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Article

Cobalt-Based MOF Material Activates Persulfate to Degrade Residual Ciprofloxacin

by
Yiting Luo
1,2 and
Rongkui Su
3,4,*
1
Hunan First Normal University, Changsha 410114, China
2
National Engineering Laboratory of Southern Forestry Ecological Application Technology, Changsha 410004, China
3
PowerChina Zhongnan Engineering Corporation Limited, Changsha 410004, China
4
College of Life and Environmental Sciences, Central South University of Forestry and Technology, Changsha 410004, China
*
Author to whom correspondence should be addressed.
Water 2024, 16(16), 2299; https://doi.org/10.3390/w16162299
Submission received: 14 July 2024 / Revised: 11 August 2024 / Accepted: 12 August 2024 / Published: 15 August 2024
(This article belongs to the Special Issue Biomass Carbon Materials in Wastewater Treatment and Resource Utilization)
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Abstract

Antibiotic residues in water environments have garnered widespread attention. Metal-organic frameworks (MOFs) have found extensive applications in water purification. This study investigates the use of a cobalt-based MOF material, zeolitic imidazolate framework-67(ZIF-67)(Co), for activating persulfate (PMS) to remove residual antibiotic ciprofloxacin (CIP) from aqueous environments. The main findings are as follows: ZIF-67(Co) exhibits insignificant adsorption capacity for CIP, and PMS alone does not degrade CIP effectively. However, ZIF-67(Co)-activated PMS demonstrates the efficient degradation of CIP, following pseudo-second-order reaction kinetics. Under optimal conditions of the catalyst dosage (15 mg) and PMS concentration (1.0 mM), the removal efficiency reaches 88% after 60 min. Comparative analysis of CIP degradation at different initial pH levels shows that the highest efficiency is reached under mildly acidic conditions, with an 86% removal rate achieved within 60 min under these conditions. Investigation into the impact of various inorganic anions on the ZIF-67(Co)-catalyzed PMS degradation of CIP reveals significant inhibition by chloride ions ( Cl − ), whereas nitrate ( NO 3 - ) and sulfate ( SO 4 2 - ) ions have minor effects on the degradation efficiency. The system demonstrates a consistent performance across different water matrices, highlighting ZIF-67(Co)/PMS as effective for ciprofloxacin removal in environmental waters. This study provides technical support for the efficient removal of antibiotic residues.

