Enhanced Ciprofloxacin Ozonation Degradation by an Aqueous Zn-Cu-Ni Composite Silicate: Degradation Performance and Surface Mechanism

Abstract

This study investigates the environmental significance of ciprofloxacin as an emerging contaminant and the need for effective degradation methods. The chemical coprecipitation method was used in this study to prepare the Zn-Cu-Ni composite silicate, serving as a heterogeneous ozonation catalyst. The catalytic activity was then evaluated by degrading ciprofloxacin (CIP). Scanning electron microscopy, X-ray diffraction, X-ray photoelectron spectroscopy, nitrogen adsorption–desorption, and Fourier transform infrared analysis (FTIR) were used to characterize the Zn-Cu-Ni composite silicate. The catalyst had a high surface area (308.137 m²/g), no regular morphology, and a particle size of 7.6 µm and contained Si-O-Si, Ni-O-Si, and Zn-O-Si. The results showed that the CIP degradation and mineralization rates (pH 7.0, CIP 3.0 mg/L, Ozone 1.5 mg/L) were significantly enhanced in the presence of the Zn-Cu-Ni composite silicate. The CIP and total organic carbon (TOC) removal rates were increased by 51.09% and 18.72%, respectively, under optimal conditions, compared with ozonation alone. The adsorption of Zn-Cu-Ni composite silicate, ozone oxidation, and ·OH oxidation synergistically promoted the efficient removal of CIP. This study provides valuable catalytic ozone technology for degradation of antibiotics in wastewater to reduce environmental pollution with potential practical applications.

1. Introduction

Antibiotics are widely used feed additives for promoting livestock growth and for treating microorganism infections in humans and animals. China is the largest antibiotic producer and consumer in the world, producing approximately 2.1 × 10⁵ tons of antibiotics annually, 85% of which are used in animal husbandry and medicine. This heavy antibiotic use has led to the detection of antibiotics nearly everywhere in the environment, from terrestrial to aquatic ecosystems, and even in surface groundwater and direct drinking water supplies. The presence of antibiotics in wastewater has emerged as a critical research area [1]. Ciprofloxacin (CIP) has a wide antibacterial spectrum, strong antibacterial activity, low toxicity, and few side effects, which make it very useful for infectious disease treatment [2]. Some studies have found that CIP does not biodegrade well in the environment. In addition, traditional sewage treatment processes cannot effectively remove active CIP, thus allowing CIP to continue to enter the environment [3]. The long-term presence of CIP in soils has been found to interfere with microorganism decomposition and inhibit plant growth [4]. Consuming water with low CIP concentrations over extended periods can lead to nervousness, nausea, vomiting, headaches, and diarrhea. However, high CIP concentrations in drinking water can cause more serious health problems, such as thrombocytopenia, acute renal failure, elevated liver enzymes, eosinophilia, and leukopenia [5].

The current primary CIP-removal methods consist of biological treatments [6], physical treatments [7,8], and chemical treatments [9]. The biological wastewater treatments are limited by the concentrations of antibiotics in the wastewater. Additionally, these treatments have several disadvantages, such as high power consumption, high treatment costs, and sludge bulking, making them unsuitable for antibiotic wastewater treatment. Traditional physical treatment methods, such as filtration and flocculation, are unable to completely degrade CIP. The chemical treatment of organic pollutants involves oxidizing the organic matter in wastewater to convert it into less harmful biodegradable substances, which are then removed from the water using oxidants. Heterogeneous catalytic ozonation is one of the most important organic chemical treatment technologies, and it has attracted attention for its ability to generate highly oxidizing hydroxyl radicals (·OH) [10].

