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Article

Highly Effective Removal of Ciprofloxacin Antibiotic from Water by Magnetic Metal–Organic Framework

by
Binbin Yu
1,2,
Hongchao Chang
2,
Wenwan Wei
2,
Hua Yu
2,*,
Zhangxin Chen
2,
Xiaoye Cheng
2,
Dan Chen
2,
Yanxian Jin
2,*,
Deman Han
2,* and
Wei Xu
3
1
School of Environment, Nanjing University, Nanjing 210008, China
2
School of Pharmaceutical and Chemical Engineering, Taizhou University, Taizhou 318000, China
3
Zhejiang Tianxiang Environmental Service Co., Ltd., Hangzhou 310011, China
*
Authors to whom correspondence should be addressed.
Water 2023, 15(14), 2531; https://doi.org/10.3390/w15142531
Submission received: 5 May 2023 / Revised: 12 June 2023 / Accepted: 9 July 2023 / Published: 10 July 2023
(This article belongs to the Section Wastewater Treatment and Reuse)
Browse Figures
Versions Notes

Abstract

:
The presence of antibiotic ciprofloxacin (CIP) in pharmaceutical wastewaters is dangerous when their concentrations exceed the allowable limits. Thus, eliminating CIP from pharmaceutical wastewaters is an essential issue. In this work, magnetic MOFs, named Fe3O4/Zn3(BTC)2 MMOF, were successfully synthesized and used for the adsorption of CIP. Compared with Cu3(BTC)2 and Fe3O4/Cu3(BTC)2 MMOF, the Fe3O4/Zn3(BTC)2 MMOF exhibited the best CIP-adsorption performance, with a maximum removal rate of 72.15% due to the large pore size, abundant adsorption sites and functional groups of MOFs, and the magnetic properties of the Fe3O4 nanorod. The influencing factors in the adsorption process, including oscillation time and pH value, were discussed, and the best adsorption performance was obtained when the pH was 3.84 and the oscillation time was 90 min. Furthermore, the removal rate of the Fe3O4/Zn3(BTC)2 MMOF still reached 31.45% after five instances of reuse, revealing its great regeneration and reusability. The results of the adsorption-kinetics studies showed that the adsorption process of CIP by Fe3O4/Zn3(BTC)2 MMOF followed the pseudo-second-order kinetic model and was mainly chemical adsorption. Based on the results above, Fe3O4/Zn3(BTC)2 MMOF is recommended as a highly efficient adsorbent for the removal of CIP from pharmaceutical wastewaters.

