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Article

Synergistic Enhancement of Oxytetracycline Hydrochloride Removal by UV/ZIF-67 (Co)-Activated Peroxymonosulfate

by
Yiting Luo
1,†,
Zhao Liu
2,†,
Mingqiang Ye
3,
Yihui Zhou
3,
Rongkui Su
4,*,
Shunhong Huang
4,
Yonghua Chen
4 and
Xiangrong Dai
5
1
Hunan First Normal University, Changsha 410114, China
2
Radiation Environmental Supervision Station of Xinjiang Uygur Autonomous Region, Urumqi 830010, China
3
Aerospace KaiTian Environmental Technology Co., Ltd., Changsha 410100, China
4
College of Life and Environmental Science, Central South University of Forestry and Technology, Changsha 410004, China
5
PowerChina Zhongnan Engineering Corporation Limited, Changsha 410004, China
*
Author to whom correspondence should be addressed.
†
These authors contributed equally to this work as co-first author.
Water 2024, 16(18), 2586; https://doi.org/10.3390/w16182586
Submission received: 25 August 2024 / Revised: 6 September 2024 / Accepted: 10 September 2024 / Published: 12 September 2024
(This article belongs to the Special Issue Water Pollution Monitoring, Modelling and Management)
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Abstract

:
This study developed a new system for removing antibiotics using UV/ZIF-67 (Co)-activated peroxymonosulfate. The presence of antibiotic organic pollutants in urban sewage presents a substantial challenge for sewage treatment technologies. Due to the persistent chemical stability of antibiotics, their low environmental concentrations, and their resistance to degradation, effectively removing residual antibiotics remains a significant issue in urban wastewater treatment. This study introduces an eco-friendly photocatalytic technology designed to enhance the removal of oxytetracycline (OTC) from municipal wastewater using a UV/ZIF-67 (Co)/PMS system. The results showed that compared with UV, UV/PMS, ZIF-67 (Co), ZIF-67 (Co)/PMS, and UV/ZIF-67 (Co) systems, the UV/ZIF-67 (Co)/PMS system had the highest OTC removal rate. When 10 mg ZIF-67 (Co) and 1 mM PMS were applied to 100 mL 30 mg/L OTC solution, the degradation efficiency reached 87.73% under 400 W ultraviolet light. Increasing the dosage of ZIF-67 (Co) and PMS can improve the removal rate of OTC, but the marginal benefit of additional dosage is reduced. The highest degradation efficiency was observed at weakly acidic pH, which may be due to potential damage to the internal structure of the catalyst and reduced performance under extreme pH conditions. The influence of chloride ions and nitrate ions on the reaction system is minimal, while bicarbonate ions exhibit a significant inhibitory effect on the removal of OTC. The UV/ZIF-67 (Co)/PMS system exhibits adaptability to various water sources, including tap water, Guitang River water, and pure water. The results of free radical identification indicate the presence of hydroxyl and sulfate groups in the UV/ZIF-67 (Co)/PMS system, both of which play important roles in the degradation of OTC. This study offers valuable insights and technical support for the green, efficient, and environmentally friendly removal of antibiotics from urban wastewater.