1. Introduction

Currently, antibiotics are classified as emerging organic pollutants [1]. In modern society, approximately only 16% of antibiotics are used for human purposes, while the remaining 84% are employed in modern livestock and aquaculture industries. Due to their low metabolic degradability, about 50% to 80% of antibiotics are discharged into the environment through urine and feces, contaminating surface water, groundwater, soil and even drinking water [2]. DU et al. studied the presence of 25 kinds of antibiotics in the coastal waters of Dalian, Bohai Sea; the results showed that the detection rate of antibiotics in this area was high, and the total mass concentration was 22.6–2402.4 ng/L [3]. Wang et al. studied the concentration, species and spatial and temporal distribution of 13 antibiotics (tetracycline, sulfonamides and fluoroquinolones) in the cultured water body of Honghu Lake and its related river networks and ponds, and found that tetracycline, oxytetracycline, chloromycin and sulfadiazine were the antibiotics with high detection rates in this area [4]. The maximum mass concentrations of tetracycline, oxytetracycline, chloromycin and sulfadiazine were 1454.8, 2796.6, 1431.3 and 499.5 ng/L, respectively. Ciprofloxacin (CIP) is a synthetic fluoroquinolone antibiotic known for its potent bactericidal effects and low adverse reactions, and, thus, is widely utilized [5,6]. However, its persistence and poor biodegradability pose challenges during wastewater treatment processes, leading to direct discharge into natural water [7,8]. This can induce antibiotic-resistant strains, distort aquatic flora and fauna and disrupt various human physiological functions, posing significant threats to both human health and ecological integrity [9]. N. Martins et al. discovered that ciprofloxacin presents a notable risk to highly sensitive aquatic species, as its levels have significantly surpassed the accepted risk threshold [10]. Biochemical analysis of Corbicula fluminea exposed to CIP in sediments showed that the increase in CIP concentration would lead to the disorder of the antioxidant system balance, which would cause oxidative damage to the gills and digestive glands [11]. Given these risks, there is an urgent need for viable methods to manage residual CIP contamination in aquatic systems [12,13].
A range of treatment methods have been established to eliminate antibiotics from wastewater, including physical, biological and chemical treatment methods [14]. Under the conditions of a 66.7 mg/L dosage, pH 5, 10 h of intercontact time and a 10 mg/L initial mass concentration of TCs, the adsorption capacity of MGO on oxytetracycline (OTC) was 292.4 mg/g [15]. WANG et al. studied the treatment of wastewater from pig farms containing antibiotics with different carbon sources such as methanol, starch and sucrose by an aerobic granular sludge (AGS) system, and found that AGS with sucrose as carbon source had the highest removal rates of tetracycline and oxytetracycline, which were 81.40% and 80.69%, respectively [16]. At room temperature, pH 6~7, a reaction time of 2 h, and an initial free chlorine concentration of 2 mg/L, GAFFNEY et al. found that free chlorine affected sulfadiazine (SDZ), sulfathiazole (STZ), sulfapyridine (SPD), sulfamethazine (SMZ) and sulfamethazine The removal rates of (SMT) and sulfamethoxazole (SMX) were 65%, 22%, 75%, 68%, 64% and 22%, respectively [17]. However, adsorption merely separates antibiotics from wastewater without fully degrading them, while biological treatment suffers from low removal efficiency due to the inhibitory effects of antibiotics on bacterial activity [18]. In contrast, advanced oxidation processes (AOPs) generate highly reactive radicals, including hydroxyl radicals (•OH), sulfate radicals ( SO 4 • − ) and superoxide radicals (O2•−), which effectively degrade antibiotics into products with lower toxicity [19]. Conventional AOPs conventionally depend on hydroxyl radical (•OH) as the primary reactive species, decomposing organic pollutants into smaller molecules such as CO2 and H2O [20,21,22]. In contrast, persulfate-based AOPs generate sulfate radicals ( SO 4 • − ) during reactions. This technology exhibits lower dependence on environmental conditions, broader pH applicability, superior stability of SO 4 • − and longer half-lives compared to •OH, enabling a more thorough degradation of organic pollutants [23,24]. Current activation methods for persulfate include photocatalysis [25], thermal activation [26] and transition metal activation [27]. For instance, Forouzesh et al. investigated zero-valent copper as a catalyst, demonstrating that thermally activated persulfate effectively generates highly oxidative radicals to degrade metronidazole (MTZ). They observed that under stable conditions, MTZ degradation rates exceeded 95%, with the involvement of both SO 4 • − and •OH radicals [28]. Key to enhancing the efficiency of persulfate-based advanced oxidation technologies is identifying efficient catalyst materials [29,30].
Metal-organic frameworks (MOFs) represent a novel class of nanomaterials composed of metal ions and organic ligands that are self-assembled through strong coordination bonds. They find extensive applications in adsorption [31], catalysis [32] and photocatalysis [33]. Among MOFs, zeolitic imidazolate framework-67(ZIF-67)(Co) is widely used in water pollution treatment due to its exceptional properties. These materials, which are synthesized via the self-assembly of tetrahedral metal ions such as cobalt (Co2+) or zinc (Zn2+) in combination with imidazole-based ligands, exhibit porous crystalline architectures [34]. MOF materials can exhibit outstanding properties and specialized functions through strategic design and modification. Due to their tunable structure and composition, MOFs and their derivatives are effective as catalysts for activating peroxydisulfate and peroxymonosulfate [35]. MOFs catalyze the production of highly oxidative radicals to degrade organic pollutants in the environment [36], leveraging their excellent thermal stability and functional characteristics [37]. As catalysts, MOFs activate persulfate or other sulfate salts to generate SO 4 • − and •OH, effectively converting target pollutants into harmless substances like water and carbon dioxide [38]. This method is straightforward to implement under various environmental conditions, demonstrating significant efficacy in pollutant removal while minimizing secondary pollution.
This study synthesizes and prepares the cobalt-based MOF material ZIF-67(Co) and investigates its application in activating persulfate to degrade residual ciprofloxacin. Key research objectives include the following: (1) developing a solvent-free synthesis technique for the ZIF-67(Co) catalyst under ambient conditions, characterizing its crystal structure through various analytical methods to identify functional groups; (2) evaluating the efficacy and kinetic characteristics of ZIF-67(Co)-activated persulfate in ciprofloxacin degradation; (3) studying the influence of the catalyst dosage (ZIF-67), the persulfate dosage (PMS), pH, inorganic anions and different water matrices on the effectiveness of ZIF-67(Co)-activated persulfate for ciprofloxacin removal. This research provides valuable technical insights into the application of ZIF-67(Co) for organic pollutant removal from wastewater. Compared to the international relevant literature, the innovation of this study lies in the development of an efficient ZIF-67(Co)/PMS system for pollutant removal.

