Insight into the performance and mechanism of magnetic Ni_0.5Cu_0.5Fe₂O₄ in activating peroxydisulfate for ciprofloxacin degradation

In recent years, the harm to human health and the environment caused by the abuse of antibiotics has attracted extensive attention (Qin et al. 2021). A large number of quinolone antibiotics, such as Norfloxacin, Ofloxacin, Ciprofloxacin and Enoxacin, coming from human excreta, wastes from agricultural food, animal production and aquaculture, industrial wastewater directly used in plants, and the pharmaceutical industry, have entered the environment and become one of the new important pollutants in the water environment. Ciprofloxacin is one of the third generation fluoroquinolones with the strongest antibacterial activity and is widely used in aquaculture or animal feeding, and also as a drug for prevention and treatment. However, after using this kind of drug, it will accumulate in animals' bodies and enter consumers through the food chains and threaten human health. At the same time, the incomplete metabolism of the drug will enter the environment, resulting in environmental pollution and huge ecotoxicological effects (Avc et al. 2020; Chow et al. 2021).

Nowadays, various traditional technologies for the removal of organic pollution from aquatic environments, such as adsorption (Avc et al. 2020), coagulation/flocculation (Zhao et al. 2020), photocatalysis (Tho et al. 2020), biodegradation (Li et al. 2021), etc., have been investigated. However, many shortcomings remain including poor efficiency, membrane clogging, high energy consumption, the transmission of pollution from one phase to another, and the production of secondary waste (Amini & Mengelizadeh 2020). In recent years, a popular method, Advanced Oxidation Technology (AOP) based on sulfate radical (SO₄^−·), for treating refractory organic pollutants in wastewater has emerged, where SO₄^−· was produced by the transfer of peroxydisulfate electrons in the activation process. In contrast to the hydroxyl radicals produced during the Fenton reaction, SO₄^−· has the following advantages: high redox potential (E₀ = 2.5–3.1 V); long half-life (30–40 μs); high stability in solution; wide pH application range and stronger ability to mineralize organic pollutants (Xia et al. 2020).

It is reported that there are many ways to effectively activate persulfate (peroxymonosulfate/peroxydisulfate), heat (Hu et al. 2019), ultrasound (Kermani et al. 2020), transition ions (Guo et al. 2020), ultraviolet radiation (Hsieh et al. 2021), and electrolysis-activated (Wang et al. 2021), but as a homogeneous activation system these methods require more energy consumption. It is found that magnesium ferrite lack of the above mentioned disadvantage and had enhanced catalytic performance for pharmaceutical products and organic dyes mineralization (Ivanets et al. 2020, 2021). Bimetallic and trimetallic iron-based systems belonging to the family of spinel with the molecular structure of MFe₂O₄ (M can be Ni, Zn, Mn, Cu, etc.) have been widely used (Gupta et al. 2020; Alhamd et al. 2021). Based on these studies and the high efficiency of bimetallic iron-based materials, researchers have recently paid more and more attention to trimetallic iron-based materials to ensure multifunctional, excellent stable catalysts, and high efficiency (Awad et al. 2019). Ni_xCu_yFe₂O₄ as a heterogeneous catalyst having many advantages of electronic properties, recyclability, easy separation, excellent photocatalytic activity, and the structure of polymetallic Ni_xCu_yFe₂O₄ catalysts is more stable, and different metals can produce synergy to improve their catalytic performance. Ni_xCu_yFe₂O₄ nanao-catalyst must show an excellent ability in activating peroxydisulfate, and can be used as a reusable catalyst in the advanced oxidation process based on sulfate radical in a wide range of pH value, for it has good magnetic separation performance. Despite the unique properties of Ni_xCu_yFe₂O₄, no study has been done on its catalytic performance for the activation of PDS to degrade ciprofloxacin.

Therefore, the main purpose of this study is to synthesize Ni_0.5Cu_0.5Fe₂O₄ by co-precipitation, an easy method to be operated, and investigate its catalytic performance in the oxidation of CIP by activated peroxydisulfate. The characterization of Ni_0.5Cu_0.5Fe₂O₄ was determined using different techniques. The effect of operating conditions including metal ratio, pH, catalyst dosage, PDS dosage, pollutant concentration, temperature, and interference ions in the degradation process of CIP was investigated. The free radicals in the system were determined by capture experiments, and proposed a possible degradation mechanism. Finally, the stability and recyclability of the catalyst were proved by cyclic experiments.