1. Introduction

In recent years, antibiotics have become widely used in clinical medicine and animal husbandry [1,2]. This extensive use has significantly advanced medical treatment and accelerated the development of animal husbandry [3,4]. However, the overuse of antibiotics has led to substantial residual levels in the environment [5,6]. These residues primarily originate from wastewater and excreta resulting from the production and application of antibiotics [7]. Industrial wastewater, domestic sewage, and excreta containing antibiotics eventually enter the aquatic environment through various pathways, including sewage treatment plants, composting, and direct discharge [8,9]. Residual antibiotics in aquatic environments can disrupt or eliminate key microbial species, impacting ecosystem health [10]. Additionally, the persistent accumulation of antibiotics can lead to the development of antimicrobial resistance and resistance genes, posing a threat to public health [11,12]. Tetracyclines (TCS) are extensively used and are among the most prevalent antibiotics in environmental pollution [13,14]. These antibiotics exhibit broad-spectrum activity against Gram-positive bacteria, Gram-negative bacteria, Rickettsia, and Chlamydia, which are known for their effective therapeutic properties. However, incomplete absorption of TCS by organisms results in over 75% being excreted as either parent compounds or metabolites through urine and feces [15]. These excreted substances ultimately enter the water environment, where they can have toxic effects on non-target organisms and pose risks to aquatic ecosystems [13]. Accumulated TCS residues can have adverse effects on microorganisms, animals, and plants. Then, they can enter the human food chain through environmental migration and transformation, posing potential risks to human health and natural ecosystems [16,17,18].
Oxytetracycline (OTC) is an important member of the tetracycline class of antibiotics and is among the most significant antibiotics in clinical and agricultural applications [19]. Approximately 70% of OTC ingested by organisms is excreted through urine and feces, leading to widespread environmental contamination and contributing to the development of antimicrobial resistance in pathogenic bacteria. This accumulation poses a threat to both ecosystem stability and human health [20]. Consequently, there has been considerable global interest in developing effective technologies for treating OTC contamination in aquatic environments [21]. Researchers are exploring various methods to remove TCs, including adsorption, coagulation, filtration, microbial degradation, and advanced oxidation processes [22,23,24,25]. Biological treatments, while common, often have limited efficacy in degrading antibiotics in wastewater due to the strong inhibitory effects of antibiotics on microbial activity [22]. In contrast, physical and chemical treatment methods can transfer antibiotics between phases but do not achieve complete removal or effective degradation [26]. Advanced oxidation processes (AOPs) are increasingly used for treating antibiotic-laden wastewater due to their broad applicability, mild reaction conditions, and rapid reaction rates [27,28,29,30]. Traditional AOPs primarily utilize reactive species such as the hydroxyl radical (•OH) to degrade contaminants [31,32,33]. Recently, the sulfate radical ( SO 4 • − ) has garnered significant attention for its superior redox potential, stability, and adaptability across a wide pH range, making it particularly effective in water pollutant removal [34,35,36].
In recent years, metal–organic frameworks (MOFs) have garnered significant research interest due to their high porosity, open metal sites, and excellent thermal stability. These materials are porous crystalline structures with a periodic network, composed of metal nodes (metal ions or metal clusters) and organic ligands connected via self-assembly [37,38]. The synthesis and design of MOFs predominantly involve transition metals, with numerous research groups exploring various transition metal ions to develop MOFs with novel structures and enhanced functionalities [39]. Among these, the metal–organic framework ZIF-67 has rapidly advanced in photocatalysis owing to its high specific surface area, diverse structural and functional attributes, and extensive pore network [40,41]. Under specific light intensities, ZIF-67 can generate electron–hole pairs, with photogenerated electrons interacting with oxygen molecules to produce reactive oxygen species for pollutant degradation [42,43]. However, single MOF materials often exhibit limited light absorption, inefficient separation/transfer of photogenerated electrons/holes, and poor cyclic stability, leading to suboptimal photocatalytic activity [44]. To address these issues, researchers have increasingly explored the integration of MOFs with advanced oxidation technologies based on persulfate [45,46].
In this study, a new system for removing antibiotics using UV/ZIF-67 (Co)-activated peroxymonosulfate (PMS) was developed. A metal–organic framework (MOF) was employed as a catalyst in advanced oxidation processes, with cobalt chosen as the metal component. ZIF-67 (Co) demonstrated exceptional adsorption performance and high pollutant degradation efficiency. Characterization of ZIF-67 (Co) was conducted using SEM, FT-IR, and XRD techniques. Oxytetracycline hydrochloride was selected as the target compound to investigate various factors and conditions (such as ZIF-67 (Co) dosage, PMS dosage, pH, inorganic anions) affecting the synergistic enhancement of OTC removal by ZIF-67 (Co) in conjunction with PMS under UV irradiation. The photocatalytic oxidation mechanism for OTC degradation was further elucidated through free radical quenching experiments. The research results will provide technical guidance and a theoretical basis for the efficient removal of residual antibiotics.