2. Materials and Methods

2.1. Instruments, and Experimental Reagents

The experimental reagents and instruments are detailed in Table 1 and Table 2.

2.2. Experimental Methods

2.2.1. ZIF-67(Co) Material Preparation

Through treatment in methanol solution, ZIF-67(Co) with high selectivity was synthesized. Next, 1.164 g Co(NO3)2·6H2O and 1.970 g 2-MI were accurately weighed and dissolved in 40 mL of MeOH. Then, the 2-MI solution was slowly added to the cobalt nitrate solution under stirring at 350 rpm for 1 h using a magnetic stirrer. The resulting mixed solution is allowed to precipitate at 22 °C for 24 h. The precipitate is washed several times with methanol and centrifuged until the supernatant is colorless and transparent, yielding a purple precipitate. Finally, the purple precipitate was dried in a vacuum oven at 60 °C for 6 h to obtain the target sample, ZIF-67(Co).

2.2.2. Material Characterization

(1) Scanning electron microscopy (SEM)
Observations revealed micrometer-scale structural features of ZIF-67(Co) by a JSM-7610FPlus scanning electron microscope.
(2) X-ray diffraction (XRD) spectroscopy
XRD patterns were obtained at scanning rate of 10°/min over a range of 5° to 90° by MiniFlex600, characterizing the crystalline structure of the analyzed materials.
(3) Fourier transform infrared (FT-IR) spectroscopy
FT-IR spectra of ZIF-67(Co) were acquired by a Nicolet iS20 Fourier transform infrared spectrophotometer, facilitating characterization of surface functional groups of the synthesized materials.

2.2.3. ZIF-67(Co) Activated Persulfate for Removal of Ciprofloxacin

Ciprofloxacin solution was placed in a 400 mL beaker and adjusted to initial pH using 0. 1 mol Cl and sodium NaOH solutions. A measured amount of ZIF-67 (Co) sample was added to the reaction solution and stirred for 60 min at 400 rpm using a magnetic stirrer. Samples were periodically withdrawn to evaluate the adsorption efficiency of ZIF-67(Co). Subsequently, peroxydisulfate was introduced into the reaction system to initiate catalytic degradation experiments. Samples were withdrawn at intervals for analysis. The absorbance of ciprofloxacin was determined by spectrophotometer at 278 nm. Then the concentration of ciprofloxacin was calculated by using the standard curve, and the degradation rate of ciprofloxacin was calculated according to Equation (1) [38].
R = C 0 − C t C 0 × 100 %
where R (−) represents the degradation efficiency of the organic pollutant, C0 (mM) is the initial CIP concentration and Ct (mM) is the CIP concentration at time t (min).

3. Results and Discussion

3.1. Characterization of ZIF-67(Co) Material

3.1.1. SEM Characterization

Figure 1 illustrates the preparation of ZIF-67(Co) observed via scanning electron microscopy (SEM). Upon inspection, uniform particles with a rhombic dodecahedral structure are clearly visible, exhibiting sharp edges. These particles display a regular polyhedral morphology with a uniform distribution and good dispersibility, as depicted in Figure 1. The synthesized ZIF-67(Co) conforms to the expected rhombic dodecahedral structure. The SEM image indicates a size distribution of particles ranging from 0.1 to 1.0 μm. While the formation of ZIF-67(Co) is governed by chemical bonding between metal ions and ligands, the choice of metal ion salts influences molecular particle size [39]. In this study, Co2+ ions sourced from Co(NO3)2·6H2O theoretically produce materials of a moderate size (<1.0 μm), consistent with the observed dimensions in Figure 1.