Iron (III) chloride hexahydrate (FeCl₃·6H₂O, AR), copper sulfate pentahydrate (CuSO₄·5H₂O, AR), nickel sulfate hexahydrate (NiSO₄·6H₂O, AR), ethanol (EtOH), tertiary butanol (TBA), sodium persulfate (Na₂S₂O₈), hydrochloric acid (HCl) and sodium hydroxide (NaOH), sodium chloride (NaCl), sodium nitrate (NaNO₃), sodium bicarbonate (NaHCO₃) were purchased from Sinopharm Chemical Reagent Co., Ltd. Ciprofloxacin hydrochloride (C₁₇H₁₈FN₃O₃·HCl, AR) was obtained from Aladdin Reagent Co., Ltd. In addition, deionized water was used during the whole experiment.

The experimental process of co-precipitation synthesis of Ni_0.5Cu_0.5Fe₂O₄ is shown in Figure 1. In brief, 0.540 g FeCl₃·6H₂O, 0.125 g CuSO₄·5H₂O and 0.131 g NiSO₄·6H₂O were weighed and dissolved in 10 mL of deionized water respectively, and stirred for 30 min. Then the above solution was transferred to a 250 mL three-neck flask and stirred constantly for 2 h at 80 °C. The pH value of the solution was maintained at 11 using 4.00 mol/L of NaOH. Then the mixture was aged at 100 °C. The solid was filtered with a filter membrane, washed to neutral with deionized water, and then dried and ground for standby. To better compare and investigate the catalytic activity of Ni_0.5Cu_0.5Fe₂O₄, CuFe₂O₄, NiFe₂O₄ and Fe₃O₄ were prepared through a similar process.

Fourier-transform infrared (FT-IR, Nicolet Magna-IR 550) spectroscopy using KBr as the reference was collected within the wavelength range of 400–4,000 cm⁻¹. The crystal properties of the as-prepared samples are characterized by powder X-ray diffraction (XRD, Philips PW1730) with Cu Kα X-ray irradiation at 40 kV and 40 mA. The morphology and element analysis of Ni_0.5Cu_0.5Fe₂O₄ was observed and determined on a scanning electron microscope equipped with an energy-dispersive X-ray (SEM-EDX, MIRA3-XMU). The nanostructure was examined by transmissions electron microscopy (TEM, JEOL JEM 2100F). The specific surface areas are characterized via N₂ adsorption-desorption, the Brunauer-Emmett-Teller (BET) and Barrett-Joyner-Halenda (BJH) methods. The magnetic analysis is carried out by using a vibrating sample magnetometer (VSM, 7400, Lake Shore, USA).

The catalytic activity of the as-prepared catalysts is determined by the degradation efficiency of CIP (20.0 mg/L, 100 mL). The reaction begins at 25 ± 2 °C in a 250 mL round bottom flask. The pH of the system was adjusted using HCl and NaOH (pH = 3.00–10.0). Then the catalyst was suspended into the system with the dosage range 0.100–0.700 g/L, and stirred for 60 min to reach adsorption-desorption equilibrium. Then a certain dosage of PDS (2.00–9.00 mmol/L) was added to be activated for producing active free radicals. 4.00 mL samples were taken every 10 min, and the solid-liquid was filtered with 0.45 filter membrane to obtain clear liquid to be determined.

To identify the free radicals in the reaction process, the capture experiments were carried out. The method is the same as mentioned above, besides adding additional ethanol (EtOH, 10.0 mmol/L) or tertbutanol (TBA, 10.0 mol/L) before the catalytic degradation process for trapping SO₄^−· and ^·OH.

The concentration of CIP during the reaction was measured by UV-Vis spectrophotometer at 272 nm wavelength. Total organic carbon (TOC) was determined by the TOC analyzer. The concentrations of released metals on the catalyst surface in the reaction solution were measured by inductivity coupled plasma-mass spectrometry (ICP-MS). A gas chromatograph/mass spectrometry (GC-MS) analysis was performed to determine the intermediates in CIP degradation process.

SEM-EDX and TEM were used to investigate the surface morphology, chemical composition and nanostructure of the Ni_0.5Cu_0.5Fe₂O₄ nano-particles, and the results are shown in Figure 2. As detected, the Ni_0.5Cu_0.5Fe₂O₄ has a particle size <100 nm. The presence of Ni, Cu, Fe and O elements in Ni_0.5Cu_0.5Fe₂O₄ catalyst were found in Figure 2(b).