2. Materials and Methods

2.1. Instruments, and Experimental Reagents

Cobalt nitrate hexahydrate (Co(NO3)2·6H2O, 99%), 2-methylimidazole (2-MI, 98%), potassium bisulfate (analytical grade), potassium chloride (analytical grade), sodium carbonate (analytical grade), potassium nitrate (analytical grade), and potassium bicarbonate (analytical grade) were purchased from Shanghai McLean Biochemical Technology Co., Ltd., Shanghai, China. Methanol (analytical grade), ethanol (analytical grade), oxytetracycline hydrochloride (95%), hydrochloric acid (analytical grade), sodium hydroxide (analytical grade), and sodium thiosulfate (analytical grade) were obtained from Sinopharm Chemical Reagent Co., Ltd, Shanghai, China.
The following equipment was used: digital display magnetic stirrer (ZGCJ-3A, Shanghai Zigui Instrument Co., Ltd., Shanghai, China), ultrapure water generator (UPT-11-40, Chengdu YOUPU Instrument and Equipment Co., Ltd., Chengdu, China), vacuum drying oven (DZ-2BCIV, Tianjin Taist Instrument Co., Ltd., Tianjin, China), desktop high-speed centrifuge (TG16-WS, Hunan Xiangyi Centrifuge Instrument Co., Ltd., Changsha, China), electronic balance (DHG-9023A, Shanghai Precision Experimental Equipment Co., Ltd., Shanghai, China), digital display pH meter (PHS-3E, Shanghai Yidian Scientific Instrument Co., Ltd., Shanghai, China), Fourier transform infrared spectrometer (Nicolet IS20, Thermo Scientific Co., Ltd., Waltham, MA, USA), X-ray diffraction spectrometer (MMI-Flex600, Rigaku Co., Ltd., Tokyo, Japan), scanning electron microscope (JSM-7610 FPlus, JEOL Co., Ltd., Tokyo, Japan), external visible spectrophotometer (UV-2700i, Shimadzu Instrument Co., Ltd., Tokyo, Japan), and photochemical reaction instrument (HF-GHX-VI, Shanghai Hefan Instrument Co., Ltd., Shanghai, China).

2.2. Preparation and Characterization of Materials

2.2.1. Preparation of ZIF-67 (Co) Material

ZIF-67 (Co) was synthesized using cobalt nitrate hexahydrate (Co(NO3)2·6H2O) as the metal center and 2-methylimidazole (2-MI) as the organic ligand, following the method described previously [47,48]. A methanol solution of Co(NO3)2·6H2O and 2-MI was prepared with a molar ratio of 1:6. The solution was stirred continuously for 1 h at room temperature using a magnetic stirrer and then allowed to settle for 24 h. After filtration, the sediment was washed multiple times with methanol and separated using a centrifuge. This washing and centrifugation process was repeated three times. Post-centrifugation, the sample stratified with a colorless, transparent upper layer and a purple target layer. The purple layer was then transferred to a vacuum drying oven, set to 60 °C, and dried for 6 h to remove residual solvents and methanol. After drying, the ZIF-67 (Co) material was obtained.

2.2.2. Characterization of ZIF-67 (Co) Materials

(1)
Scanning Electron Microscopy (SEM)
Scanning electron microscopy (SEM) provides detailed images of the surface and internal features of materials, including surface morphology, crystal size, and distribution, as well as pore shape and size. In this study, the microstructure of ZIF-67 (Co) was examined using a JSM-7610 FPlus scanning electron microscope (JEOL Co., Ltd., Tokyo, Japan). This allowed for an in-depth analysis of the material’s surface and internal characteristics.
(2)
X-ray Diffraction (XRD) Analysis
X-ray diffraction (XRD) is employed for both qualitative and quantitative analysis of the crystalline structure of materials. The XRD spectra were recorded using a MiniFlex600 diffractometer (Rigaku Co., Ltd., Tokyo, Japan), with a scanning rate of 10°/min over a 2θ range of 5° to 90°. This analysis provides information on the crystal structure and phase purity of the ZIF-67 (Co) material.
(3)
Fourier Transform Infrared (FT-IR) Spectroscopy
Fourier Transform Infrared (FT-IR) spectroscopy is used to identify functional groups and chemical bonds within a material. The FT-IR spectra were obtained using a Nicolet IS20 spectrometer (Thermo Scientific Co., Ltd., Waltham, MA, USA). This technique enabled the detection of characteristic vibration peaks of ZIF-67 (Co), facilitating the analysis and identification of the corresponding functional groups present in the material.