Among the existing methods, heterogeneous catalytic ozonation has attracted attention for its ability to generate highly reactive hydroxyl radicals (·OH), which break down organic pollutants into less harmful compounds. The catalytic performance during the heterogeneous catalytic ozonation process primarily depends on the catalyst structural characteristics and the chemical pollutant compositions [11]. In recent years, many researchers have prepared a large number of catalysts using different methods to remove various antibiotics [12,13,14]. Among these catalysts, metal oxidation catalysts [15] and metal mineralization catalysts [16] have attracted wide attention. Although metal oxides have excellent catalytic ozonation performances for refractory organic removal, their particle agglomeration and excessive metal oxidation can lead to a significant decrease in catalytic efficiency. Spinel and perovskite catalysts are the most common for metal mineralization. The spinel (chemical formula [17]: AB₂O₄, where A is Zn, Ni, Cu, Co, and other transition metals and B is Fe, Mn, and Ti plasma) surface has a high percentage of hydroxyl groups and numerous lattice defects and active sites that enable ozone absorption and catalysis. Perovskite [18] is an inorganic compound with the chemical formula of ABX₃ (where A represents the metal ions, e.g., alkali metals and alkaline earth metals and B represents the cations with six coordination numbers, e.g., titanium and manganese). The A/B ion ratio can be changed to alter the catalyst’s properties and promote the spontaneous formation of a large number of radical oxygen species (ROS) in ozone. This is conducive for the removal of refractory organic compounds [19]. However, there are some problems due to its complex synthesis, such as ion dissolution and the oxidation of cationic components. This results in a catalytic efficiency decrease [19,20].

The purpose of this study is to prepare a stable catalyst using Zn, Cu, and Ni transition metals using a relatively simple method to catalyze ozonation and remove CIP from water. A composite catalyst is prepared in this study using the chemical coprecipitation method. The obtained Zn-Cu-Ni composite silicate is then characterized using different techniques (e.g., SEM, XRD, FT-IR, BET). The effects of the catalyst dosage, solution pH, ciprofloxacin dosage, and tert-butanol on the ciprofloxacin removal rate and mineralization degree are then studied. Furthermore, a possible reaction mechanism for CIP ozonation with the Zn-Cu-Ni composite silicate is discussed. This study provides valuable catalytic ozone technology to reduce environmental pollution with potential practical applications.

2. Materials and Methods

2.1. Chemicals

Ciprofloxacin (CIP) with a purity of >98% was purchased from the Shanghai Macklin Biochemical Co., Ltd. (Shanghai, China). Zinc nitrate, copper nitrate, and nickel nitrate were purchased from the Tianjin Komeo Chemical Reagent Co., Ltd. (Tianjin, China). Sodium silicate was purchased from the Tianjin Zhiyuan Chemical Reagent Co., Ltd. (Tianjin, China), and sodium thiosulfate was purchased from the Tianjin Guangfu Technology Development Co., Ltd. (Tianjin, China). Tret-butanol (TBA) was purchased from the Tianjin Damao Chemical Reagent Factory (Tianjin, China). All chemicals used were of analytic grade and did not require further purification. The solution pH was adjusted using hydrochloric acid (Beijing Chemical Plant, Beijing, China) and sodium hydroxide (Fuchen Chemical Reagent Co., Ltd., Tianjin, Chian).

2.2. Preparation of Catalysts

A chemical coprecipitation method was used to prepare the composite of the Zn-Cu-Ni silicate catalyst particles. The preparation process involved adding a sodium silicate solution (0.3–0.8 mol/L) dropwise to a mixed nitrate solution containing zinc, copper, and nickel in a 1:1:1 molar ratio. The addition was stopped once the solution reached pH 7, while stirring was maintained at 100–300 rpm to ensure a complete reaction. This solution was then titrated using NaOH to a pH of 12, and the solution was incubated for 24 h at 40 °C. Solids were repeatedly washed with ultrapure water until the electrical conductivity was zero and then dried at 60 °C for 24 h. The dry solid catalyst was crushed and passed through 300-mesh sieves, then stored in a vacuum desiccator. In addition, the Zn-Ni composite silicate, zinc silicate, copper silicate, and nickel silicate catalysts were prepared using the same method.

