Activation of peroxymonosulphate using a highly efficient and stable ZnFe2O4 catalyst for tetracycline degradation

Tetracycline (TC) is a widely used antibiotic that adversely affects ecosystems and, therefore, must be removed from the environment. Owing to their strong ability to oxidise pollutants, including antibiotics, and selectivity for these pollutants, an improved oxidation method based on sulphate radicals (SO4·−) has gained considerable interest. In this study, a novel technique for removing TC was developed by activating peroxymonosulphate (PMS) using a ZnFe2O4 catalyst. Using the co-precipitation method, a ZnFe2O4 catalyst was prepared by doping zinc into iron-based materials, which increased the redox cycle, while PMS was active and facilitated the production of free radicals. According to electron paramagnetic resonance spectroscopy results, a ZnFe2O4 catalyst may activate PMS and generate SO4·−, HO·, O2·−, and 1O2 to eliminate TC. This research offers a new method for creating highly effective heterogeneous catalysts that can activate PMS and destroy antibiotics. The study proposes the following degradation pathways: hydroxylation and ring-opening of TC based on the products identified using ultra-performance liquid chromatography-mass spectrometry. These results illustrated that the prepared ZnFe2O4 catalyst effectively removed TC and exhibited excellent catalytic performance.

Among the transition-metal-based catalysts, Fe-based catalysts are frequently utilised to trigger PMS because of their high effectiveness, safety, non-toxicity, and low price.Fe-based catalysts that can activate PMS to breakdown organic pollutants include magnetic Fe 3 O 4 , α-Fe 2 O 3 , γ-Fe 2 O 3 , and δ-FeOOH 23 .Although low valence Fe 2+ and Fe 0 are readily oxidised, the slow cycle of Fe 2+ /Fe 3+ results in low PMS activation efficiency 24 .Therefore, to eliminate these negative effects, bimetallic oxide catalysts have been proposed as substitutes for improving catalytic activity and stability.In the past, there were many studies on the degradation of organic pollutants by persulfate using Fe-Mn, Fe-C O bimetallic oxides as catalysts 20,22,24 , but the leaching of C O and Mn ions will cause secondary pollution to the environment, therefore, this study focused on environmental pollution-free Fe-Zn bimetallic oxide catalysts.
In this study, ZnFe 2 O 4 catalyst was synthesised using the co-precipitation reaction, and its performance in TC degradation was evaluated.The primary goals of this study were to (1) investigate the physicochemical characteristics of the catalyst and discuss its catalytic effectiveness in PMS systems, (2) investigate the effect of various environmental conditions on TC degradation, and (3) examine the reactive oxygen species generated in the ZnFe 2 O 4 /PMS system and elucidate the TC degradation process.Our study provides a new perspective on finding improved, inexpensive, and eco-friendly catalysts.

Synthesis of ZnFe 2 O 4 .
Using a co-precipitation method, ZnFe 2 O 4 was synthesised.First, FeCl 3 •6H 2 O and ZnCl 2 were ultrasonically dispersed in 50 mL of DI water for 30 min.The resultant mixture was then placed in a water bath at 50 °C and stirred magnetically.NaOH was added to the solution until the pH reached 9, and the suspension was continuously stirred and maintained at 50 °C for 1 h.The ZnFe 2 O 4 composites were then centrifuged, separated, and washed with ethanol and ultrapure water to reach a pH of 7. The obtained composites were dried at 60 °C for 24 h, pulverised, passed through an 80-mesh sieve, and then calcined at 600 °C for 2 h under N 2 gas.
Characterization of the catalyst.The X-ray diffraction (XRD) pattern were measured between 5 and 80° at 40 kV and 250 mA.The morphology of the ZnFe 2 O 4 composites was obtained using scanning electron microscopy (SEM; TESCAN MIRA LMS) equipped with energy-dispersive X-ray spectroscopy (EDS).X-ray photoelectron spectroscopy (XPS) was performed using a Thermo Scientific K-Alpha photoelectron spectrometer system.