2.3. Experimental Methods

In each catalytic experiment, 30 mg/L OTC solution was placed into a 100 mL quartz reaction vessel. ZIF-67 (Co) material was then added to the remaining 100 mL OTC solution. The reactor was placed in a dark reaction chamber and stirred for 5 min. Following this, a mercury lamp was turned on and set to 400 W, and 1 mL of 0.1 mol/L PMS solution was added. At predetermined time intervals, 1 mL samples were withdrawn and immediately quenched with 1 mL of 0.1 mol/L sodium thiosulfate solution. The absorbance of OTC was measured before and after the catalytic reaction using a UV–Vis spectrophotometer, with the wavelength set to 352 nm. The concentration of oxytetracycline hydrochloride was determined based on a standard curve. The effects of catalyst dosage, PMS dosage, solution pH, and inorganic anions on the degradation efficiency were systematically evaluated.

2.4. Removal Efficiency Calculation

The removal efficiency was calculated based on the absorbance measurements before and after the catalytic reaction using a UV–Vis spectrophotometer set to a wavelength of 352 nm. The concentration of OTC corresponding to the measured absorbance values was determined. By comparing the concentrations of OTC before and after adsorption and photocatalytic degradation, the removal efficiency of the ZIF-67 (Co) material was calculated. The formula for removal efficiency is given by Equation (1):
R = C 0 − C t C 0 × 100 %
where R (-) represents the removal efficiency of the target compound, C0 (mM) is the initial concentration of OTC, and Ct (mM) is the concentration of OTC at time t.

3. Results and Discussion

3.1. Characterization of ZIF-67 (Co) Material

3.1.1. SEM Analysis

The SEM image of ZIF-67 (Co) is shown in Figure 1. The material was synthesized at room temperature using cobalt nitrate hexahydrate and 2-methylimidazole in a molar ratio of 1:6. The SEM image reveals that the synthesized material consists of rhombic dodecahedral nanocrystals with diameters ranging from 0.1 to 1.0 μm. The observed crystal size is moderate, consistent with the morphology of ZIF-67 (Co) reported in the literature [49]. Figure 1 indicates that the material exhibits good dispersion and a smooth surface.

3.1.2. XRD Analysis

To further confirm the formation of ZIF-67 (Co), its crystal structure was analyzed via X-ray diffraction (XRD), as shown in Figure 2. The material exhibits a prominent diffraction peak at 2θ = 7.32°, corresponding to the (011) crystal plane index according to the standard card. Additionally, strong and sharp diffraction peaks observed at 12.7° and 18.0° correspond to the (112) and (222) crystal planes, respectively, in the ZIF-67 (Co) standard card. The XRD pattern aligns well with the expected lattice parameters, consistent with previously reported results. Combined with SEM analysis, these findings confirm the successful synthesis of ZIF-67 (Co) [50].

3.1.3. FT-IR Characterization

Figure 3 shows the infrared spectrum of the ZIF-67 (Co) material sample, and the composition of organic groups and chemical bonds of the sample are analyzed in detail. Since ZIF-67 (Co) is connected by a “node” composed of cobalt ions and 2-methylimidazole as a “bridge”, the main band in the IR spectrum can be attributed to the contribution of ligand 2-methylimidazole. From the spectra, we observed that there were absorption peaks at 756 cm−1 and 1630 cm−1, which corresponded to the C=N bond on the imidazole ring of the sample molecule. The absorption peaks at 991 cm−1 and 1141 cm−1 correspond to the C-N bond and further analysis showed the characteristic peak at 1303 cm−1, corresponding to the C=C bond on the imidazole ring, respectively. In addition, the stretching vibration of the imidazole ring was found at 1418 cm−1. It was found that the infrared absorption of the C-H bond of the aromatic ring and the C-H bond of the aliphatic hydrocarbon chain appeared at 2922 cm−1 and 3134 cm−1 in the high wavenumber region, respectively; Finally, the infrared absorption of the N-H bond on imidazole ring was observed at 3431 cm−1. Based on the above spectral data and analysis, consistent with the results previously reported, we confirmed the successful synthesis of ZIF-67 (Co) material and accurately analyzed the distribution of organic groups and chemical bonds in the material [49].