2.3. Analysis Methods

The sample morphologies were analyzed using a scanning electron microscope (SEM, Zeiss Sigma 5000, Jena, Germany). X-ray diffraction (XRD, Bruker D8 FOCUS, Karlsruhe, Germany) was used to analyze the crystalline shape of the catalyst. A nitrogen gas adsorption analyzer (Micromeritics 2460, Norcross, GA, USA) and the Brunauer–Emmett–Teller (BET) method were used to analyze the specific surface area, total pore volume, and average pore size of the catalysts. A Fourier transform infrared (FT-IR) spectrum instrument (Bruker Tensor 27, Karlsruhe, Germany) was used to measure the infrared spectra of the catalysts under ambient conditions. X-ray photoelectron spectroscopy (XPS, Thermo ESCALAB 250Xi, Waltham, USA) with Al Kα as the source was used to determine the composite surface components. A Shimadzu LC-20AT high performance liquid chromatograph (HPLC) with 4.6 mm × 250 mm C18 Waters columns was used at room temperature to determine the CIP concentration. To elute the CIP sample, 1 mL/min of 0.1% formic acid and acetonitrile (8:2, v/v) were pumped simultaneously into the sample. The point of zero charge (pH_pzc) of the synthesized composite was measured by a Zeta potential Analyzer (Malvern, Nanao-Z, Zen 2600). The reaction rate was evaluated by a first-order kinetics model, as seen in Equation (1) [20]:

(1)$\mathrm{l}\mathrm{n}\left({\displaystyle \frac{{\mathrm{C}}_{\mathrm{t}}}{{\mathrm{C}}_0}}\right)=-{\mathrm{k}}_{\mathrm{obs}}\cdot \mathrm{t}$

where C₀ and C_t are the initial CIP concentration and the CIP concentration at time t, respectively, and k_obs represent the apparent rate constant.

2.4. Catalytic Ozonation Procedure

CIP catalytic ozonation degradation experiments were conducted in batches in a modified 1 L glass flask. A microporous aeration head was used to bubble ozone into the reaction system to achieve the desired concentration. The ozone concentration in the water was adjusted by controlling the gas flow rate and aeration time. Sodium indigo disulfonate spectrophotometry was used to determine the ozone concentration in water [21]. When the ozone reached the designed concentration, the CIP and the catalyst were rapidly added, and the reactor was then sealed. A magnetic stirrer was used to continuously mix the suspension. An Na₂S₂O₃ solution at a concentration of 0.1 mol/L was used to immediately quench the suspension collected from the reactor at different time intervals. The quenched suspension was passed through a 0.45 μm filter prior to analysis. HPLC was used to determine the residual CIP concentration in water.

3. Results and Discussion

3.1. Characterization of the Catalysts

The Zn-Cu-Ni composite silicate morphologies and compositions were observed using SEM (Figure 1), and the red star portions were analyzed using EDS. Figure 1a shows that the Zn-Cu-Ni composite silicate consisted of some aggregated nanoparticles with irregular morphology and a particle size around of 7.6 μm. However, the elemental mapping images show that Zn, Cu, and Ni were homogeneously distributed on the surface of the Zn-Cu-Ni composite silicate (Figure 1c–e). In addition, the EDS results show that this area was mainly composed of Zn, Cu, and Ni, and they accounted for 41.03%, 36.08%, and 22.35%, respectively.

The different material structures were investigated using XRD (Figure 2a). The Zn-Ni composite silicate, zine silicate, nickel silicate, and copper silicate exhibited no characteristic peaks. This result demonstrated that the Zn-Ni composite silicate, zine silicate, nickel silicate, and copper silicate had poor crystallinities and were amorphous. However, the typical characteristic peaks at 12.8°, 25.8°, 34.1°, and 38.9° were observed on the Zn-Cu-Ni composite silicate. These characteristic peaks corresponded to the (020), (040), (211), and (231) crystal planes of Cu₂(OH)₃NO₃, respectively, according to the Joint Committee on Chemical Analysis by Powder Diffraction Methods (JCPDS) Card No. 84-0599.