3.1.2. XRD Characterization

Figure 2 presents the XRD pattern of ZIF-67(Co). The significant diffraction peaks observed at 2θ = 7.32°, 10.36°, 12.70°, 14.68°, 16.42° and 17.98° correspond to the crystallographic indices (011), (002), (112), (022), (013) and (222) of ZIF-67(Co) standard cards [40], confirming the successful synthesis of ZIF-67(Co).

3.1.3. FT-IR Characterization

The ligand 2-MI notably influences the infrared absorption spectra of ZIF-67(Co) [41]. Analysis of the characterization graph (Figure 3) reveals characteristic peaks at 758 and 1629 cm−1 corresponding to the C=N bond on the imidazole ring. Additionally, a new absorption peak attributed to the C-N single bond appears in the range of 990–1142 cm−1, exhibiting favorable spectral properties. At 1302 cm−1, the C=C double bond on the imidazole ring displays a heightened spectral response. The peak at 1415 cm−1 arises from stretching vibrations of the imidazole ring. Furthermore, distinctive peaks at 2920 and 3131 cm−1 correspond to the C-H bonds on the aromatic ring and aliphatic hydrocarbon chain, respectively. The peak at 3431 cm−1 represents the N-H bond on the imidazole ring [42], confirming the successful synthesis of ZIF-67(Co).

3.1.4. XPS Characterization

Figure 4 shows the complete measurement of ZIF-67 involving Co 2p, C 1s, N 1s and O 1s peaks. For the spectrum of Co 2p (Figure 4b), the two main peaks at 778.78 and 795.08 eV (with an interval of about 16 eV) belong to Co 2p3/2 and Co 2p1/2, respectively [43]. The two distinguishable companion peaks of the main peak of Co 2p3/2 are located near 784.74 eV and 788.33 eV, and the intensity at 784.74 eV is much higher than that at 788.33 eV. The oxidation state of Co (II) ions is generally difficult to determine, and the energy gap between 2p main peak and accompanying peak of Co (II) ions can be an important criterion for judging the oxidation state of Co (II) ions. That is, the energy gap of Co (II) ions is about 6.0 eV between the 2p main peak and the accompanying peak of Co (II) ions, while Co (III) cations typically have an energy gap of 9–10 eV [44,45]. Therefore, Co (II) is the main form present in the prepared ZIF-67 material.

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

3.2.1. Adsorption Efficiency of ZIF-67(Co) on CIP

At room temperature and an initial pH of 5.0, the adsorption efficiency of ZIF-67(Co) on CIP was investigated. As shown in Figure 5, the adsorption process reaches saturation at around 10 min. When the dosage of ZIF-67(Co) was 5, 10, 15 and 20 mg, the removal efficiency for CIP was approximately 20%, 28%, 31% and 36%, respectively. With the increase in the ZIF-67(Co) dosage, the adsorption removal rate of ciprofloxacin increased gradually. These results indicate the saturation of adsorption within approximately 10 min and that the adsorption efficiency increases with the increase in the ZIF-67(Co) dosage. MOF, as a material with high porosity and adjustability, has an adsorption effect on ciprofloxacin and other antibiotics.

3.2.2. PMS Degradation Capability on CIP

Under room temperature conditions and an initial pH of around 5.0, the degradation effect of PMS alone on ciprofloxacin (CIP) was experimentally studied. Various volumes (0.5, 1.0, 2.0 and 3.0 mL) of 0.1 mol/L PMS were added separately to a 200 mL CIP solution at 20 mg/L without ZIF-67(Co). As shown in Figure 6, when only PMS was added, the degradation rates of CIP after 60 min were approximately 8% at 0.25 mM PMS, 11% at 0.50 mM PMS, 16% at 1.00 mM PMS and 20% at 1.50 mM PMS. These results showed that the removal effect of CIP by PMS alone was not obvious.