1. Introduction

Undoubtedly, the advancement of medical technologies has contributed to the increasing lifespans of human beings and animals. Antibiotics have been widely applied to treat bacterial diseases in humans, livestock, and fish [1,2,3,4]. Approximately 100,000 to 200,000 tons of antibiotics are used worldwide to treat human diseases yearly. Ciprofloxacin (CIP), a second-generation fluoroquinolone antibiotics, has been designated as a broad-spectrum antibiotic [5]. However, the extensive use, abuse, and random discard of antibiotics worsen the fate of water bodies and lead to uncontrolled effluent disposal in waste streams in original or metabolized form. Recently, CIP have been found in aquatic environments worldwide, which has been linked to a range of unhealthy symptoms, including diarrhea, headaches, vomiting, tremors, etc. [6,7]. Thus, the treatment of pharmaceutical wastewaters containing CIP has gradually attracted significant attention. A wide range of treatment processes, such as catalytic oxidation, electro-Fenton photocatalytic oxidation, and adsorption have been applied to remove CIP from water [8,9,10,11,12,13,14,15,16]. Compared to these technologies, adsorption has been marked as an excellent and promising approach due to its lower expenses, higher efficiency, and convenience of use. However, the adsorption treatment of this type of antibiotic was rarely reported in previous studies, or the adsorption efficiency was not satisfactory. Thus, an effective adsorbent is urgently needed to investigate the removal of CIP from pharmaceutical wastewaters.
The adsorption efficiency mainly depends on the properties of the adsorbent, which include porosity, specific surface area, availability, functional group, and regenerability. Various adsorbents have been used for CIP adsorption, including nanotubes, TiO2 nanotubes, magnetic biochar, magnetic mesoporous carbon, and others [17,18,19,20]. For example, in 2015, Zhuang et al. synthesized a long TiO2 nanotube/reduced graphene oxide (rGO-TON) hydrogel and used it for the removal of CIP from water [18]. This adsorbent exhibited good adsorption and regeneration capacity due to its porous property and 3D structure. However, most of the aforementioned nanoparticles (NPs) are prone to aggregation, blocking channels, and showing poor adsorption ability. To overcome these drawbacks of NPs for adsorption, the introduction of surface linkers is highly desirable.
Metal–organic frameworks (MOFs), a new class of porous materials composed of inorganic salts and organic ligands, have received considerable attention in studies on adsorption recently [21,22,23,24]. Some of their key features, such as their enormous surface area, their well-defined and tunable pore size, and their capacity to offer various functionalities, endows them with superior adsorption capacity. Also, the amine groups or aromatic rings in the structure of these compounds enhance their ability to absorb antibiotics on their surfaces. Several MOFs were recently employed for antibiotics remediation, such as HKUST1, MIL-53, MIL-100, UIO-66, and Cu3(BTC)2 [25,26,27,28,29,30,31,32], which showed remarkable affinities with antibiotics. Compared to other MOFs, benzenetricarboxylic acid (BTC)-like MOFs are suitable for the removal of antibiotics from pharmaceutical wastewaters due to their high water-solution stability, biocompatibility, and greener synthesis [33]. However, these pure BTC-like MOFs adsorbents are in the powder state and difficult to separate and recycle from large volumes of water, restricting their application in wastewater treatment. Thus, the introduction of magnetic nanoparticles to form magnetic metal–organic framework (MMOF) composites to enhance the effect of separation is a good strategy.
Inspired by reports on BTC-like MOFs and magnetic nanoparticles, in this work, magnetic metal–organic frameworks (MMOFs) were synthesized and applied for CIP removal in pharmaceutical wastewaters. Nontoxic Cu2+ and Zn2+ were chosen as metal joints to synthesize Cu3(BTC)2 and Zn3(BTC)2 through a hydrothermal method. Next, they were loaded on the surface of the magnetic Fe3O4 nanorod. The CIP-adsorption abilities of Cu3(BTC)2, Fe3O4/Cu3(BTC)2 MMOF, and Fe3O4/Zn3(BTC)2 MMOF in water were compared by using HPLC. The Fe3O4/Zn3(BTC)2 was found to be the most effective, and the adsorption process is displayed in Scheme 1. Further, various factors influencing the adsorption process, including the solution pH, oscillation times, adsorption kinetics, concentration, and recyclability were investigated to obtain a clear understanding of the effect of Fe3O4/Zn3(BTC)2 MMOF on its adsorption performance. The prepared Fe3O4/Zn3(BTC)2 MMOF not only retains the original large pore size, adsorption sites, and functional groups of MOFs, but also combines the magnetic properties of Fe3O4. More importantly, with the low toxicity of Fe3O4/Zn3(BTC)2 MMOF, the adsorption process of CIP from pharmaceutical wastewaters can become safer and greener. The purpose of this work is to develop an effective, environmentally friendly, and economical method to remove antibiotics from pharmaceutical wastewaters and, further, to provide a reference for the improvement of adsorption technologies.

2. Materials and Methods

2.1. Reagent and Materials

Ferric chloride nonahydrate (FeCl3·6H2O, AR), ferrous sulphate heptahydrate (FeSO4·7H2O, AR), copper acetate (C4H6CuO4), zinc acetate hydrate (C4H6O4Zn·2H2O, AR) were obtained from Sigma Chemical Co. (St. Louis, MO, USA). The NH3·H2O, 1,3,5-benzenetricarboxylic acid (H3BTC), N,N-Dimethylformamide (DMF), acetic acid, methanol, and ethanol were received from Shanghai Chemical Reagents Corporation (Shanghai, China). Ciprofloxacin (CIP, >98%) was purchased from Anpel laboratory Technologies Inc. (Shanghai, China). Ultrapure water was used throughout this study.

