Epsilon-MnO₂ simply prepared by redox precipitation as an efficient catalyst for ciprofloxacin degradation by activating peroxymonosulfate

Ciprofloxacin (CIP) is one of the third-generation fluoroquinolone antibiotics. Due to its good clinical efficacy and strong antibacterial activity, it has become one of the most commonly prescribed antibiotics in medicine (Anirudh Gupta 2019; Alhaddad & Amin 2022; Enyoh & Wang 2023). However, more than 75% of CIP cannot be absorbed by humans and animals, resulting in a large amount of CIP residues being released into the water environment (Huang et al. 2018a). Recently, it has been detected frequently in surface water, groundwater and even drinking water, with the CIP concentrations ranging from ng L⁻¹ to mg L⁻¹ (Chun-Hui Shen 2019; Hu et al. 2020; Omuferen et al. 2022). Unfortunately, due to the poor biodegradability and high ecotoxicity of CIP, it is impossible to effectively remove it from water treatment by various traditional sewage technologies. In recent years, it has been found that the emergence of antibiotic-resistant bacteria and resistant genes due to the extensive use of antibiotics will pose a severe threat to human health and ecosystem security (Yu et al. 2020; Falyouna et al. 2022). When the World Health Organization revised antibiotic drugs in 2017, it classified some antibiotics as ‘prudent use’ in an attempt to reduce their use. In addition to effective control at the source, developing treatment technologies to remove such antibiotics from water and wastewater systems is essential.

At present, various methods have been investigated to reduce CIP from water, such as membrane filtration (Nasrollahi et al. 2022), adsorption (Arif et al. 2022), biodegradation (Shah et al. 2022) and advanced oxidation process (AOP) (Salari et al. 2022). Among them, the AOP based on sulfate radical has aroused much concern in recent years due to its advantages of economy, high efficiency, environmental protection, safety and stability (Tian et al. 2022). Persulfate (PS) includes peroxymonosulfate (PMS) and peroxydisulfate (PDS), both of which contain –O–O– bonds in their molecules. The method for activating PS degradation is to couple and excite PS through specific physical or chemical approaches to break its –O–O– bond and produce strong oxidizing ⁠, which reacts with organic substances through dehydrogenation, electron addition, electron transfer and other ways to trigger a series of free radical chain reactions, to realize the oxidative degradation of organic pollutants. Currently, PS can be activated in various ways, such as UV activation (Ghauch et al. 2017), thermal activation (He et al. 2021a), ultrasonic activation (Gujar et al. 2023), transition metal ion activation (Fe²⁺, Mn²⁺, Cu²⁺, Ag⁺, Co²⁺, Ce³⁺, etc.) (Huang et al. 2009; Gao et al. 2018; Yoon et al. 2022) and heterogeneous catalysts activation (Tian et al. 2022; Zhou et al. 2023). UV activation, thermal activation and ultrasonic activation all require external energy. Transition metal ion activation has difficulties in recovery and secondary pollution. In contrast, heterogeneous catalytic materials have become a research hotspot due to their high efficiency, low cost and low energy consumption (Zhang et al. 2021).

Transition metal-based heterogeneous materials have been reported to be effective in activating persulfates (Rama et al. 2018; Wang & Wang 2018), but cobalt-based or copper-based materials may cause health hazards due to the immersion of toxic ions in the aqueous phase, silver-based materials are expensive, which will limit their practical application. In contrast, manganese-based materials have obvious advantages such as many valence states, high oxygen migration efficiency, abundant resources, low price, friendly environment and strong safety (Huang & Zhang 2019). Manganese oxide has been reported as an effective catalyst for ozone oxidation (He et al. 2021b; Issaka et al. 2022) and also exhibited excellent performance in catalytic degradation of pollutants by hydrogen peroxide (Hussain et al. 2021; Zhen et al. 2023).