3.2.3. Degradation of Ciprofloxacin by ZIF-67(Co)/PMS System

First, 15 mg of ZIF-67(Co) and 200 mL of CIP solution with a concentration of 20 mg/L were mixed. After adsorption equilibrium, H2O2/PMS with a concentration of 1.0 mM was added to investigate the degradation efficiency of CIP under the pH 5.0 conditions in this study. As showed in Figure 7, the removal efficiencies of CIP by ZIF-67(Co), PMS, ZIF-67(Co)/H2O2 and ZIF-67(Co)/PMS were 16.0%, 0.01%, 21.7% and 87.8% after 60 min, respectively. This indicates that the ZIF-67(Co)/PMS system achieves a higher efficiency in removing CIP due to the enhanced radical production from PMS catalyzed by ZIF-67(Co).
The degradation of CIP using the ZIF-67(Co)-catalyzed PMS system was fitted using linear regression. The fitting showed R2 values of 0.4563 for zero-order kinetic, 0.7262 for first-order kinetic and 0.9463 for second-order kinetic equations, indicating that the CIP degradation in the ZIF-67(Co)/PMS system follows the pseudo-second-order kinetic equation.

3.3. Impact of ZIF-67(Co) Dosage on Experimental Results

Under controlled conditions of 1.0 mM PMS, an initial of pH 5.0, and room temperature, experiments were conducted with varying dosages of ZIF-67(Co) (5.0, 10.0, 15.0 and 20.0 mg). As depicted in Figure 8, at the 5 mg catalyst dosage, the removal rate of CIP after 60 min was approximately 79%; at 10 mg, it was 83%; at 15 mg, approximately 88%; and at 20 mg, about 84% after 360 min. The removal rate of CIP increased with increasing the catalyst dosage from 5 mg to 15 mg, as more active sites were available for better CIP degradation. However, increasing the dosage from 15 mg to 20 mg showed minimal change or a slight decrease in the CIP removal rate due to material aggregation reducing the number of active sites [46]. The results show that higher amounts of catalyst are not necessarily better. Figure 8 presents the kinetic analysis of different ZIF-67(Co) dosage levels. The reaction rate constants (k) for the catalytic degradation of CIP were 0.0079 L·mol−1·min−1 at 5 mg, 0.0087 M−1·min−1 at 10 mg, 0.0099 M−1·min−1 at 15 mg and 0.0083 M−1·min−1 at 20.0 mg. Therefore, considering degradation effectiveness, reaction rate and economic factors, 15 mg was identified as the optimal dosage. Hence, the quantity of the catalyst added does not necessarily correlate with a better performance.

3.4. Impact of PMS Dosage on Experimental Results

Under pH 5.0 and room temperature conditions, with a catalyst dosage of 15.0 mg ZIF-67(Co), the effect of PMS dosage on CIP was examined. PMS concentrations of 0.25, 0.5, 1.0, 1.5 and 2.0 mM were added to the CIP reaction solution, as shown in Figure 8. At 0.25 mM PMS concentration, the removal rate of pollutants reached only 59% in 60 min; increasing the concentration to 0.5 mM resulted in an 80% removal rate; at 1.0 mM, the removal rate reached approximately 88% and at 1.5 mM and 2.0 mM, it stabilized around 85%. The activation of PMS generates SO 4 • − and •OH, and an appropriate increase in the PMS concentration produces more radicals for effective pollutant reaction. However, excessive PMS competes for reaction sites, leading to reduced reaction rates and removal efficiencies. Additionally, interactions between high concentrations of sulfate and persulfate produce sulfate radicals and sulfate ions, which undergo self-decomposition reactions, thereby reducing the sulfate content in the solution [47].
The kinetics of CIP degradation with varying PMS dosages are shown in Figure 9. Reaction rate constants (k, in units of L mol−1 min−1) under conditions of 0.25, 0.50, 1.0, 1.5 and 2.0 mM PMS were determined as 0.0036, 0.0089, 0.0096, 0.0066 and 0.0068, respectively. It is evident that both the reaction rate and the degradation efficiency were poor, at 0.25 mM PMS, due to insufficient dosage. Under the conditions of 1.5 and 2.0 mM, the degradation rate of PMS is higher than 0.5 mM and lower than 0.5 mM, respectively. The reason is that excessive H SO 5 − can quench the SO 4 • − and •OH generated by the catalyzing PMS solution, producing low oxidation SO 5 • − and, thereby, reducing the effective utilization of SO 4 • − and •OH [48,49]. At PMS dosages of 0.5 mM and 1.0 mM, the reaction rates were 0.0089 M−1 min−1 and 0.0096 M−1 min−1, respectively. Further economic and degradation efficacy analyses suggest that the optimal PMS dosage is 1.0 mM, which ensures the effective removal of CIP with a higher reaction rate.