3.2. UV/ZIF-67 (Co)/PMS System for OTC Removal

3.2.1. Removal Effect of UV/ZIF-67 (Co)/PMS on OTC

At room temperature, 100 mL (30 mg/L) of oxytetracycline hydrochloride (OTC) solution was added to 100 mL of the quartz test tube, and then 10 mg of ZIF-67 (Co) material and 1 mM of PMS were added. The reaction lasted for 10 min under 400 W light. Samples were taken at 2 min, 4 min, 6 min, 8 min, and 10 min, respectively, and 1 mL (0.1 mol/L) of sodium thiosulfate solution was added to quench. Adjust the UV–VIS spectrophotometer wavelength to 352 nm, and measure the change in OTC concentration. The pre-experimental study showed that the adsorption performance of ZIF-67 (Co) on OTC was not significant, and the adsorption–desorption equilibrium was reached at 2 min, and the adsorption time was selected as 5 min in the subsequent experiment. Figure 4 is obtained by drawing the Ct/C0~t diagram. It can be seen from Figure 4 that PMS alone has little effect on OTC removal. The 10 min removal rate of OTC in ZIF-67 (Co) alone is 30.00%, the 10 min removal rate of OTC in UV alone is 16.20%, the 10 min removal rate of OTC in UV/PMS system is 42.30%, the 10 min removal rate of OTC in ZIF-67 (Co)/PMS system is 74.50%, and the 10 min removal rate of OTC in UV/ZIF-67 (Co)/PMS system is 87.73%. The results showed that ZIF-67 (Co) could activate PMS to rapidly remove OTC, and the UV/ZIF-67 (Co)/PMS system had a better OTC removal effect, indicating that the active substances produced by PMS activation played a major role. Lin et al. found that the effect of removing rhodamine B by ZIF-67 (Co) adsorption is not significant, but after adding PMS, the removal efficiency of Rhodamine B is significantly increased, which is consistent with our results [51]. The reaction kinetics for OTC removal using UV/ZIF-67 (Co)-activated PMS were analyzed, revealing R2 values of 0.8611, 0.9333, and 0.9919 for zero-order, first-order, and second-order kinetics, respectively. These results indicate that the reaction kinetics conform more closely to the second-order reaction model.

3.2.2. Effect of ZIF-67 (Co) Dosage on OTC Removal Efficiency

As illustrated in Figure 5, with all other conditions held constant, increasing the dosage of ZIF-67 (Co) from 5.0 mg to 10.0 mg resulted in an increase in the OTC removal rate from 71.81% to 87.73%, a rise of 15.92%. However, further increases in dosage to 15.0 mg and 20.0 mg resulted in only marginal increases in removal efficiency of 1.25% and 4.08%, respectively. This trend indicates that while higher doses of ZIF-67 (Co) generally improve OTC removal, the incremental benefit diminishes beyond 10.0 mg. These findings suggest that the removal rate of OTC improves with the dosage of ZIF-67 (Co) up to a certain point, after which additional increases in dosage have a diminishing effect. This observation is consistent with our previous report, which indicated that more ZIF-67 (Co) does not necessarily equate to better performance [44]. Consequently, considering both efficiency and cost-effectiveness, 10.0 mg of ZIF-67 (Co) is identified as the optimal dosage for subsequent experiments.

3.2.3. Effect of PMS Dosage on OTC Removal Efficiency

When other conditions are kept constant, the effect of varying PMS dosages on OTC removal efficiency was investigated. As depicted in Figure 6, at a PMS concentration of 0.25 mM, the OTC removal rate is limited to 57.25%. Increasing the PMS concentration to 0.50 mM raises the OTC removal rate to 67.00%, likely due to insufficient degradation of OTC at this lower concentration. At higher concentrations of PMS, specifically 1.00 mM and 2.00 mM, the removal rates increase to 87.73% and 91.14%, respectively. Although the removal rate improves significantly at these higher concentrations compared to 0.25 mM and 0.50 mM, the rate of increase diminishes with further increases in PMS concentration. This trend suggests that while a higher PMS dosage generally enhances OTC removal, the incremental benefit diminishes beyond a certain concentration. This diminishing effect may be attributable to the quenching of free radicals by excessive PMS [52]. Therefore, considering both economic and environmental factors, a PMS concentration of 1.00 mM is recommended for use in subsequent experiments.