The synthesis route was of great importance to the physical and chemical properties and structural performance of the catalyst. For decades, a series of synthesis techniques have been used to synthesize manganese oxides, such as hydrothermal (Wang et al. 2015a), solvothermal (Ren et al. 2014), sol–gel (Zhang et al. 2019), template (Huang et al. 2018b) and redox precipitation (Wei et al. 2023) methods. Hydrothermal and solvothermal synthesis was characterized by high pressure, long period and strict requirements for reactants. There were many steps and uncontrollable factors in the process of sol–gel synthesis, which was difficult to industrialize; The purity of the products synthesized by the template method was not high, and the selection of template agent was very demanding. These shortcomings limited the practical application of many manganese oxides. The synthesis method with high efficiency, low cost and simple operation has become an urgent problem to be solved in the development of new manganese oxide catalysts. Redox precipitation method effectively avoided complex working procedures and special equipment requirements, and it was worth studying because of its simple process, fast reaction speed and diverse morphology of samples.

In recent years, manganese oxides, such as MnO₂, Mn₃O₄ and MnO(OH), have attracted much attention in the field of catalysts because of their physical and chemical properties. Wang et al. (2015b) found that α-MnO₂ showed shape dependence when used to degrade phenol; Saputra et al. (2013b) concluded through research that Mn₃O₄ had better performance than Fe₃O₄ and Co₃O₄ in activating PMS to degrade pollutants. He et al. (2020) synthesized MnO(OH) with the same crystal structure to degrade 2,4-dichlorophenol and found that different morphologies would lead to different physical and chemical properties, thus providing guidance for rational design of catalysts. However, there were few studies on the activity of a series of manganese oxides with different crystal structures in environmental catalysis. At the same time, we also found that the research on MnO₂ was mostly focused on polycrystals. Epsilon-MnO₂ showed low crystallinity due to its own structural fracture (i.e. De Wolff fracture) and structural defect (i.e. micro-twin) (Wang et al. 2015a), and its catalytic performance in water treatment was rarely touched.

Herein, four different manganese (hydroxyl) oxides were prepared by hydrothermal and redox coprecipitation methods, and the influence of catalyst structure on catalytic performance was studied by systematic characterization. The degradation of CIP by PMS activated by ε-MnO₂ with low crystallinity was systematically studied to explore the green catalytic materials that were easy to prepare and highly active to degrade antibiotics.

All chemicals are used directly after purchase without any treatment. Manganese chloride (MnCl₂·4H₂O), manganese sulfate (MnSO₄·H₂O), potassium permanganate (KMnO₄), L-histidine (C₆H₉N₃O₂) and ciprofloxacin (C₁₇H₁₈FN₃O₃) were bought from Shanghai Zhongqin Chemical Reagent Co., Ltd (Shanghai, China). Anhydrous ethanol (CH₃CH₂OH), sulfuric acid (H₂SO₄), methanol (CH₃OH), tert-butanol (C₄H₁₀O), sodium hydroxide (NaOH), potassium nitrate (KNO₃), phenol, potassium dihydrogen phosphate (KH₂PO₄), humic acid (HA), sodium chloride (NaCl), sodium carbonate (Na₂CO₃) and nitrobenzol (NB) were provided by Tianjin Damao Chemical Reagent Co., Ltd (Tianjin, China). PMS (KHSO₅·0.5KHSO₄·0.5 K₂SO₄, PMS), 5, 5-dimethyl-1-pyrrole Lino-N-oxide (DMPO), 2,2,6,6-tetramethylpiperidine (TEMP) were purchased from Aladdin Reagent Co., Ltd (Shanghai, China).

The valence heterostructure manganese oxides (MnOx) were prepared by hydrothermal method. Typically, 0.79 g (5 mmol) of KMnO₄ was dissolved in 50 mL of a mixed solvent of ethanol and water, 0.99 g (5 mmol) of MnCl₂·4H₂O was added, stirred for about 30 min until the mixture was uniform and transferred to 100 mL of polytetrafluoroethylene (PTFE) liner stainless steel jacket reactor for 160° hydrothermal treatment for 12 h. The brown precipitates were washed three times with deionized water and anhydrous ethanol, respectively and then dried at 70° C for 24 h. In the experiment, the volume ratio of ethanol and water in the solvent was adjusted to 0:100, 50:50 and 100:0, respectively, to prepare nanomaterials with different crystal forms and morphologies, which were recorded as MnOx-1, MnOx-2 and MnOx-3.