3.5. Impact of pH Conditions on CIP Degradation

The catalyst ZIF-67(Co) was introduced at a dosage of 15 mg, with PMS added at 1.0 mM, and the effects of different pH values on CIP removal were examined at room temperature. As depicted in Figure 10, under an initial pH of 3, CIP achieved a removal rate of 79% over 60 min. At initial pH values of 7, 9 and 11, CIP removal rates remained consistent at 82%. Notably, at pH 5, CIP removal reached 86%, indicating the enhanced degradation of CIP in weakly acidic solutions. This can be attributed to the inhibitory effect of excessive OH− under alkaline conditions, which hinders the decomposition of H SO 5 − , thereby reducing the availability of SO 4 • − necessary for CIP degradation. Furthermore, the accumulation of OH− on the catalyst surface strengthens the electrostatic repulsion between catalyst and persulfate, leading to a decrease in the SO 4 • − concentration [50,51]. In general, ZIF-67(Co)/PMS has a wide range of adaptability to pH. Zhang et al. also found that the FeCo/N-MOF/PS system has good adaptability to pH, which is consistent with our conclusion [46].

3.6. Impact of Anions on Experimental Outcomes

At a dosage of 15 mg for ZIF-67(Co) and 1.0 mM for PMS, 10 mM and 50 mM dosages of NaCl, KNO3 and K2SO4 were added to the system at room temperature to investigate the effects of three tradition inorganic anions, Cl − , NO 3 − and SO 4 2 − , on the CIP removal efficiency at lower and higher anion concentrations, respectively. As shown in Figure 11, within 60 min, the control group achieved an 88% removal rate. However, with the addition of 10 mM anions, chloride ( Cl − ) exhibited a significant inhibitory effect on CIP degradation, resulting in a removal rate of approximately 70% after 60 min. At the same concentration (10 mM), sulfate ( SO 4 2 − ) and nitrate ( NO 3 − ) did not significantly inhibit CIP degradation, yielding removal rates of 86% and 83%, respectively, after 60 min of reaction. At a higher concentration of 50 mM anions, chloride ( Cl − ) exerted a more pronounced inhibitory effect on CIP degradation, with a removal rate of only 60% after 60 min. Sulfate ( SO 4 2 − ) and nitrate ( NO 3 − ) at this concentration showed comparable inhibitory effects on CIP removal, with removal rates reaching 78% for both after 60 min of reaction.
Through the above research, it was found that Cl − can effectively inhibit the degradation ability of PMS on CIP under the catalysis of ZIF-67(Co). NO 3 − and SO 4 2 − did not have a significant inhibitory effect. Yang et al. [52] found that this phenomenon may be due to the reaction between Cl − and the active free radicals SO 4 • − and •OH in the solution, and the low oxidation properties of the products Cl• and ClOH−• leading to a decrease in the effective utilization rate of strong oxidative free radicals used for degrading CIP.

3.7. Impact of Different Environment Water

The influence of tap water and outdoor pond water on CIP degradation was tested. Figure 12 showed that the degradation efficiency of CIP reached approximately 84% using tap water and around 80% using outdoor pond water within 60 min. Compared with the degradation rate of 88% in the control group, the difference was not obvious. It is evident that tap water and outdoor pond water have no significant inhibitory effect. This suggests that the ZIF-67(Co)/PMS system can effectively remove ciprofloxacin under environmental water conditions. However, 10 mg/L humic acid (HA) has a certain inhibitory effect on CIP degradation, which may be due to the reaction between humic acid and free radicals, resulting in a decrease in free radicals’ utilization for CIP degradation.