2.2. Apparatus

The prepared samples were synthesized and characterized in detail using multiple spectroscopic and analytical techniques. For characterization, scanning-electron microscopy (SEM, S-4800 apparatus Carl Zeiss Microscopy, Hitachi, Tokyo, Japan), X-ray diffractometer (XRD, D8 advanced, Bruker, Karlsruhe, Germany), Brunauer–Emmett–Teller (BET, ASAP2020HD88, McMuritik, Shanghai, China), Zetasizer Nano (DLS, ZS90, Malvern, Malvern, Britain), and vibrating sample magnetometer (VSM, 7404 apparatus, LakeShore, Columbus, Ohio, America) were used.
For CIP removal from water, high-performance liquid chromatograph (HPLC, Agilent 1100, Agilent, Kasrul, Germany) measurements were carried out with an Agilent XDB-C18 HPLC column (250 × 4.6 mm i.d. stainless steel) packed with 5 μm octadecyl (C18)-bonded silica (pore size = 80 Å).

2.3. Synthesis of Fe3O4 Nanorods

The Fe3O4 nanorod particles were prepared through simple coprecipitation [34]. Briefly, 2.7 g FeCl3·6H2O (0.5 mM) and 2.7 g FeSO4·7H2O were dissolved in 100 mL of ultrapure water and heated to 30 °C. Next, 15 mL NH3·H2O was slowly added to the solution, stirred, and heated to 80 °C for 30 min. The Fe3O4-nanorod precipitation in the solution was obtained through external magnetic separation and washed several times with ultrapure water and ethanol. The synthesized Fe3O4 nanorods were dispersed in 20 mL of ethanol solution and sonicated for 30 min to exfoliate into flakes and form stable aqueous dispersions.

2.4. Synthesis of Cu3(BTC)2 and Zn3(BTC)2

To obtain Cu3(BTC)2, 0.43 g Cu(Ac)2 was dissolved in 20 mL ultra-pure water, and 0.25 g H3BTC was dissolved in 20 mL DMF-ethanol mixed solution. Next, solutions were mixed under continuous stirring and heated to 70 °C for 4 h. After the reaction, the blue Cu3(BTC)2 precipitate was obtained by centrifugation and rinsing with ultrapure water, followed by drying at 60 °C. The synthesis of Zn3(BTC)2 was similar to that of Cu3(BTC)2, except for use of 0.51 g Zn(Ac)2 instead of 0.43 g Cu(Ac)2.

2.5. Synthesis of Fe3O4/Cu3(BTC)2 and Fe3O4/Zn3(BTC)2

Firstly, 50 mg H3BTC was dissolved in 20 mL DMF-ethanol solution (1:1). Next, 10 mL Fe3O4-nanorod ethanol solution (2.5 mM) was added to H3BTC solution and ultrasonicated for 30 min. Subsequently, 20 mL Cu(OAc)2 solution (10 mM) was dropped into the solution; the reaction temperature was 70 °C under mechanical stirring for 4 h. The Fe3O4/Cu3(BTC)2 was obtained by centrifugation, washing, and vacuum drying. The synthesis of Fe3O4/Zn3(BTC)2 was similar to that of Fe3O4/Cu3(BTC)2, except for use of Zn(OAc)2 solution (10 mM) instead of Cu(OAc)2 solution (10 mM).