Catalytic activity test.
To evaluate the catalytic performance and reusability of ZnFe 2 O 4 , we determined its TC removal efficiency.To achieve an equilibrium between adsorption and desorption, the catalyst (0.2 g/L) was uniformly suspended in the TC solution (20 mg/L) at 25 °C and continuously shaken for 30 min.To initiate the catalytic oxidation, PMS was introduced into the reaction solution.We collected 2 mL of sample solutions at a time, filtered them through a 0.22 mm membrane, and measured their TC content using an ultraviolet-visible spectrophotometer at 360 nm.An ultra-performance liquid chromatography-mass spectrometry (UPLC-MS) system was used to identify the TC oxidation intermediates.Reactive oxygen species (ROS) were detected using electron paramagnetic resonance (EPR).After the reaction, the catalyst was collected, washed with ethanol and water, and used in a subsequent cycle to determine its suitability for reuse.
The SEM was used to examine the surface morphology and particle size distribution of ZnFe 2 O 4 .Figure 2 shows an SEM image of a synthetic ZnFe 2 O 4 catalyst at various magnifications.Evidently, ZnFe 2 O 4 nanoparticles with hexagonal and spherical structures are uniformly dispersed.After the reaction, the surface pores of the catalyst become larger.The homogenous distribution of the ZnFe 2 O 4 particles may help in establishing contact between the catalyst and oxidant, facilitating the activation of PMS 25 .Moreover, the surface of ZnFe 2 O 4 contains several pores, which help adsorb TC on the catalyst surface.

TC degradation in various systems. The degradation of tetracycline under different Fe-based catalysts
was illustrated in Table 1.TC removal efficiency was higher (63%) than other Fe-based catalysts, when Fe-Zn catalysts was employed in the same reaction conditions.So, the TC removal efficiencies were investigated in various systems to evaluate the catalytic effectiveness of ZnFe 2 O 4 -activated PMS for TC degradation (Fig. 3a).ZnFe 2 O 4 alone was found to remove 15% of TC within 30 min because of TC adhering to the large surface area of the catalyst.Despite being a powerful oxidant (E = 1.82 V), PMS alone could remove only 30% of the TC in a 60-min period due to insufficient catalyst for PMS activation and oxygen radical production.The TC removal   Catalytic activity and reaction parameter effects.The effects of the catalyst amount, PMS addition amount, and starting pH on TC decomposition were investigated to determine the catalytic oxidation efficiency of ZnFe 2 O 4 .The TC degradability improved as the catalyst quantity was increased from 0.1 to 0.5 g L −1 , as shown in Fig. 3b.With a catalyst concentration of 0.1 g L −1 , 53% TC was removed in 60 min.This is attributed to the increased number of active sites available for PMS activation as the catalyst dose increases to facilitate TC degradation 26 .When the catalyst concentration was increased to 0.2 g L −1 , TC removal increased to 78% in 60 min.However, no discernible improvement was observed on further increasing the catalyst amount to 0.5 g L −1 , which may be attributed to the ability of the catalyst to bind excess radicals that tend to aggregate [27][28][29][30] .
Figure 3c shows the effect of the PMS concentration on the TC removal efficiency.Using 20 mg L −1 PMS, the TC elimination was 42% within 60 min and reached to 78% when using 100 mg L −1 PMS.This could be because increasing the PMS concentration increases the contact between PMS and the catalyst, thus producing more free radicals 31 .Nevertheless, the effectiveness of TC elimination decreased when the PMS concentration was increased further.Because the excess PMS could quench SO 4 • − and HO• to form SO 5 • − (Eqs. 1 and 2) with weak oxidation ability 32 , the produced SO 4 • − or SO 5 • − should also have a self-quenching ability (Eqs.3 and 4) [33][34][35][36] .Therefore, in future studies, 100 mg/L PMS should be used as the optimal concentration.
(1) www.nature.com/scientificreports/ A key factor influencing TC removal is the initial pH of the reaction solution.Figure 3d shows that the TC removal efficiencies within 60 min were 85%, 81%, 78%, 73%, and 64% at different pH values of 3.0, 5.0, 7.0, 9.0, and 11.0, respectively.According to these findings, ZnFe 2 O 4 -activated PMS could dissolve PMS over a broad pH range, and its removal effectiveness decreased with increasing pH.Generally, SO4• − , which can be manufactured in large quantities for TC degradation, is the primary active species under acidic conditions (Eqs. 5 and 6) 37,38 .However, SO 4 • − tends to react with OH − to form HO• under alkaline conditions (Eq.7) 39 .To dissociate TC chemical bonds, the oxidative potential of the HO• radical should be smaller than that of the SO 4 • − radical.A higher concentration of OH − can also cause HO• to interact with OH − , resulting in radical annihilation and reduced TC degradation efficiency.