The Zn-Cu-Ni composite silicate chemical structure was measured using XPS (Figure 2b), revealing characteristic peaks that corresponded to Zn 2p, Cu 2p, Ni 2p, and O 1s. The Cu 2p spectrum of the Zn-Cu-Ni composite silicate had two primary peaks, namely, Cu 2p_2/3 (at 935.6 eV) and Cu 2p_1/2 (at 954.68 eV). It also contained satellite peaks at 941.79 (S1), 944.34 (S2), and 963.32 (S3) eV. The Cu 2p_2/3 spectrum had two deconvolution peaks that appeared at 934.75 and 936.47 eV, and these contributed to the overall Cu 2p_2/3 intensities of 42.6% and 57.4%, respectively (Figure 2c). Importantly, the peak at 934.75 eV corresponded to Cu⁺, and the peaks at 936.47 and 954.68 eV corresponded to Cu²⁺. As shown in Figure 2d, the Zn-Cu-Ni composite silicate had two measurable Zn 2p peaks, with binding energies located at 1022.52 and 1045.50 eV, and these corresponded to Zn²⁺ and Zn³⁺, respectively. The contributions of Zn²⁺ and Zn³⁺ relative to the total Zn intensity were 59.4% and 40.6%, respectively. In addition, the Ni 2p spectrum had two deconvolution peaks at 856.64 and 862.51 eV that corresponded to Ni²⁺ and Ni³⁺, respectively (Figure 2e). The Ni²⁺ and Ni³⁺ contributions relative to the total Ni intensity were 76% and 24%, respectively [15]. Figure 2f shows the O 1s, which is fine general diagram of the sample, with the results primarily attributed to surface hydroxyl groups. According to the related literature [22], the oxygen with a binding energy of less than 531 eV was the lattice oxygen, and the oxygen with a binding energy between 531 eV and 533 eV was the chemisorbed oxygen, which was related to the compound oxygen vacancy concentration. The XPS analysis results of the catalyst showed that there were lattice oxygen and chemisorbed oxygen in the catalyst.

Figure 3 shows the N₂ adsorption–desorption isotherms and pore size distribution curve of the Zn-Cu-Ni composite silicate. The N₂ adsorption–desorption isotherm of the Zn-Cu-Ni composite silicate catalyst is type IV according to the International Union of Pure and Applied Chemistry classification, and the hysteresis ring is the H3 type. These results indicated that the Zn-Cu-Ni catalyst surface contained mesoporous structures. According to the literature [23], this type of structure can adsorb ozone or organic molecules in water, promote the occurrence of chemical reactions, and improve the degradation efficiency of organic compounds. Table 1 lists the Zn-Cu-Ni BET values. The Zn-Cu-Ni composite specific surface area was determined to be 308.137 m²/g.

The Zn-Cu-Ni composite silicate surface functional groups were analyzed using a Fourier transform infrared analysis (FTIR). As shown in Figure 4, the peaks near 1633 cm⁻¹ and 3421 cm⁻¹ corresponded to the H-O-H bending vibration absorption peak and hydroxyl vibration absorption peak, respectively [24]. This hydroxylation structure allows dissolved ozone to adsorb on the catalyst surface, enabling more efficient removal of organic matter. The bands at 1455 cm⁻¹ were attributed to CuO bending and stretching modes [25]. The absorption peak at 1040 cm⁻¹ originated from Si-O-Si. The absorption peak at 900 cm⁻¹ originated from Ni-O-Si [26]. The absorption peak at 600 cm⁻¹ originated from Zn-O-Si [27]. The peak at approximately 468 cm⁻¹ may have been the vibration peak of Cu-O or Zn-O [27].