Manganese oxide (MnOx-4) was synthesized by the redox coprecipitation method. 0.004 mol (0.6321 g) KMnO₄ was added to 200 mL of 0.03 mol/L MnSO₄·H₂O solution and stirred for 30 min. Afterward, the solution was placed in a collector magnetic stirrer at 75 °C and went for 5 h to obtain a black precipitate. The precipitate was washed for several times alternately with deionized water and anhydrous ethanol until the supernatant was neutral, then it was put into a 60 °C constant temperature drying oven for 24 h after suction filtration and grounded to powder after cooling, which was denoted as MnOx-4.

The microstructure of the catalysts was studied by X-ray diffractometer (XRD, X'pert pro) using the scanning angle (2θ) at 10–90° and Cu–Kα radiation at 40 kV and 40 mA. The microtopography of the catalysts was characterized by scanning electron microscopy (SEM, Apreo S) and transmission electron microscope (TEM, FEI Tecnai F 30). The sample elements and surface topography were analyzed using energy-dispersive methods (EDS, EDAX). The chemical bonds and functional groups of the catalysts were measured by a Fourier-transform infrared spectrometer (FTIR, VERTEX 70, Bruker). X-ray photoelectron spectroscopy (XPS, PHI 5702) was mainly used to characterize the elemental composition and valence state of the catalyst surface. The Brunauer–Emmett–Teller (BET) surface areas were determined by N₂ adsorption tester (JW-BK200C). Electron paramagnetic resonance spectroscopy (EPR, Bruker EMX plus) was used to identify free radicals in solution and solid oxygen vacancies. The detection parameters were as follows: microwave frequency 9.8434 GHz, scanning width 100 G, central field 3,505.00 G, modulation amplitude 1 G, time constant 5.12 ms and scanning time 60 s.

The catalytic performance of the prepared four kinds of MnOx was investigated by activating PMS to degrade CIP in water. All degradation experiments were carried out in 250 mL beakers on a constant temperature magnetic stirrer (speed 1,200 rpm). First, a certain amount of catalyst was added to the 200 mL prepared target pollutants (10 mg·L⁻¹), after the catalyst reached adsorption equilibrium with the solution, a certain amount of PMS was added to start the timing. Five millilitres (5 mL) sample was taken from the beaker at regular intervals, filtered through a 0.45 μm microfiltration membrane and quenched with methanol. The CIP concentration was determined by measuring the absorbance at the maximum absorption wavelength of 277 nm with an ultraviolet spectrophotometer (UV-2800A).

When exploring the influence of different parameters on the degradation of pollutants, the single-variable principle was used for experimental analyses. During the investigation, 0.1 mol·L⁻¹ H₂SO₄ and NaOH solutions were used to regulate the pH of the solution to study the influence of the initial pH value. To check the effect of quenchers on degradation experiments, MeOH, TBA, L-His, phenol, KH₂PO₄ and NB were selected as quenchers and set blank control at the same time. Samples were taken at regular intervals for testing after the degradation began. To explore the stability and reuse performance of the catalyst, the cyclic degradation test of the catalyst was carried out. After a single reaction, the catalyst was recovered and washed repeatedly, then put into a constant temperature drying oven at 80 °C for drying and reuse.

When the synthesized catalyst was further applied to the removal of CIP from actual water, the raw water obtained from Liujiaxia Reservoir in the upper reaches of the Yellow River was first filtered with 0.45 μm PTFE filter, which was used to dissolve CIP and configured into a solution of target concentration, and the removal effect of ε-MnO₂/PMS on CIP was observed.

The FTIR of the four manganese oxides is shown in Figure 1(b). It could be seen that the absorption peaks of the four manganese oxides appeared at 3,425 and 1,633 cm⁻¹, which could be related to the stretching vibration and bending vibration of hydroxyl. The absorption peaks of γ-MnO(OH) at 1,150 and 1,085 cm⁻¹ were attributed to the bending vibration of hydroxyl, and the absorption peaks at 2,674 and 2,080 cm⁻¹ were assigned to the stretching vibration of hydroxyl. This indicated that there were abundant hydroxyl groups on the surface of the four catalysts. The absorption peaks in the 400–800 cm⁻¹ corresponded to the vibrational absorption of Mn–O metal bonds in manganese oxides. FTIR showed that the Mn–O metal bonds had been formed in the four catalysts, and a large number of hydroxyl groups existed on the surface of the catalysts.