2.6. Adsorption Experiments

The removal of CIP from water by adsorption on Cu3(BTC)2, Fe3O4/Cu3(BTC)2, and Fe3O4/Zn3(BTC)2 was performed in an incubator shaker. A rotary shaker at 150 rpm generated the degree of shaking for adsorption. Different concentrations of CIP were prepared by diluting stock solution (1000 mg/L) with double-distilled water. In a typical experiment, 50 mL of the CIP drug solution (20–100 mg/L) was positioned on an incubator shaker at room temperature. Next, 40 mg MOFs were added to form a suspension. Next, the solution was placed on shaker at 150 rpm for 180 min. At specific intervals for 5, 10, 30, 60, 90, 120, and 180 min, 1.0 mL of the supernatant was aspirated with a specification syringe and filtered by 0.45 μm syringe filter of MCE membrane. Next, the concentrations of CIP in the solution after equilibrium were determined by HPLC (C18 ODS column) with a UV wavelength at 275 nm. Finally, the quantity of CIP adsorbed (qe; mg/g) and the removal efficiency (R) were determined according to Equations (1) and (2), as follows:
Qt = (C0 − Ct) V/m,
% R = [(C0 − Ct) × 100]/C0,
where C0 (mg/L) and Ct (mg/L) are the CIP’s initial and final concentrations, respectively, V (L) is the volume of solution, and m (g) is the MOF-adsorbent quantity. All experiments were performed in duplicate to minimize errors, and the average value of the two replicates was used in the models.
To analyze the adsorption capacity of CIP by MOFs, the adsorption kinetic of CIP was obtained according to quasi-first-order (Equation (3)) and quasi-second-order kinetic equations (Equation (4)), as follows:
qt = qe(1 − e−k1t),
qt = k2qe2t/(1 + k2qet),
where t is the adsorption time (h), and qe and qt (mg g−1) are the absorption capacity at equilibrium and time t. The k1 (h−1) and k2 (g·mg−1·h−1) are adsorption rate constant.

2.7. Chromatographic Conditions for CIP

The chromatographic separation was achieved with 0.2% v/v acetic acid aqueous solution (mobile phase A) and methanol (mobile phase B). The A/B mobile phase with a volumetric ratio of 20/80 was used at injection-flow rate of 1 mL/min. Injection volume was 10 μL. Column temperature was 30 °C.

2.8. Recyclability Experiments

The recyclability of the adsorbent was used to determine the commercial applicability of the as-synthesized MOF. After adsorption of CIP, MOF was thoroughly washed with water and methanol, centrifuged and dried. The recovered MOFs were again used as adsorbents for the adsorption of CIP in aqueous phase under the same conditions as the first adsorption. The absorbance of CIP drug solution was assessed using HPLC for each cycle. The procedure was followed identically for five successive cycles.