FT-IR is shown in Figure 3(a), the peak around 3,400 cm⁻¹ can be attributed to the stretching vibration peak of O-H. Two main ferrite characteristic bands were detected: octahedral metal stretching inherent in the low-frequency band (415–455 cm⁻¹) and tetrahedral stretching vibration in the high-frequency band (570–630 cm⁻¹) (Wahaab et al. 2020).

XRD was further conducted to study the crystallinity of the Ni_0.5Cu_0.5Fe₂O₄ nano-particles, and Figure 3(b) indicates that the XRD patterns were indexed to pure cubic spinel structure with a change lattice parameter of ‘a’ and its space group: Fd-3 m (277). According to ICDD card for Ni and Cu ferrites, the peaks appeared at 2θ range of 31.22°, 35.81°, 38.70°, 43.51°, 53.52°, 57.44° and 63.20° can be attributed to the (220), (311), (320), (400), (422), (511) and (440) major lattice planes in the XRD patterns, which confirms the formation of spinel cubic structure.

The specific surface area and pore volume of the Ni_0.5Cu_0.5Fe₂O₄ nano-particles were analyzed by N₂ adsorption-desorption isotherm experiment in Figure 3(c) and 3(d). The specific surface area of the tested samples is 131 g/m². And through the BJH experiment results, we can see that the tested samples have mesoporous structure, of which the pore diameters are less than 10.0 nm. Meanwhile, the as-prepared catalysts showed H₃ hysteresis ring (P/P₀ > 0.400) and IV N₂ adsorption-desorption isotherms, which further proved that these samples had mesoporous structure, according to the IUPAC classification.

Figure 4(a) shows the degradation efficiency of CIP with different catalysts (Fe₃O₄, CuFe₂O₄, NiFe₂O₄, and Ni_0.5Cu_0.5Fe₂O₄) to activate PDS. It is obvious that the degradation efficiency and kinetic rate are very low (6.50% and 1.30 × 10⁻³min⁻¹), without adding any catalyst, only PDS exists. When catalysts were added, the final degradation efficiency and kinetic data were 23.2% and 8.60 × 10⁻³ min⁻¹ for Fe₃O₄; 61.8% and 1.60 × 10⁻² min⁻¹ for CuFe₂O₄, 65.8% and 2.05 × 10⁻² min⁻¹ for NiFe₂O₄, 80.2% and 2.98 × 10⁻²min⁻¹ for Ni_0.5Cu_0.5Fe₂O₄ respectively. These results indicated that Ni_0.5Cu_0.5Fe₂O₄ had the strongest catalytic efficiency.

Ni_0.5Cu_0.5Fe₂O₄ has redox cycles of Fe³⁺/Fe²⁺, Ni³⁺/Ni²⁺ and Cu³⁺/Cu²⁺. Therefore, the ratio between copper/nickel/iron Ni_0.5Cu_0.5Fe₂O₄ is an important variable to obtain the optimal efficiency. In theory, spinel will have the molecular structure of MFe₂O₄(Fe³⁺/M²⁺ = 2:1). Changing the Fe³⁺ will destroy the balance of Fe/Ni/Cu in Ni_0.5Cu_0.5Fe₂O₄ and contribute a different effect on degradation efficiency (Pham et al. 2018). Figure 4(b) shows when the molar amount of Fe³⁺ increased from 0.500 to 2.00 mol (with Cu²⁺ and Ni²⁺ quantified at 0.500 mol), the degradation efficiency increased from 57.2% to 80.2%, and the corresponding law appears in the kinetic rate, increased from 1.66 × 10⁻² min⁻¹ to 2.98 × 10⁻² min⁻¹. However, the continuous increase of the number of Fe³⁺ (3.00 mol) will lead to an adverse impact on the degradation efficiency and kinetic rate: decreased down to 53.8% and 1.49 × 10⁻² min⁻¹. According to this result and analysis, the optimum ratio of the three metal ions can be determined at Fe³⁺ : Cu²⁺ : Ni²⁺ = 2 : 0.5 : 0.5.

Figure 4(c) and the inset show two processes in the removal of CIP: adsorption and degradation. Adsorption-desorption equilibrium can be achieved in 60 min, and the adsorption and degradation rate is 56.1%, 80.2% (93.1% of CIP and 55.4% total organic carbon were removed). Besides, the ratio of the dissolved iron of Ni_0.5Cu_0.5Fe₂O₄ into solution was 0.0004 wt.% of Fe ions, 0.202 wt.% of Cu ions and 1.19 wt.% of Ni ions.