According to the measurement of N₂ adsorption and desorption, the four samples in Figure 2(h) presented a typical type-IV isotherm, which indicated that all four catalysts were mesoporous materials with average pore diameters ranging from 2 to 50 nm, and there were obvious H₃-type hysteresis loops between the adsorption and desorption isotherms of Mn₃O₄ and ε-MnO₂ in the range of P/P₀ of 0.4–1.0 and 0.8–1.0, respectively. It could be inferred that these two catalysts were composed of slit-like pores of flaky particles (Yang et al. 2001). As shown in Table S1, the BET surface area of ε-MnO₂ was 156.43 m²/g, which was 8.25 times that of α-MnO₂, 25.35 times of γ-MnO(OH) and 6.66 times of Mn₃O₄. The pore volume of ε-MnO₂ was 0.42 cm³/g, which was 7.5 times that of α-MnO₂, 28.49 times of γ-MnO(OH) and 2.43 times of Mn₃O₄.

It has been reported that the catalytic activity of manganese oxides of the same type was affected by the structure, especially the tunnel size and crystallinity (Saputra et al. 2013b; Huang et al. 2019). For instance, α-MnO₂ with (2 × 2) tunnel structure had a larger size and higher degradation efficiency than β-MnO₂ with (1 × 1) tunnel structure and γ-MnO₂ with (1 × 1) and (1 × 2) tunnel structure. The four catalysts prepared were α-MnO₂ with (2 × 2) tunnel structure, γ-MnO(OH) with (1 × 1) tunnel structure, Mn₃O₄ with spinel-like structure and ε-MnO₂ with (1 × 1) tunnel and (2 × 1) tunnel in a matrix of random coexistence, respectively. The degradation efficiency followed the order ε-MnO₂ > Mn₃O₄ > α-MnO₂ > γ-MnO(OH), which was inconsistent with the reported results. It might be that the mesoporous nanostructures of ε-MnO₂ exhibited more active sites due to its defects. The ε-MnO₂ with the highest reaction efficiency was found to have the lowest crystallinity among the four catalysts, which was also different from the conclusions in the literature. Combined with the EPR spectrum of ε-MnO₂ in Figure 3(d), a sharp symmetric peak appears at g = 2.011, which could be considered as the solid defect existing in the structure of ε-MnO₂ led to the formation of oxygen vacancy (Zhao et al. 2020), thus improving the catalytic activity.

By comparing the SEM of four nanoparticles, it was found that the reactivity of nanorods and spherical nanoparticles was higher than that of nanorods and spherical nanoparticles, which was consistent with the findings of Wang and Jing et al. (Wang et al. 2015b; Jing et al. 2017), that is, the nanoflower shape shows the greatest ability to activate PMS, which was conducive to the heterogeneous reaction on the catalyst surface.

The order of BET surface area and pore volume was ε-MnO₂ > Mn₃O₄ > α-MnO₂ > γ-MnO(OH), which was consistent with the results of CIP degradation by activated PMS. The larger surface area meant more exposed active sites, and the larger void volume could reduce the diffusion resistance, thus showing better adsorption performance and catalytic performance (Liu et al. 2020), which was beneficial to the activation of PMS.

In addition, it was reported that Mn(III) content played an important role in Mn-based catalysts (Meng et al. 2014), because it was generally believed that was generated by Mn (III) reaction (Equation (3)), while was generated by Mn (IV) reaction (Equation (4)), and was higher than oxidation–reduction potential (Wen-Da et al. 2016). Unfortunately, the content of Mn(III) in the four catalysts did not show a significant correlation with the degradation efficiency of pollutants. However, it was found that the Mn(III)/Mn(Ⅳ) ratio in the catalyst was positively correlated with the degradation efficiency of CIP, which might be related to the redox reaction occurring during the degradation of manganese oxides.