3. Results and Discussion

3.1. Characterization of Cu3(BTC)2, Fe3O4/Cu3(BTC)2, and Fe3O4/Zn3(BTC)2

The SEM was used to investigate the size and the surface morphology of synthetic materials. As shown in Figure 1A, the as-prepared Fe3O4 materials had a uniform rod-like structure with a smooth surface and a diameter of about 400 nm. Figure 1B demonstrates that the Cu3(BTC)2 MOF exhibited a spherical structure and a smooth surface with an approximate diameter of 100 nm. Figure 1C and Figure 1D, respectively, show the morphologies of the Fe3O4/Cu3(BTC)2 and Fe3O4/Zn3(BTC)2 MMOFs. It can be seen that the smooth surfaces of the Fe3O4 materials became rough and formed composite materials; some agglomeration was also observed in the SEM images due to presence of magnetic attraction.
The XRD patterns of the synthesized MOFs are presented in Figure 2. The well-resolved characteristic peaks centered at 2θ = 30.1°, 35.4°, 37.1°, 53.4°, 56.9°, and 62.5° were assigned to the diffraction from the (220), (311), (400), (422), (333), (440), and (622) planes of the face-centered cubic (fcc) lattice of the Fe3O4, respectively [35]. Moreover, the diffraction peaks at 9.3°, 11.6°, 13.4°, 17.4°, and 26.1° corresponded to the (220), (222), (400), (333), and (731) faces of the Cu3(BTC)2 structure [36], and the major diffraction peaks at 10.1°, 16.4°, and 19.3° were related to the (200), (400), and (420) diffraction planes for the Zn3(BTC)2 structure [37]. These results demonstrate the successful formation of the targeted MOF on the Fe3O4 surface.
Based on the exploration of the Zn3(BTC)2 structure, a schematic diagram of the Zn3(BTC)2 growth mechanism is illustrated in Figure 3. At a low concentration of Zn2+ and H3BTC (approx. 0.2 mmol), crystal growth is mainly dominated by the oriented attachment mechanism [38]. The nuclei were formed by consuming the Zn2+ ions and H3BTC. Subsequently, the crystals preferred to attach along with the same crystal facets for the best lattice match, after which the nuclei quickly attached and grew into large crystals in two-dimensional organic frameworks.
The surface area of the adsorbent plays a vital role in the adsorption of harmful pollutants from water and wastewater. Thus, four adsorbents were tested using the BET multi-point method (BET) and N2 adsorption–desorption to investigate their porosity. As shown in Table 1, four adsorbents showed different pore sizes, and the average pore size of the Fe3O4/Zn3(BTC)2 MMOF was the largest. Further, the adsorption and desorption curves are shown in Figure 4, where an obvious IV isotherm for Cu3(BTC)2 MOF, Zn3(BTC)2 MOF, Fe3O4/Cu3(BTC)2 MMOF, and Fe3O4/Zn3(BTC)2 MMOF can be observed, which refers to the presence of a mesoporous structure. Increasing the pore size of the adsorbent appropriately facilitated the adsorption of reactive molecules on the surface of the adsorbent. By comparison, the Fe3O4/Zn3(BTC)2 MMOF was predicted to have the best CIP-adsorption effect.
The magnetic properties of the Zn3(BTC)2 MOF, Fe3O4 nanorod, Fe3O4/Cu3(BTC)2 MMOF, and Fe3O4/Zn3(BTC)2 MMOF were analyzed with a vibrating sample magnetometer (VSM) at room temperature. As shown in Figure 5, the Zn3(BTC)2 exhibited no magnetic properties, while the magnetization curves exhibited magnetic hysteresis loops. and the saturation-magnetization values were determined as 70.9, 27.9, and 24.4 emu/g for the Fe3O4 nanorod, Fe3O4/Cu3(BTC)2 MMOF, and Fe3O4/Zn3(BTC)2 MMOF, respectively. Compared to the as-synthesized Fe3O4 nanorod, the saturation magnetization of the Fe3O4/Cu3(BTC)2 MMOF and Fe3O4/Zn3(BTC)2 MMOF were reduced, but they still persisted at a relatively high level, which was beneficial for the subsequent magnetic separation.

3.2. Effect of Oscillation Time on CIP Adsorption

As the main aim of this work was to create an effective MMOF for the removal of CIP from water, the CIP-adsorption performances of the Cu3(BTC)2 MOF, Zn3(BTC)2 MOF, Fe3O4/Cu3(BTC)2 MMOF, and Fe3O4/Zn3(BTC)2 MMOF were examined by changing the oscillation time. Samples were taken at 5, 10, 30, 60, 90, 120, and 180 min, respectively, to measure the removal efficiency of the CIP according to the standard curve, as shown in Figure 6 (the pH of solution was 3.84, the concentration of the CIP was 20 mg/L, and the dosage of the MOF adsorbent was 40 mg). As shown in Figure 6A, at the same initial concentration, the MOFs’ CIP-removal rate was more than halved, while the oscillation time increased within 0 to 30 min. Furthermore, the MOFs’ CIP-removal rate slowed down gradually when the oscillation-time increase exceeded 90 min. Interestingly, the CIP-removal rates of the Cu3(BTC)2 and Zn3(BTC)2 were about the same, and the CIP-removal rate of the Fe3O4/Zn3(BTC)2 MMOF was the highest.
Further, the MOFs’ CIP-adsorption quantity was investigated. As shown in Figure 6B, the adsorption quantity of CIP increased rapidly within 30 min, and the maximum adsorption capacity of the Fe3O4/Zn3(BTC)2 MMOF was 7.22 mg/g. After 90 min, the adsorption process essentially reached an equilibrium. This trend was predictable because in the first 30 min, the adsorption-solution concentration was larger and the adsorbents had larger porosity, which gave the CIP and the adsorbents more opportunities to touch. After 90 min, the adsorption sites were close to saturation. The powerful adsorption performance of the Fe3O4/Zn3(BTC)2 MMOF is attributable to the retained large pore size, the adsorption sites and functional groups of the Zn3(BTC)2, and the magnetic properties of the Fe3O4 nanorod. Thus, the Fe3O4/Zn3(BTC)2 MMOF was selected for further experiments.