3.2. Catalyst Activities and Stabilities

The catalytic activity of the catalyst was evaluated using control tests according to three processes, namely, ozonation alone, catalytic ozonation, and adsorption on catalysts. The specific experimental results are shown in Table 2. The CIP-removal curve versus the reaction time is illustrated in Figure 5a. The adsorption efficiency in the presence of the Zn-Cu-Ni composite silicate after 30 min was 10.64%. In addition, the CIP-removal rate was 34.35% after 30 min of ozone oxidation alone. This result indicated that ozonation alone had a limited influence on CIP degradation. Under the same experimental conditions, compared with ozonation alone, the catalytic ozonation of CIP-removal rates were in the following order: Zn-Cu-Ni composite silicate > Zn-Ni composite silicate > zinc silicate > nickel silicate > copper silicate. Hence, the Zn-Cu-Ni composite silicate had the best catalytic performance, which was 50% more than that of ozonation alone, 7.3% more than that of Zn-Ni composite silicate, 11.44% more than that of zinc silicate, 12.44% more than that of nickel silicate, and 14.52% more than that of copper silicate. The result of the CIP-removal kinetic correlation value R² in all catalytic systems showed that different catalytic systems agreed more with the pseudo-first-order kinetic model. The apparent rate constant (k_obs) of catalytic ozonation followed the order of Zn-Cu-Ni composite silicate > Zn-Ni composite silicate > zinc silicate > nickel silicate > copper silicate > ozonation alone.

The mineralization efficiencies of each catalyst for CIP in the different catalytic ozone oxidation experiments were shown in Figure 5b. The TOC-removal rate was 21.63% after 30 min of ozone oxidation alone. However, the TOC-removal rate by the Zn-Cu-Ni composite silicate adsorption properties was only 5.31%. The TOC-removal rate was significantly increased during catalytic ozonation. The TOC-removal rates by the Zn-Cu-Ni composite silicate, the Zn-Ni composite silicate, the zinc silicate, nickel silicate, and copper silicate were increased to 40.6%, 38.34%, 32.90%, 32.64%, and 32.38%, respectively. The Zn-Cu-Ni composite silicate had the best performance with a 40.16% TOC removal, which was consistent with the CIP-removal efficiency. These experimental results showed that Zn-Cu-Ni composite silicate was an effective catalyst and incorporated into the ozonation process well.

3.3. Catalyst Dosage Influence on the Catalytic Ozonation Process

Catalysts were the key to the catalytic effect and are cost-effective for water treatment. Therefore, a discussion regarding the optimum amount of catalyst used during the catalytic ozonation experiment was required. As seen in the Figure 6, the CIP-removal efficiency was 75.72%, and the TOC-removal efficiency was 32.68%, respectively, when 100 mg/L of catalyst was added. Catalyst dosage increase to 500 mg/L resulted in a CIP-removal efficiency increased to 82.66%, while the TOC-removal efficiencies increased to 40.26%. However, there were no further improvements in these efficiencies when increasing the catalyst dose to 800 mg/L. Xu et al. [28] reported that an increasing catalyst concentration in solution increased its specific surface area, and this promoted ozone molecule decomposition into ·OH. This result agreed with the fact that the CIP-removal effect increased with a catalyst increase. Additionally, as the catalyst dosage increases, the probability of particle collision also increases, which influences the catalytic effect.

3.4. Influence of CIP Initial Concentration on the Catalytic Ozonation Process

Figure 7 shows the effect of the initial concentration on the CIP- and TOC-removal efficiencies during the Zn-Cu-Ni composite silicate catalytic ozonation process. The CIP- and TOC-removal efficiencies decreased with increases in the initial CIP concentration. The CIP- and TOC-removal efficiencies were 82.66% and 40.65%, respectively, with an initial CIP concentration of 3 mg/L. The CIP- and TOC-removal rates decreased to 72.85% and 34.38%, respectively, as the initial CIP concentration increased to 15 mg/L. It was concluded that when the initial CIP concentration was low, the dissolved ozone in water and the amount of ·OH produced by ozone decomposition were high, which degraded CIP rapidly. As the initial CIP concentration increased, the ozone was consumed rapidly. In addition, the intermediates produced in the reaction eliminated ozone and ·OH, resulting in decreases in the CIP- and TOC-removal efficiencies. The TOC removal was lower compared to the CIP catalytic oxidation, as it produced many intermediates that were more resistant to oxidation than CIP [9].