In summary, it was not difficult to see that the chemical properties and reactivity of manganese oxides were highly dependent on their structure. High BET surface area could provide more reaction sites; ε-MnO₂ with low crystallinity might have the best activation performance because of its own defects leading to the formation of oxygen vacancy. The oxidation capacity of manganese oxides was positively correlated with the ratio of Mn(III)/Mn(Ⅳ) content, which might indicate that high Mn(III)/Mn(Ⅳ) content could promote the redox reaction in the process of manganese oxides activation of PMS.

The process of ε-MnO₂ activating PMS to degrade CIP was influenced by many reaction factors. The effects of catalyst dosage, PMS dosage, initial pH of solution, reaction temperature, common anions in water and actual water on CIP degradation were discussed, and the optimal reaction parameters of the system were determined to provide a reference for engineering practice.

Figure s1a showed when the dosage of ε-MnO₂ was 0.05, 0.1, 0.2, 0.4, 0.6 and 1.0 g·L⁻¹, the degradation efficiencies of CIP after 30 min of reaction reached 66.7, 83.8, 92.4, 95.3, 95.9 and 97.1%, respectively. Obviously, the increase of catalyst concentration could promote the degradation of CIP, because with the increase of catalyst content in the reaction system, more active sites would be generated in the system, and more active components would be produced to remove more CIP. However, when the catalyst dosage was higher than 0.2 g/L, the removal rate of CIP increased slowly. Therefore, 0.2 g/L was selected as the catalyst concentration of the system.

There were often various inorganic anions and natural organic matter in actual water, such as Cl⁻, ⁠, ⁠, etc., which would affect the activation efficiency of PMS by quenching free radicals and competing with the active site of the catalyst. Humic acid (HA) was the most common natural organic matter in water, mainly consisting of aromatic ring, hydroxyl group and phenolic hydroxyl group, which might adsorb on the surface of the material to occupy the active site or compete for free radicals in water, thus affecting the removal effect of pollutants. Figure S1e shows the effect of 5 mmol/L Cl⁻, ⁠, and HA, as well as the Yellow River water as solvents on the degradation of CIP by ε-MnO₂-activated PMS. Only and HA among them had inhibitory effects on the system, the other had little effect. As shown in the figure, significantly inhibited the degradation of CIP in the system, which might be due to the competitive reaction of both and CIP, resulting in the formation of intermediates (Equation (11)) with low or no oxidation performance (Lin & Tsai 2019). In addition, the addition of would increase the alkalinity of the solution, which was not conducive to the degradation of CIP according to the research conclusion of the influence of PH value. HA had a certain inhibitory effect on CIP degradation, which might be that HA occupied the active site of MnO₂ and affected its binding with PMS to form reaction intermediates, or that HA competed with CIP to react with active substances, leading to a decrease in CIP degradation efficiency. In addition, ɛ-MnO₂/PMS also had excellent removal performance in CIP solution with Yellow River water as solvent, which indicated that ε-MnO₂/PMS system had a good application prospect.

In order to explore the stability of the catalyst, the prepared catalyst ε-MnO₂ was tested for cyclic degradation. It was apparent from Figure S2 that ε-MnO₂ was relatively stable and still exhibited excellent catalytic performance after four cycles of degradation experiments. The removal rate of CIP in the fourth cycle in 30 min was 77.4%, 15.7% lower than that in the first cycle. The reasons could be attributed to two aspects: with the increase in catalyst cycle degradation times, the active components on the catalyst surface might be lost, resulting in a gradual reduction of active sites. In addition, the small molecule intermediates generated in the process of organic degradation gathered on the catalyst surface, which hindered the contact of the catalyst with PMS and CIP, thus reducing the removal rate of CIP.

To explore the main reaction active species in the system, tert-butanol (TBA), methanol (MeOH) and L-histidine (L-His) were used as quenchers to investigate the effect of CIP degradation in the ε-MnO₂/PMS system. According to the literature (Hao et al. 2019), the reaction rate constants of MeOH and TBA with •OH and were shown in Table s4, in which the reaction rate constant of MeOH and •OH was 50 times that of ⁠, while the reaction rate constant of TBA and •OH was 1,000 times that of ⁠. When the solution contains two free radicals simultaneously, it could be considered that TBA only quenched •OH, while MeOH quenched •OH and ⁠. The contribution of the two free radicals was judged by comparing the effects of two kinds of quenching agents on the degradation of CIP in the system. L-histidine could be used to quench singlet oxygen (Chen et al. 2018) with a second-order rate constant of 3 × 10⁷ (D₂O, pD = 7) (Wilkinsan 1981). So MeOH (3 M), TBA (3 M) and, L-His (0.2 mM) were used to identify the presence of active species such as ⁠, •OH and, ¹O₂ in the system, respectively.