The following experiments are the catalytic process after adsorption-desorption equilibrium, and the degradation rate is calculated with the adsorption-desorption equilibrium concentration as the initial concentration.

The value of pH is a basic factor in AOPs. As shown in Figure 5(a), the degradation efficiency increased from 52.8% to 80.2%, the constant kinetic rate increased from 1.29 × 10⁻² min⁻¹ to 2.98 × 10⁻² min⁻¹ at the range of pH = 3.00–6.00. For pH = 6.00–10.0, the degradation efficiency and constant kinetic rate decreased from 80.2%, 2.98 × 10⁻² min⁻¹ to 40.6%, 1.09 × 10⁻² min⁻¹. The degradation efficiency of CIP reached the highest at pH = 6.00. The inhibition of CIP degradation at pH < 6.00 may be due to scavenging reasons of H⁺ on SO₄^−· and ^·OH. And metal ion leaching will also affect the proportion of three metals in the catalyst and destroy the structure of the catalyst (Zhang et al. 2019). The degradation efficiency of CIP changed in pH = 3–10 can be made based on zeta potential of Ni_0.5Cu_0.5Fe₂O₄ (pHpzc is about 6.7). Therefore, the surface of Ni_0.5Cu_0.5Fe₂O₄ is positive at pH< pHpzc, while it will be negative at pH > pHpzc. Lower pH may promote the formation of hydrogen bond between H⁺ and O-O group in PDS, so the positive charge around PDS will hinder the electron transfer between PDS and positively charged catalyst (Wang et al. 2019). When the pH increases to alkaline, it can be inferred that the number of surface hydroxyl groups (OH⁻) of Ni_0.5Cu_0.5Fe₂O₄ catalyst will increase, and the electrostatic repulsion force will reduce the opportunity of interaction between Ni_0.5Cu_0.5Fe₂O₄ and PDS, resulting in a significant decrease in the degradation efficiency of CIP (Li et al. 2017). When the pH value increases, SO₄^−· could react with ^·OH to form other mid-bodies, such as SO₄²⁻, HSO₄⁻ and O₂, which could cut down the consistence of active free radicals and further debase the degradation efficiency of the system (Zhang et al. 2019).

The initial CIP concentration on its degradation process in Ni_0.5Cu_0.5Fe₂O₄/PDS system was evaluated in Figure 5(d). Under the condition of constant catalyst and oxidant dosage, the degradation efficiency and the kinetic rate increased from 62.4% and 1.93 × 10⁻² min⁻¹ to 84.2% and 3.23 × 10⁻² min⁻¹ with the increase of initial concentration in the low concentration range (10.0–100 mg/L). But with the initial concentration of CIP increased to 300 mg/L, the degradation efficiency and the kinetic rate decreased to 63.8% and 1.72 × 10⁻² min⁻¹. It can be explained that the amount of SO₄^−· produced in the system is constant, so the number of CIP molecules that can be decomposed by these amounts of SO₄^−· is also certain. In addition, the formation of intermediates produced by CIP decomposition occupies the catalyst surface and has competitive behavior with the target pollutants. At lower concentration, the active sites of the catalyst are sufficient, and this competition is not enough to inhibit the degradation of CIP, but when the concentration increases, the degradation efficiency and kinetic rate of CIP are significantly inhibited.

Figure 5(e) shows the distinction on CIP degradation caused by temperature variation (10 °C, 25 °C, 50 °C). The degradation efficiency reached 42.6%, 51.8%, 57.9% in 10 min, and the kinetic rate increased from 5.55 × 10⁻² min⁻¹ to 8.65 × 10⁻² min⁻¹. In general, the temperature can promote the degradation of CIP, for heating can decompose PDS into SO₄^−· and ^·OH through thermolytic cleavage of O-O bond (Hu et al. 2019).