3.5. Possible Reaction Mechanism During the Zn-Cu-Ni Catalytic Ozonation Process

3.5.1. The Initial pH Influence on the Catalytic Ozonation Process

The ozone decomposition rate and ·OH formation are greatly affected by the initial pH value of the water in the catalytic ozonation system. In addition, the electron gain and loss of hydroxyl groups on the catalyst surface are different under different pH conditions [29]. Several research results have shown that a neutral-charged catalyst surface is beneficial for ozone decomposition to ·OH [30,31]. As shown in Figure 8a, the CIP-removal rate after 15 min of catalytic ozonation tests followed the order of pH 8 > pH 7 > pH 9 > pH 10 > pH 6 > pH 5 with values of 84.70%, 82.66%, 79.12%, 78.83%, 74.64%, and 71.39%, respectively. The Zn-Cu-Ni composite silicate/O₃ system accelerated the O₃ to ·OH conversion, and the inflection point at pH 8 showed the maximum contribution of the catalyst over a pH range of 5–10. The pH_pzc of the Zn-Ni-Cu was 8.03 by the Zeta potential analyzer. According to the literature [32], when the pH of the solution is lower than pH_pzc, the positive charge on the catalyst surface is destroyed due to protonation, which is not conducive to the catalytic process of surface ozone molecules. Conversely, when the solution pH is higher than the pH_pzc, the surface hydroxyl groups on the catalyst become deprotonated, causing most of the electrophilic H in the surface hydroxyl groups to be released into the solution. This reduces the interactions between the surface hydroxyl groups and ozone. Moreover, when the solution pH is equal to the pH_pzc, the charges of metal oxide surface and surface hydroxyl groups are zero, which can promote ozone decomposition to produce ·OH generation.

The TOC removal showed a similar trend. The TOC-removal efficiency improved with an increase in the initial pH value from 5 (34.07%) to 8 (40.60%). Even if the intermediates produced during CIP degradation were difficult to oxidize by ozonation, the selectivity of the reaction of ·OH with organic compounds was lower. Hence, these intermediates could be further oxidized or even mineralized. Therefore, increasing the ·OH output was beneficial for improving the TOC-removal rate.

3.5.2. Radical Scavenging Experiment

Tert-butanol (TBA) is a well-known ·OH scavenger that can capture the ·OH radicals produced during the ozone decomposition process [33]. The TBA and ·OH reaction rate constant was 6.0 × 10⁸ M⁻¹ S⁻¹, which was much higher than the TBA and ozone reaction rate constant of 3.0 × 10⁻³ M⁻¹ S⁻¹ [34]. In the catalytic ozonation system, TBA reacts instantly with hydroxyl radicals to form inert intermediates that inhibit the chain reaction of ozone decomposition to hydroxyl radicals. Figure 9a shows the effects of TBA on CIP degradation. The results showed that when the TBA concentration increased from 0 to 0.06 mM, the CIP degradation efficiency decreased from 84.12% to 53.67% after a 15 min reaction time. Hence, during the CIP catalytic ozonation process, the TBA reaction with hydroxyl radicals produced highly selective and inert intermediates that terminated the chain reaction of free radicals, thus reducing the CIP degradation rate [9]. In addition, it was proven that there existed a ·OH mechanism during the CIP ozonation process that was catalyzed by the Zn-Cu-Ni composite silicate. Moreover, ESR analysis (Figure 9b) revealed the signals of DMPO reacts with OH adducts (DMPO-OH) with peaks in the 1:2:2:1 range, indicating that ·OH was the primary active species in Zn-Cu-Ni composite silicate catalytic ozonation of CIP.