3.8. Degradation Mechanism

Free radical quenching experiments were used to test the active species involved in the degradation of CIP by ZIF-67(Co)-activated PMS. As is well known, methanol (MeOH) is a quencher for SO 4 • − and •OH, while tert butanol (TBA) can quench •OH. This study added specific concentrations of methanol and tert butanol as quenchers to the system. Figure 13 shows that without adding any quencher, the degradation rate of CIP can reach 88% at 60 min. After adding 100 mM MeOH to the system, the final degradation rate of CIP solution was 61.1%; after adding 100 mM TBA to the system, the CIP degradation rate was 85.8% at 60 min. The results showed that TBA had almost no inhibitory effect on the system, while methanol had an obvious inhibitory effect. This suggests that •OH hardly participates in the degradation reaction of the system. Therefore, SO 4 • − is the main active substance involved in the reaction. Our results have been consistent with the research of Su et al. [53]. Su et al. studied the degradation of dimethyl phthalate by persulfate activated by a novel MOF catalyst (Fe-MOF-74 @ SiO2 @ MIP) [53]. The findings indicated that Fe-MOF-74 @ SiO2 @ MIP demonstrated strong activation potential for PS, with SO 4 • − being the predominant species.

4. Conclusions

This study explores the removal efficiency of Co-based MOFs for the quinolone antibiotic ciprofloxacin. The research systematically investigates the impact of the MOF dosage, potassium persulfate (PMS) addition, pH values, types of anions and natural water environments on the CIP removal efficiency. ZIF-67(Co) material was synthesized via a solvent-based method at room temperature, featuring a rhombic dodecahedral structure. ZIF-67(Co) exhibits a modest adsorption capacity for CIP, while the addition of sole persulfate (PMS) does not effectively degrade CIP. However, activation of ZIF-67(Co) with PMS proves efficient in degrading CIP, adhering to a pseudo-second-order reaction kinetics model. Under conditions of a catalyst dosage of 15 mg and a 1.0 mM PMS addition, the CIP degradation efficiency reaches 88% within 60 min of reaction. Comparative analysis of CIP degradation at different initial pH levels reveals the highest efficiency is reached under weakly acidic conditions, with an 86% CIP removal rate achieved within 60 min under these conditions. Investigation into the influence of various inorganic anions on ZIF-67(Co)-catalyzed PMS degradation of CIP indicates significant inhibition by chloride ions ( Cl − ), whereas nitrate ( NO 3 − ) and sulfate ( SO 4 2 − ) exhibit minor effects on the degradation efficiency. The system demonstrates consistent degradation rates across different water conditions, highlighting the robust removal efficacy of the ZIF-67(Co)/PMS system for ciprofloxacin in environmental water matrices. This study contributes to supporting the use of MOF materials for the catalytic oxidation of antibiotic residues. However, the practical application of MOF as a catalytic material still needs a lot of research.

Author Contributions

Conceptualization, Y.L. and R.S.; methodology, Y.L. and R.S.; formal analysis, Y.L. and R.S.; data curation, Y.L. and R.S.; writing—original draft, Y.L.; writing—review and editing, R.S.; 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), Hunan Provincial Natural Science Foundation of China (2024JJ7094) and Key Project of Scientific Re-search Project of Hunan Provincial Department of Education (23A0225).

Data Availability Statement

Data is contained within the article.

Acknowledgments

The authors thank all the participants who devoted their free time to participate in this study. This research was also funded by 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).

