sizes of the as-prepared catalyst were obtained by the ultra-high resolution transmission scanning electron microscope (SEM; SU8020, Hitachi Limited, Tokyo, Japan) and the transmission electron microscope (TEM; FEI Talos F200S, FEI Company, Hillsboro, OR, USA). Types and contents of elements in material micro-area composition were analyzed by an X-ray energy dispersive spectrometer (EDS; HORIBA EX-350, Hitachi Limited, Tokyo, Japan ). The crystal structures and phases of as-prepared catalysts were characterized by an X-ray diffractometer (XRD, XD-6, Beijing Pgeneral, Beijing, China) using Cu-Kα radiation (36 kV, 20 mA). The chemical bonds and functional groups of samples were identified by Fourier transform infrared spectrometer (FTIR, Nicolet IS10, Waltham, MA, USA). X-ray photoelectron spectroscopy (XPS; Escalab 250Xi, Thermo Fisher Scientific, Inc., Waltham, MA, USA) was used to detect the surface chemical states and compositions of the elements of catalyst. The magnetic property of the CuFe 2 O 4 @BC catalyst was characterized by a vibrating sample magnetometer (SQUID-