3.2.4. Effect of pH on OTC Removal Efficiency

The pH of wastewater can fluctuate and plays a crucial role in influencing the speciation of organic compounds and the formation of key active species. This study examined the effect of pH on the removal efficiency of oxytetracycline hydrochloride (OTC) using PMS activated by ZIF-67 (Co) at pH values of 3.0, 5.0, 7.0, 9.0, and 11.0. As shown in Figure 7, the removal efficiencies of OTC at these pH values were 82.17%, 87.56%, 85.05%, 83.66%, and 81.39%, respectively. The results indicate that OTC removal is slightly more efficient under mildly acidic conditions, although the differences are not substantial. This suggests that the UV/ZIF-67 (Co)-activated PMS system is effective across a broad pH range. While it is commonly believed that alkaline conditions favor photocatalytic processes by promoting hydroxyl radical formation, this study’s results suggest that hydroxyl radicals may not be the sole or predominant active species in this system. Additionally, it was observed that OTC concentrations decreased by 0.09%, 37.74%, 32.40%, 25.21%, and 17.97% at pH values of 3.0, 5.0, 7.0, 9.0, and 11.0, respectively, after a 5 min treatment with ZIF-67 (Co). At pH values of 3.0 and 11.0, the adsorption of OTC onto ZIF-67 (Co) significantly decreased, possibly due to the material’s instability under highly acidic or alkaline conditions, which may impair its structural integrity and adsorption capability. Overall, the UV/ZIF-67 (Co)-activated PMS technology demonstrates good performance in mildly acidic and neutral conditions.

3.2.5. Effect of Inorganic Anions on OTC Removal Efficiency

With all other experimental conditions held constant, the impact of common inorganic anions on OTC degradation was assessed by introducing Cl−, NO3−, and HCO3− into the reaction system. As shown in Figure 8, the reaction rate constant remained relatively unchanged with the addition of Cl− and NO3−, indicating minimal effects on the overall reaction process [53]. However, the OTC removal rate decreased by 5.48% in the presence of 1 mM HCO3− and by 24.32% in the presence of 10 mM HCO3−. This decline is likely due to HCO3− quenching active radicals such as S O 4 • − and •OH in the solution, thereby reducing their availability for OTC degradation. While NO3− and Cl− may generate secondary radicals under UV excitation, these secondary radicals are less reactive compared to the primary radicals like S O 4 • − and •OH. Consequently, despite the formation of secondary radicals, their lower reactivity compared to the primary radicals reduces the overall reaction efficiency [54,55,56].

3.2.6. Effect of Different Environment Water on OTC Removal Efficiency

OTC degradation experiments were conducted in pure water, tap water, and Guitang River water, using 100 mL of a 30 mg/L OTC solution, 10 mg ZIF-67 (Co), and 1 mM PMS, without pH adjustment and at room temperature. The tap water is sourced from the Changsha Water Supply Company (Changsha, China), while the Guitang River water is taken from the Guitang River, which flows through Changsha City. As shown in Figure 9, the OTC removal rates were 87.73% in pure water, 88.91% in tap water, and 90.82% in Guitang River water. These results indicate that the UV/ZIF-67 (Co)/PMS system exhibits good adaptability to various environmental waters. However, Guitang River water appears to have a slight inhibitory effect on the adsorption and removal of OTC by ZIF-67 (Co). This may be attributed to competition between suspended solids and complex organic substances present in Guitang River water for adsorption sites, which can reduce the efficacy of OTC removal. Despite this, the UV/ZIF-67 (Co)/PMS system achieves a higher OTC removal rate in Guitang River water compared to pure water. This enhanced performance could be due to the generation of additional secondary free radicals from the suspended solids and complex substrates in Guitang River water under UV irradiation, which potentially accelerates OTC degradation [57,58].