It is found that Ni_0.5Cu_0.5Fe₂O₄ has superior activating ability in this study. The mechanism of CIP degradation in Ni_0.5Cu_0.5Fe₂O₄/PDS system is proposed as the following Equations (13)–(21) (Li et al. 2017). Firstly, CIP molecules will be adsorbed to the reaction site on the surface of Ni_0.5Cu_0.5Fe₂O₄. Then PDS is activated by electron transfer between redox pairs on the catalyst surface to form SO₄^−·, and then SO₄^−· can be transferred into ^·OH by the reactions with OH⁻ and H₂O. Finally, the degradation reaction took place on the catalyst surface and produced intermediate products, CO₂ and H₂O. Intermediates from CIP degradation were identified by GC-MS, and possible degradation pathways were suggested in Figure 6, Pathway I: SO₄^−· and ^·OH attack on the piperazine ring and P1 (m/z = 334) was found. Followed by the ‘-CO’ and ‘-CH₂CH₂NH₂’ lose, the P2 (m/z = 334) was generated. Pathway II: The ‘C-F’ bond on the quinolone ring is hydroxyl substituted to form P5 (m/z = 330), and then further decarboxylated to form P6 (m/z = 285). Pathway III: Decarboxylation occurred and P3 formed (m/z = 288). Pathway IV: a hydroxylation process, hydroxyl radical attacked the quinolone ring of CIP to produce P4 (m/z = 348). Finally, the above intermediates could be decomposed into other smaller products even CO₂ and H₂O.

The magnetic separation characteristics of the catalyst were further studied. From the magnetic hysteresis loops of Figure 8(a), Ni_0.5Cu_0.5Fe₂O₄ exhibits a distinctly symmetric hysteresis loop with a satisfactory magnetization saturation value (∼8.6 emu/g), so it can be easily separated by a magnet and the result was shown in the inset photograph of Figure 8(b). It is obvious that when the magnet exists, the system gradually becomes clear from turbidity, which implies that the catalyst has good magnetic retrievability.

As shown in Figure 8(b), the degradation efficiency and kinetic rate are reduced compared with the new catalyst after three cycles of experiments. The reason contributing to this phenomenon can be explained: On the one hand, the leaching of metal ions, the destruction of active surface, and the conglomeration of catalyst. On the other hand, the main contaminant and its intermediate will occupy the catalyst sites at the same time and competitive adsorption occur. However, there is no big gap between the three cycle runs, the degradation efficiency is almost maintained at 60.0%. From the FT-IR spectrometer comparison in Figure 3(a) before and after use, it can be seen that two main ferrite characteristic bands still exist, which proves that the structure of the catalyst has not changed significantly after three times of use. Based on the analysis above, Ni_0.5Cu_0.5Fe₂O₄ has excellent stability and recyclability for activating PDS in CIP degradation.

Utilizing a simple co-precipitation method to synthesize magnetic Ni_0.5Cu_0.5Fe₂O₄ nano-catalyst for activating PDS in degradation of CIP was studied. Two processes exist in the removal of CIP: adsorption and degradation. The experiments show that compared with heterogeneous systems (Fe₃O₄/PDS, CuFe₂O₄/PDS, NiFe₂O₄/PDS), the degradation efficiency of Ni_0.5Cu_0.5Fe₂O₄/PDS is much higher (80.2%) at pH = 6.00, 0.500 g/L of Ni_0.5Cu_0.5Fe₂O₄, 5.00 mmol/L of PDS, CIP concentration of 20.0 mg/L at 25 ± 2 °C for 50 min. Besides, the ratio of the dissolved iron of Ni_0.5Cu_0.5Fe₂O₄ into solution was 0.0004% of Fe ions, 0.202% of Cu ions and 1.19% of Ni ions. The presence of Cl⁻, NO₃⁻, HCO₃⁻ in the reaction solution can reduce the degradation efficiency. Free radical capture experiments emphasized that the two reactive species (SO₄^−· and ^·OH) were produced in the present process, and the important reactive species in the degradation of CIP by Ni_0.5Cu_0.5Fe₂O₄/PDS. The Ni_0.5Cu_0.5Fe₂O₄ is proven a magnetic material, so the catalyst can be easily separated from the solution and used in recycling experiments after the reaction. The results show that with increasing the number of cycles from 1 to 3, the degradation efficiency decreased and was maintained near 60%. It should be noted that this work provides an easily prepared and practical catalyst for the development of magnetic retrievability catalyst for degradation organic pollutant molecules based on the practical needs.

The authors declare that they have no conflict of interest.

This project was supported by the National Key Research and Development Program of China (2017YFD0800301), Liaoning Province Education Administration (No. LJ2020008, LQ2020023, and LQ2020027), Program for Liaoning Innovative Research Team in University (LT2020).

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