Effects of different anions on TC degradation. Water and wastewater contain various inorganic anions
that affect tetracycline removal.Therefore, the effects of Cl − , CO 2 3 , and H 2 PO − 4 on the rate of TC degradation in the ZnFe 2 O 4 /PMS system were studied.Figure 4a shows that 1 mM of Cl − had a slight effect on TC degradation; however, 5-10 mM of Cl − could improve TC removal efficiency to 87%.Furthermore, high levels of Cl − may transfer electrons to PMS, resulting in sulphate radicals and superabundant chlorine species (Eqs.8 and 9) [40][41][42][43] , which may participate in the TC degradation process 44 .
Similarly, Fig. 4c shows that H 2 PO 4 -degrades TC rapidly, which may be due to the transformation of SO 4 • − into the more reactive H 2 PO 4 • − as shown in(Eqs.11 and 12) 48 .

Reusability of ZnFe 2 O 4 in catalytic reaction.
The most crucial factor in practical applications is the capacity of the catalyst to be reused.To investigate the reusability of ZnFe 2 O 4 , four cycling runs were performed under ideal experimental conditions.As shown in Fig. 5, the TC degradation efficiencies reduced from 77 to 66% in 60 min after 4 cycles, indicating the good reusability of the ZnFe 2 O 4 catalyst.A minor metal ion overflow on the catalyst may have decreased its activity.Furthermore, TC decomposition may have been hampered by intermediate products absorbed by the catalyst 49 .
Catalytic mechanism.To explore the catalytic mechanism of ZnFe 2 O 4 , ROS involved in the ZnFe 2 O 4 /PMS system were investigated using EPR spectroscopy.DMPO was used to capture SO 4 • − , HO•, and O 2 • − using spin trapping, and TEMP was used to detect 1 O 2 .As shown in Fig. 6a, the DMPO-HO• and DMPO-SO 4 • − adducts showed their characteristic peaks when the time was increased from 0 to 10 min.Moreover, the DMPO-O 2 • − adduct signal in Fig. 6b indicates that O 2 • − may also be involved in TC degradation.Moreover, the TEMP-1 O 2 adduct signal was detected at 10 min, implying the presence of 1 O 2 in the reaction (Fig. 6c).These findings demonstrated that PMS might be triggered by ZnFe 2 O 4 producing some active substance that removes TC.
XPS was used to analyse changes in Zn and Fe valence states in untreated and treated ZnFe 2 O 4 catalysts to further investigate their role in PMS activation.The C 1s, O 1s, Fe 2p, and Zn 2p peaks of both new and used catalysts in Fig. 7 indicate the good stability of ZnFe 2 O 4 .The peaks at 284.8, 530.8, 711.5, and 1021.6 eV in Fig. 7a correspond to C 1s, O 1s, Fe 2p, and Zn 2p, respectively.Table 2 shows the relative element contents before and after the reaction.The C 1s orbital in the samples before and after the reaction has similar components, which can be identified as C-O, C-C, O=C-O, and C=O based on the peak patterns and binding energies of 286.3, 284.8, 288.9, and 287.2 eV, respectively (Fig. 7b and Table 3).In Fig. 7c, the energy difference between the spin-orbit splitting peaks (2p3/2 and 2p1/2) is approximately 23 eV, and the spectral peak area ratio (2p3/2:2p1/2) is approximately 2:1.The energy position of Zn 2p spectral peaks and the database suggest that 1021.9 eV should (4)  correspond to ZnO.Based on the Fe 2p spectrum in Fig. 7d, the Fe species in the catalyst should be Fe 3+ with the lowest peak intensity at 708.9 eV.The peaks at 714.5 and 719.3 eV are surface and satellite peaks of the catalyst, respectively, whereas those at 710.0, 711.0, 712.0, and 713.0 eV correspond to the four typical multiple cleavages of Fe 3+ with relative contents listed in Table 4. Based on these results, the valence states of Fe and Zn in the catalyst have not changed substantially.However, following the reaction, the carbon content rose, whereas the concentrations of Fe and Zn decreased.Although the catalyst contains only Fe 3+ , its content at a low binding energy increased after the reaction.
Degradation pathways of TC.To elucidate the TC decomposition process, the main products were qualitatively analysed using the ESI Q Orbitrap HRMS.