3.3. Effect of pH on CIP Adsorption

The initial pH of the solution had a different influence on the surface charge and the degree of ionization of the adsorbent. Thus, the adsorption performance on the CIP of the Fe3O4/Zn3(BTC)2 MMOF at different pH was studied (the solution concentration was 20 mg/L, and the dosage of Fe3O4/Zn3(BTC)2 MMOF was 40 mg). As shown in Figure 7, when the pH was 3.84, the removal rate (Figure 7A) and -adsorption quantity (Figure 7B) of the CIP by the Fe3O4/Zn3(BTC)2 MMOF were the highest, reaching 63.06% and 5.19 mg/g., respectively. The removal rate and the adsorption quantity were lowest when pH was 10.12. The reason for this may be that the amino group of the CIP was protonated in an acidic solution, which can adsorb to the surfaces of negatively charged Fe3O4/Zn3(BTC)2 MMOFs via electrostatic interaction. In the alkaline solution, the amino group was deprotonated and the CIP molecules mainly took the form of CIP-, which resulted in the reduced adsorption capacity.
The Zeta potential of the Fe3O4/Zn3(BTC)2 MMOF was tested at different pH values of 3.84, 6.82, and 10.12. As shown in Figure 8, the surface of the Fe3O4/Zn3(BTC)2 MMOF was negatively charged in the whole tested pH range. The CIP molecules took the form of cationic and zwitterionic species when the pH < 5.90 and the pH value ranged from 5.90 to 8.8; the main form of the CIP was anionic species when the solution pH was higher than 8.89 [39]. Thus, when the pH value was 3.84, the electrostatic attraction between the negatively charged surface of the Fe3O4/Zn3(BTC)2 MMOF and the CIP cations benefited the adsorption capacity. However, when the pH was 10.12, the electrostatic repulsion between the anionic CIP species and the negatively charged Fe3O4/Zn3(BTC)2 MMOF resulted in lower adsorption capacity. Therefore, the acidic condition is more favorable for CIP adsorption by Fe3O4/Zn3(BTC)2 MMOF.

3.4. Regeneration and Reusability of Fe3O4/Zn3(BTC)2 MMOF

In addition to the adsorption properties, the regeneration and reusability of adsorbents are also important factors for practical applications. The Fe3O4/Cu3(BTC)2 MMOF and Fe3O4/Zn3(BTC)2 MMOF were selected to conduct an experiment on the continuous adsorption performance of the CIP. Methanol was used as the desorbing agent for the desorption of the CIP and the regeneration of the adsorbents. As shown in Figure 9A, after five cycles of adsorption, the CIP-removal rate of the Fe3O4/Zn3(BTC)2 MMOF remained at 31.45%, whereas the CIP-removal rate of the Fe3O4/Cu3(BTC)2 MMOF was only 16.50%. The recovery of the Fe3O4/Zn3(BTC)2 MMOF was also higher than that of the Fe3O4/Cu3(BTC)2 MMOF (Figure 9B). Further, the SEM images of the adsorbents before and after five cycles of adsorption are shown in Figure 10. It can be observed that after five cycles, irregular particles were attached to the surface of the Fe3O4/Cu3(BTC)2 MMOF, and the morphology was completely changed, which may have resulted in blocked channels. The Fe3O4/Zn3(BTC)2 MMOF lost its rod-like structure, but there was still a uniform pore structure on the surface, which was beneficial for the adsorption of CIP. These results suggest that the Fe3O4/Zn3(BTC)2 MMOF adsorbent exhibited great regeneration and reusability, which may be ascribed to the magnetic Fe3O4 nanorod.
To further demonstrate the good stability of the Fe3O4/Zn3(BTC)2 MMOF, we detected the leaching of Zn ions from the Fe3O4/Zn3(BTC)2-CIP solution during the adsorption process with ICP, and the results are shown in Figure 11. It is clear that the leaching rate of the Zn ions was very low during the adsorption process, and it was still only 14.4% even after 120 min of oscillation. The results further proved that the Fe3O4/Zn3(BTC)2 had good stability as an adsorbent of CIP.