Conflicts of Interest

Author Rongkui Su 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 02299 g001
Figure 1. SEM.
Figure 1. SEM.
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Figure 2. XRD.
Figure 2. XRD.
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Figure 3. FT-IR.
Figure 3. FT-IR.
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Figure 4. XPS Spectrum (a) Survey Spectrum, (b) Co 2p, (c) C 1s, (d) N 1s, (e) O 1s.
Figure 4. XPS Spectrum (a) Survey Spectrum, (b) Co 2p, (c) C 1s, (d) N 1s, (e) O 1s.
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Figure 5. Adsorption efficiency of ZIF-67(Co) on CIP.
Figure 5. Adsorption efficiency of ZIF-67(Co) on CIP.
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Figure 6. Removal efficiency of CIP by adding only PMS.
Figure 6. Removal efficiency of CIP by adding only PMS.
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Figure 7. Degradation of ciprofloxacin by ZIF-67(Co)/PMS system.
Figure 7. Degradation of ciprofloxacin by ZIF-67(Co)/PMS system.
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Figure 8. Impact of ZIF-67(Co) dosage on CIP removal efficiency.
Figure 8. Impact of ZIF-67(Co) dosage on CIP removal efficiency.
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Figure 9. Impact of PMS dosage on CIP removal efficiency.
Figure 9. Impact of PMS dosage on CIP removal efficiency.
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Figure 10. Impact of pH on CIP removal efficiency.
Figure 10. Impact of pH on CIP removal efficiency.
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Figure 11. Impact of anions on CIP removal efficiency ((a). 10 mM and (b). 50 mM).
Figure 11. Impact of anions on CIP removal efficiency ((a). 10 mM and (b). 50 mM).
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Figure 12. Effects of different environment water and HA conditions on CIP degradation.
Figure 12. Effects of different environment water and HA conditions on CIP degradation.
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Figure 13. Radical quenching experiment.
Figure 13. Radical quenching experiment.
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Table 1. Instruments.
Table 1. Instruments.
NameModelManufacturer
Digital magnetic stirrerZGCJ-3AShanghai Zigui Instrument Co., Ltd., Shanghai, China
Ultra-pure water purifierUPT-11-40Chengdu Youpu Instrument Equipment Co., Ltd., Chengdu, China
Vacuum drying ovenDZ-2BCIVTianjin Test Instrument Co., Ltd., Tianjin, China
Desktop high-speed centrifugeTG16-WSHunan Xiangyi Centrifuge Instrument Co., Ltd., Changsha, China
Electronic balanceDHG-9023AShanghai Precision Experimental Equipment Co., Ltd., Shanghai, China
Digital acidity meterPHS-3EShanghai Yidian Scientific Instrument Co., Ltd., Shanghai, China
Fourier transform infrared spectrometerNICOLET iS20Thermo Scientific, Waltham, MA, USA
XRDMiniFlex600Rigaku, Tokyo, Japan
SEMJSM-7610FPlusJeol, Tokyo, Japan
UV visible spectrophotometerUV-2700iShimadzu Instrument Co., Ltd., Tokyo, Japan
Pipette100–1000 μLThermo Scientific, Waltham, MA, USA
Table 2. Experimental reagents.
Table 2. Experimental reagents.
NameChemical Formula or AbbreviationSpecificationsManufacturer
Cobalt Nitrate HexahydrateCo(NO3)2·6H2O99%Shanghai McLean Biochemical Technology Co., Ltd., Shanghai, China
2-Methylimidazole2-MI98%
Potassium Monopersulfate PMSAR
Cobalt Nitrate HexahydrateCo(NO3)2·6H2O≥88.5%
MethanolMeOHAnalytical Reagent (AR)China National Pharmaceutical Group Chemical Reagent Co., Ltd., Shanghai, China
Hydrochloric AcidHClAR
Sodium HydroxideNaOHAR
Potassium NitrateKNO3AR
Potassium SulphateK2SO4AR
Ciprofloxacin HydrochlorideC17H18FN3O3·HClARBeijing Solaibao Technology Co., Ltd., Beijing, China
Potassium ChlorideKClARTianjin Hengxing Chemical Reagent Co., Ltd., Tianjin, China
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Luo, Y.; Su, R. Cobalt-Based MOF Material Activates Persulfate to Degrade Residual Ciprofloxacin. Water 2024, 16, 2299. https://doi.org/10.3390/w16162299

AMA Style

Luo Y, Su R. Cobalt-Based MOF Material Activates Persulfate to Degrade Residual Ciprofloxacin. Water. 2024; 16(16):2299. https://doi.org/10.3390/w16162299

Chicago/Turabian Style

Luo, Yiting, and Rongkui Su. 2024. "Cobalt-Based MOF Material Activates Persulfate to Degrade Residual Ciprofloxacin" Water 16, no. 16: 2299. https://doi.org/10.3390/w16162299

APA Style

Luo, Y., & Su, R. (2024). Cobalt-Based MOF Material Activates Persulfate to Degrade Residual Ciprofloxacin. Water, 16(16), 2299. https://doi.org/10.3390/w16162299

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