3.5.3. Surface Mechanism of Catalytic Ozonation by the Zn-Cu-Ni Composite Silicate

According to the previous discussion, the Zn-Cu-Ni composite silicate showed good adsorption performance and catalytic activity during CIP heterogeneous catalytic ozonation. According to the literature [35], the surface hydroxyl groups of catalysts could promote the generation of ·OH. According to FTIR results, Zn-Cu-Ni composite silicate has abundant hydroxyl group. The TBA experiment and ESR result indicated that the predominant reactive species in the Zn-Cu-Ni composite silicate catalytic ozonation process was ·OH. To illustrate the possible interaction in the Zn-Cu-Ni composite silicate/O₃ reaction system for removing CIP, a simplified mechanism showing the catalytic ozonation of CIP with the Zn-Cu-Ni composite silicate catalyst is shown in Figure 10. Ozone could directly attack CIP in an aqueous solution to generate CO₂ and H₂O, but the rate of direct reaction was much less than that of the oxidation of ·OH. The existence of ·OH could promote continuous CIP oxidation and the oxidation of its intermediates, which was beneficial for TOC-removal rate improvements. In addition, Zn-Cu-Ni composite silicate adsorption was beneficial for CIP and TOC removal.

Consequently, there may be several pathways during the ozonation process catalyzed by the Zn-Cu-Ni composite silicate: (1) The catalyst promoted ozone decomposition to produce a large amount of ·OH. The ·OH rapidly removed CIP and the intermediates. (2) Organic molecules (CIP or intermediates) adsorbed on the catalyst surface were oxidized by gaseous or aqueous ozone. (3) The adsorption of both ozone and organic molecules (CIP or intermediates) on the Zn-Cu-Ni composite silicate catalyst surface and their direct and indirect reactions occurred between ozone and CIP.

4. Conclusions

Zn-Cu-Ni composite silicate catalysts were successfully prepared using the chemical coprecipitation method and showed considerable catalytic activity for CIP degradation and mineralization. The CIP-removal rates of catalysts Zn-Cu-Ni composite silicate, Zn-Ni composite silicate, zinc silicate, nickel silicate, and copper silicate were 84.38%, 77.08%, 72.95%, 71.94%, and 69.86%, respectively, and the TOC-removal rates were, respectively, 40.6%, 38.34%, 32.90%, 32.64%, and 32.38%. Hence, Zn-Cu-Ni composite silicate has the best catalytic effect in the catalytic system. The CIP-removal rate with the Zn-Cu-Ni composite silicate catalysts reached 84.38% after 15 min, which was 51.09% higher than that of ozonation alone. The TOC-removal efficiency improved by approximately 18.73% compared to ozonation alone. The CIP- and TOC-removal efficiencies in aqueous solution increased during Zn-Cu-Ni composite silicate catalytic ozonation with an increase in the catalyst dose and the initial solution pH. However, as the CIP initial concentration increased, the catalyst capability for CIP degradation and TOC removal decreased. The CIP removal was attributed to a synergistic effect of adsorption, ozone oxidation, and ·OH oxidation. This study provides valuable insights into an inexpensive, viable, and promising catalyst for the effective catalytic ozonation of antibiotics in wastewater, with potential practical applications.

Author Contributions

Conceptualization, Y.L., J.L. and R.G.; methodology, Y.L., J.L. and R.G.; software, Y.L., R.G., C.W. and W.W.; validation, Y.L., Y.C., W.W. and H.Z.; formal analysis, Y.L., J.L., W.W. and H.Z.; investigation, Y.L., J.L., R.G. and C.W.; data curation, Y.L. and R.G.; writing—original draft preparation, Y.L.; writing—review and editing, Y.L. and W.W.; supervision, Y.L. and Y.C.; project administration, Y.L. and W.W.; funding acquisition, Y.L., W.W. and H.Z. All authors have read and agreed to the published version of the manuscript.