3.5. CIP-Adsorption Kinetics and Adsorption Mechanism

In order to further explore the kinetic mechanism of the CIP adsorption by the Fe3O4/Zn3(BTC)2 MMOF, quasi-first-order and quasi-second-order kinetic equations were used to fit the experimental data. The parameters of the kinetic model of the CIP adsorption by the Fe3O4/Zn3(BTC)2 MMOF are shown in Table 2. The correlation coefficient of the pseudo-first-order kinetic model was 0.952, while it was 0.971 for the pseudo-second-order kinetic model. Furthermore, the qe calculated by the pseudo-second-order model was consistent with the experimental adsorption capacities qexp. These results suggest that the pseudo-second-order kinetic model is well suited to modeling CIP adsorption by Fe3O4/Zn3(BTC)2 MMOF compared to the pseudo-first-order kinetic model. That is, the adsorption of CIP by Fe3O4/Zn3(BTC)2 MMOF is mainly a chemical adsorption process.
Based on the results above, the mechanism of the CIP’s adsorption by the Fe3O4/Zn3(BTC)2 MMOF adsorbent was analyzed. Firstly, CIP is a hydrophobic molecule, and its flat shape easily induces intermolecular aggregation to form stable polymers [40]. Thus, the larger pore size of Fe3O4/Zn3(BTC)2 MMOF appropriately facilitates the adsorption of CIP. Secondly, the coordination bonds between the open Zn(II) sites in the Fe3O4/Zn3(BTC)2 MMOF and -COOH groups in CIP create stronger adsorption capacity. Thirdly, the organic ligands of Fe3O4/Zn3(BTC)2 and the aromatic ring of CIP stimulate adsorption via π–π interactions, while the carboxyl group of CIP, as an electron-absorbing group, can enhance π–π interactions. Additionally, as shown in Figure 12, the H-bond between the H atom of the BTC linker and O atom of the CIP supports the adsorption of micro-pollutants from wastewater [27]. Under different pH values, the adsorption performance of the CIP changed to varying degrees; it was best when the pH value was 3.84. Based on the Zeta potential of the Fe3O4/Zn3(BTC)2 MMOF, the surface of the Fe3O4/Zn3(BTC)2 MMOF was negatively charged in the whole tested pH range. The CIP was in three states in the solution: a cationic form (with a protonated amine group in solutions with pH values below 5.90 ± 0.15), an anionic form (with a deprotonated carboxylic acid group in solution with pH values above 8.89 ± 0.11), and a zwitterionic form [39]. Furthermore, the CIP molecules took the form of cationic and zwitterionic species when the pH < 5.90; the main form of the CIP was anionic species when the solution pH was higher than 8.89. Thus, when the pH value was 3.84, the electrostatic attraction between the negative charged surface of the Fe3O4/Zn3(BTC)2 MMOF and the CIP cations benefited the adsorption capacity. These results suggest that the electrostatic interaction between the Fe3O4/Zn3(BTC)2 and the CIP played a decisive role. When the CIP molecule assumes a cationic state, it facilitates the formation of new H bonds, originating between the H atoms of the CIP molecule and the oxygen atoms of the BTC linkers. Thus, new H bonds further increase the adsorption capacity between Fe3O4/Zn3(BTC)2 MMOF and CIP. More importantly, magnetic Fe3O4 nanorod makes Fe3O4/Zn3(BTC)2 convenient for recycling. Due to these advantages, high-efficiency CIP adsorption was realized by the Fe3O4/Zn3(BTC)2 MMOF.