3.3. Free Radical Identification

To elucidate the reaction mechanism of the UV/ZIF-67 (Co) activation of PMS for the degradation of OTC, ethanol (EtOH) and tert-butyl alcohol (TBA) were employed as quenching agents. EtOH and TBA are commonly used to identify the presence of S O 4 • − and •OH, respectively. In this experiment, 100 mL of a 30 mg/L OTC solution, 10 mg of ZIF-67 (Co), and 1 mM PMS were treated with varying volumes of ethanol (0.25, 1.25, 2.50 mL) and tert-butyl alcohol (0.25, 1.25, 2.50 mL) at room temperature. The results, presented in Figure 10, show that after the addition of 0.25, 1.25, and 2.50 mL of EtOH, the OTC removal rates decreased to 73.92%, 69.56%, and 67.27%, respectively. These reductions correspond to decreases of 13.91%, 19.17%, and 20.46% compared to the removal rates observed without EtOH, suggesting the involvement of S O 4 • − in the reaction. Similarly, with the addition of 0.25, 1.25, and 2.50 mL of TBA, the OTC removal rates were 77.10%, 74.74%, and 72.10%, respectively, indicating decreases of 10.63%, 12.99%, and 15.63% compared to the rates without TBA. This suggests the presence of •OH in the system. These findings demonstrate that the UV/ZIF-67 (Co)/PMS system generates two types of active free radicals: S O 4 • − and •OH, which is consistent with previous studies [59]. PMS generates sulfate radicals under the activation of UV/ZIF-67 (Co), which react with hydroxide ions in solution to produce hydroxyl radicals. These free radicals contain unpaired electrons and have high reactivity, making them easy to react with OTC and achieve the removal of OTC.

4. Conclusions

In this study, a new system for removing antibiotics using UV/ZIF-67 (Co)-activated peroxymonosulfate (PMS) was developed. ZIF-67 (Co) material was synthesized and characterized, and OTC was selected as the target antibiotic pollutant for the degradation experiments. The effects of ZIF-67 (Co) dosage, PMS dosage, pH, and inorganic anions on the system were investigated. The active species produced in the system were identified. The results are summarized as follows:
(1) XRD analysis revealed that the combination of cobalt nitrate hexahydrate and 2-methylimidazole resulted in the formation of a complete framework material. FT-IR provided vibrational characteristic peaks and identified the functional groups present in ZIF-67 (Co). SEM showed that ZIF-67 (Co) had a rhombic dodecahedral nanocrystal morphology with medium-sized crystals.
(2) Compared with UV, UV/PMS, ZIF-67 (Co), ZIF-67 (Co)/PMS, and UV/ZIF-67 (Co) systems, the UV/ZIF-67 (Co)/PMS system had the highest OTC removal rate. The main reason is that the active free radicals generated by PMS activated by UV/ZIF-67 (Co) play an important role in OTC removal. Under the conditions of room temperature, 10 mg ZIF-67 (Co), 1 mM PMS, 30 mg/L OTC, 400 W UV light, and 100 mL reaction solution, the degradation efficiency of OTC reached 87.73% within 10 min. This demonstrates the feasibility of the UV/ZIF-67 (Co)/PMS system for the effective removal of organic pollutants.
(3) The results indicate that increasing the dosage of ZIF-67 (Co) and PMS can improve the OTC removal rate. However, as the dosage increases, the marginal benefits of additional ZIF-67 (Co) and PMS on OTC removal gradually decrease. The degradation efficiency of OTC is highest at weakly acidic pH. Extreme acidic or alkaline conditions may damage the internal structure of materials, thereby potentially affecting the adsorption and catalytic performance of the system. Cl⁻ and NO3⁻ have a minimal effect on the reaction system, while HCO3⁻ has a certain inhibitory effect on the removal of OTC. The UV/ZIF-67 (Co)/PMS system can adapt to various environmental water and conditions.
(4) In the free radical quenching experiment, it was observed that an increase in the concentration of ETOH and TBA led to a significant decrease in the OTC removal rate. This indicates the presence of two active substances in the system, namely, S O 4 • − , and •OH. These active substances can quickly react with OTC, leading to OTC degradation.
In summary, the UV/ZIF-67 (Co)-activated peroxymonosulfate system demonstrates excellent performance in removing organic pollutants and holds significant potential for practical applications.