Data Availability Statement

Data will be made available on request.

Conflicts of Interest

Author Rong Guo was employed by the company Jiangsu Kejia Design Group Co., Ltd. The remaining authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Footnotes

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Figures and Tables

Figure 1. (a,b) the morphologies of the Zn-Cu-Ni composite silicate, (c–e) the elemental mapping images of Zn-Cu-Ni composite silicate, (f) EDS analysis results of Zn-Cu-Ni composite silicate.

Figure 2. (a) XRD patterns of the different materials; (b) XPS spectra analysis of the Zn-Cu-Ni composite silicate; XPS spectra of Cu 2p (c), Zn 2p (d), Ni 2p (e), and O1s (f).

Figure 3. (a) The N2 adsorption-detachment isotherm of the Zn-Cu-Ni composite silicate and (b) the pore size distribution curve.

Figure 4. FTIR spectrum of the Zn-Cu-Ni composite silicate.

Figure 5. (a) Degradation of CIP by catalytic ozonation and single ozonation; (b) degradation of TOC by catalytic ozonation and single ozonation. Experimental conditions: pH = 7.0, [CIP]0 = 3.0 mg/L, [Ozone]0 = 1.5 mg/L, catalyst dose = 500 mg/L, T = 25 °C.

Figure 6. (a) Influence of the catalyst dosage on CIP-removal rate, (b) influence of catalyst dosage on TOC-removal rate. Experimental conditions: pH = 7.0, [CIP]0 = 3.0 mg/L, [Ozone]0 = 1.5 mg/L, T = 25 °C.

Figure 7. (a) Influence of CIP initial concentration on the effect of catalytic ozone oxidation, (b) influence of CIP initial concentration on TOC-removal rate. Experimental conditions: pH = 7.0, [Ozone]0 = 1.5 mg/L, catalyst dose = 500 mg/L, T = 25 °C.

Figure 8. (a) Influence of initial pH on the catalytic ozone oxidation effect, (b) influence of initial pH on TOC-removal rate. Experimental conditions: [CIP]0 = 3.0 mg/L, [Ozone]0 = 1.5 mg/L, catalyst dose = 500 mg/L, T = 25 °C.

Figure 9. (a) Influence of tert-butanol on the catalytic ozone oxidation effect, (b) ESR analysis of the catalytic ozone oxidation system. Experimental conditions: pH = 7.0, [CIP]0 = 3.0 mg/L, [Ozone]0 = 1.5 mg/L, catalyst dose = 500 mg/L, T = 25 °C.

Figure 10. Reaction mechanism of the Zn-Cu-Ni composite silicate process.

Table 1

Specific surface area and pore size of different materials.

Parameter	BET Surface Area, m2/g	Single Point Hole Volume, cm3/g	Average Aperture, nm
Zn-Cu-Ni composite silicate	308.137	0.357	7.79
Zn-Ni composite silicate	299.147	0.322	6.14
Zinc silicate	298.132	0.301	6.01
Nickel silicate	263.644	0.284	5.98
Copper silicate	271.568	0.293	6.09

Table 2

Experimental results under different experimental conditions.

	CIP-Removal Rates (%)	Rate Constant (kobs) of Catalytic Ozonation (min−1)	Correlation Value R2	TOC-Removal Rate (%)
Ozone alone	34.35	0.126	0.9899	21.63
Zn-Cu-Ni composite silicate/O3	84.38	0.505	0.9983	40.6
Zn-Ni composite silicate/O3	77.08	0.461	0.9858	38.34
Zinc silicate/O3	72.95	0.436	0.9927	32.9
Nickel silicate/O3	71.94	0.43	0.9958	32.64
Copper silicate/O3	69.85	0.416	0.9974	32.38

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