It has been reported that the –OH group was essential in the heterogeneous activation of PMS by metal-based catalysts (Liu et al. 2015). In order to determine the role of the –OH group on the catalyst surface, KH₂PO₄ was added to the system as a masking agent to study its effect on CIP degradation. KH₂PO₄ could easily combine with transition metals to mask the –OH group on the catalyst surface (Khan et al. 2017), preventing the interaction of the –OH group with ⁠. After 10.0 mM was added to the ε-MnO₂/PMS system, the degradation efficiency of CIP decreased from 91.9 to 15.9%, which indicated that the –OH group on the catalyst surface played an important role in controlling the interaction between peroxides and catalysts.

To further determine the production of reactive oxygen species, EPR tests were performed on ε-MnO₂/PMS system and PMS system alone using DMPO and TEMP as spin trapping agents. and •OH could form stable adducts with DMPO, which could be detected by electron paramagnetic resonance (EPR) spectroscopy, so DMPO was used as a trapping agent for and •OH, and TEMP was a spin capture agent for ¹O₂ (Xu et al. 2021). As shown in Figure 5(b), the mixed signals of DMPO- and DMPO-•OH and the triplex signal of TEMP-¹O₂ (1:1:1) were observed in the spectrum of the ε-MnO₂/PMS system, but no obvious signal was detected when only PMS existed indicating that PMS produced ⁠, •OH and ¹O₂ in solution under the activation of ε-MnO₂, and the quantity of was higher than that of •OH and ¹O₂ in terms of relative spectral intensity.

By comparing the XPS spectra before and after the ε-MnO₂ reaction (Figure 3(a)–3(c)), it was found that the manganese content of different valence states changed, Mn(III) decreased with Mn(Ⅳ) increasing, Mn(III)/Mn(Ⅳ) decreased from 5.85 to 4.82. The relative content of O_ads also decreased, and the O_ads of ε-MnO₂ participated in the reaction. The proportion of Mn and O species changed slightly before and after ε-MnO₂ reaction, indicating that ε-MnO₂ could regenerate itself and maintain high activity while participating in the reaction.

An environmentally friendly catalytic material, ε-MnO₂ was synthesized by the redox precipitation method and used to activate PMS to degrade the antibiotic CIP. The results showed that the CIP degradation rate could reach 93% within 30 min after the optimization of reaction parameters. The high activity of ε-MnO₂ was related to its physicochemical properties of the surface, such as the specific surface area, the oxygen vacancy formed by the defect and the Mn(III)/Mn(Ⅳ) content ratio. After four cycles of degradation experiments, the efficiency decreased by 13%, which confirmed that the catalyst had good stability. The quenching experiment and EPR result showed that sulfate radicals, hydroxyl radicals and singlet oxygen jointly participated in the degradation of CIP. The main free radical was ⁠. The hydroxyl group on the surface of ε-MnO₂ was helpful to the contact between PMS and the catalyst, which could promote the activation of PMS. The redox reaction between Mn⁴⁺ and Mn³⁺ occurred during the activation of PMS by ε-MnO₂, which could realize the self-repair in situ and recycle of the catalyst, which provided the theoretical basis and technical support for the removal of antibiotics and had great potential for practical engineering application.

This study was financially supported by Gansu Province Higher Education Industry Support Plan Project (2022CYZC-40), Natural Science Foundation of Gansu Provincial Department of Science and Technology (22JR5RA316), Major Scientific and Technological Achievements Cultivation Project of Lanzhou Jiaotong University (2022CG07) and Tianyou Innovation Team of Lanzhou Jiaotong University (TY202005).

All relevant data are included in the paper or its Supplementary Information.

The authors declare there is no conflict.