Author Contributions

Conceptualization, Y.L. and R.S.; methodology, Y.L. and R.S.; formal analysis, Z.L., Y.Z. and R.S.; investigation, M.Y. and R.S.; resources, R.S.; data curation, S.H. and R.S.; writing—original draft, Y.L., Z.L. and R.S.; writing—review and editing, M.Y., X.D., Y.C., Y.Z. and S.H.; supervision, X.D. and Y.C.; project administration, R.S.; funding acquisition, R.S. and Y.L. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by Major Program Natural Science Foundation of Hunan Province of China (2021JC0001), Hunan Provincial Natural Science Foundation of China (2023JJ31010, 2024JJ7647, 2024JJ7094), and Key Project of Scientific Research Project of Hunan Provincial Department of Education (23A0225). This research was also funded by National Nature Science Foundation of China (52000183), Research project funded by the Education Department of Hunan Province (22B0883), Hunan Province Environmental Protection Research Project (HBKYXM-2023038), National Local Joint Engineering Research Center of Heavy Metals Pollutants Control and Resource Utilization Open Fund (ES202380056), and Scientific Research Foundation for Talented Scholars of CSUFT (2020YJ010).

Data Availability Statement

Data are contained within the article.

Acknowledgments

The authors thank all the participants who devoted their free time to participate in this study.

Conflicts of Interest

Authors Mingqiang Ye and Yihui Zhou were employed by the Aerospace KaiTian Environmental Technology Co., Ltd. Author Xiangrong Dai was employed by the company PowerChina Zhongnan Engineering Corporation Limited. 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.

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Water 16 02586 g001
Figure 1. SEM of ZIF-67 (Co).
Figure 1. SEM of ZIF-67 (Co).
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Figure 2. XRD of ZIF-67 (Co).
Figure 2. XRD of ZIF-67 (Co).
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Figure 3. FT-IR of ZIF-67 (Co).
Figure 3. FT-IR of ZIF-67 (Co).
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Figure 4. Effects of different reaction systems on OTC removal efficiency (The dashed line represents the boundary point between dark and light).
Figure 4. Effects of different reaction systems on OTC removal efficiency (The dashed line represents the boundary point between dark and light).
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Figure 5. The effect of ZIF-67 (Co) dosage on OTC removal efficiency.
Figure 5. The effect of ZIF-67 (Co) dosage on OTC removal efficiency.
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Figure 6. Effect of PMS dosage on OTC removal efficiency.
Figure 6. Effect of PMS dosage on OTC removal efficiency.
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Figure 7. Effect of pH on OTC removal efficiency.
Figure 7. Effect of pH on OTC removal efficiency.
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Figure 8. Effect of anions on OTC removal efficiency ((a) 1 mM and (b) 10 mM).
Figure 8. Effect of anions on OTC removal efficiency ((a) 1 mM and (b) 10 mM).
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Figure 9. Effects of different environment water on OTC removal efficiency.
Figure 9. Effects of different environment water on OTC removal efficiency.
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Figure 10. Effect of free radical quencher ((a) ETOH, (b) TBA) on OTC degradation.
Figure 10. Effect of free radical quencher ((a) ETOH, (b) TBA) on OTC degradation.
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Luo, Y.; Liu, Z.; Ye, M.; Zhou, Y.; Su, R.; Huang, S.; Chen, Y.; Dai, X. Synergistic Enhancement of Oxytetracycline Hydrochloride Removal by UV/ZIF-67 (Co)-Activated Peroxymonosulfate. Water 2024, 16, 2586. https://doi.org/10.3390/w16182586

AMA Style

Luo Y, Liu Z, Ye M, Zhou Y, Su R, Huang S, Chen Y, Dai X. Synergistic Enhancement of Oxytetracycline Hydrochloride Removal by UV/ZIF-67 (Co)-Activated Peroxymonosulfate. Water. 2024; 16(18):2586. https://doi.org/10.3390/w16182586

Chicago/Turabian Style

Luo, Yiting, Zhao Liu, Mingqiang Ye, Yihui Zhou, Rongkui Su, Shunhong Huang, Yonghua Chen, and Xiangrong Dai. 2024. "Synergistic Enhancement of Oxytetracycline Hydrochloride Removal by UV/ZIF-67 (Co)-Activated Peroxymonosulfate" Water 16, no. 18: 2586. https://doi.org/10.3390/w16182586

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