Conclusions
The co-precipitation method was used to synthesise a highly active ZnFe 2 O 4 catalyst, which was subsequently used as a PMS activator to degrade TC.The ZnFe 2 O 4 catalyst was studied using XRD, SEM, and XPS.The effects of the changes in the amount of catalyst, PMS concentration, and initial pH on TC decomposition efficiency were studied under various conditions.Under the optimised reaction conditions, the TC degradation efficiency reached 78%.Inorganic anions (H 2 PO 4 -H 2 PO − 4 , CO 2− 3 , and Cl -) can promote TC degradation to certain extent.In the cyclic experiments, ZnFe 2 O 4 was found to be catalytically active and stable.In addition, the EPR and XPS results revealed the presence of numerous active substances, including SO 4 • -, HO•, O 2 • -, and 1 O 2 , in the TC mineralization process.The potential TC degradation mechanisms of the ZnFe 2 O 4 /PMS system were hypothesised to depend on the products identified by UPLC-MS.These results demonstrated that the prepared ZnFe 2 O 4 catalyst effectively removed TC and exhibited excellent catalytic performance.

Figure 2 .
Figure 2. SEM images of ZnFe 2 O 4 particles before and after the reaction.Higher magnification suggests the presence of agglomerated nanoparticles.

Figure 8
shows the possible degradation pathways discussed below.The initial compound (TC) has an ion peak at m/z 445, corresponding to the proton-ionised form of [M + H] + .The first possible reaction pathway involves the formation of the m/z 461 product via oxidative hydroxylation of the "A" ring of TC, m/z 477 via hydroxylation of the "D" ring, and m/z 449 via oxidative "D" ring opening and removing the carbonyl group.Further oxidation could occur when the C-C bonds are broken and side chains are removed to yield m/z 378 and 394.In addition, m/z 366 could be obtained via the oxidative ring opening of the "B" ring.The second possible reaction pathway involves the formation of product m/z 461 via hydroxylation of the "D" ring, product m/z 495 via the "D" ring opening, and products m/z 376 and 422 via oxidative breaking of the C-C bonds.The product m/z 366 was then obtained by oxidising the "B" ring and removing the carboxyl group.The third possible reaction channel involves the TC breakdown via the C-N bond and demethylation of dimethylamine to procure m/z 431, the hydroxylation of the "A" ring of TC to obtain m/z 463, and the oxidative opening of the "D" ring to procure m/z 364.The splitting of the rings to form tiny molecules of acids and amines, as well as H 2 O, CO 2 , NO − 3 , and NH + 4 , indicate oxidative breakdown and complete degradation of the compounds.

Table 2 .
Relative content of the elements in the samples/at.%.

Table 3 .
Relative content of each chemical bond of C1s/at.%.

Table 4 .
Relative content of each chemical bond of Fe 3+ /at.%.