4. Conclusions

In summary, four adsorbents, Cu3(BTC)2 MOF, Zn3(BTC)2 MOF, Fe3O4/Cu3(BTC)2 MMOF, and Fe3O4/Zn3(BTC)2 MMOF, were successfully prepared. The CIP-removal rate in water by the Fe3O4/Zn3(BTC)2 MMOF reached 72.15%, which was superior to those of the other adsorbents and, thus, the Fe3O4/Zn3(BTC)2 MMOF was selected as the best adsorbent for CIP adsorption. The acidic conditions and prolonged oscillation time improved the adsorption efficiency. The CIP adsorption by the Fe3O4/Zn3(BTC)2 MMOF followed the pseudo-second-order kinetic model. On account of the original large pore size, adsorption sites, and functional groups of the MOFs and the magnetic properties of Fe3O4, the Fe3O4/Zn3(BTC)2 MMOF adsorbent exhibits a stronger adsorption capacity, great regeneration, and reusability for CIP. More importantly, all the ingredients of the adsorbent are harmful, making the adsorption process safer and greener. This highly effective and safe adsorbent may be a promising candidate for the removal of CIP antibiotics from water.

Author Contributions

Conceptualization, B.Y. and Y.J.; methodology, W.W., H.C., X.C. and D.C.; investigation, H.Y. and Z.C.; writing—original draft preparation, B.Y. and W.X.; writing—review and editing, H.Y. and D.H.; supervision, D.H.; project administration, B.Y.; funding acquisition, B.Y. and Y.J. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the NNSF of China (grant no. 22202146, 2272115, 22202145, 22202147), Zhejiang Provincial Natural Science Foundation of China (grant no. LGG21B070001), Key Research and Development Plan of Zhejiang Province (2021C03022), and National Innovation Training program for college students (grant no. 202210350038).

Data Availability Statement

Data are contained within the article.

Conflicts of Interest

The authors declare no conflict of interest.

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Water 15 02531 sch001 550
Scheme 1. Schematic illustration of CIP-adsorption process of Fe3O4/Zn3(BTC)2 MMOF in pharmaceutical wastewaters.
Scheme 1. Schematic illustration of CIP-adsorption process of Fe3O4/Zn3(BTC)2 MMOF in pharmaceutical wastewaters.
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Figure 1. SEM images of (A) Fe3O4 nanorod, (B) Cu3(BTC)2, (C) Fe3O4/Cu3(BTC)2, and (D) Fe3O4/Zn3(BTC)2.
Figure 1. SEM images of (A) Fe3O4 nanorod, (B) Cu3(BTC)2, (C) Fe3O4/Cu3(BTC)2, and (D) Fe3O4/Zn3(BTC)2.
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Figure 2. XRD pattern of Fe3O4, Fe3O4/Cu3(BTC)2, and Fe3O4/Zn3(BTC)2 (the red dots represent the fcc lattice of Fe3O4, the green squares represent the fcc lattice of Cu3(BTC)2, the purple flowers represent the fcc lattice of Zn3(BTC)2).
Figure 2. XRD pattern of Fe3O4, Fe3O4/Cu3(BTC)2, and Fe3O4/Zn3(BTC)2 (the red dots represent the fcc lattice of Fe3O4, the green squares represent the fcc lattice of Cu3(BTC)2, the purple flowers represent the fcc lattice of Zn3(BTC)2).
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Figure 3. Schematic diagram of the Zn-BTC MOF’s growth mechanism.
Figure 3. Schematic diagram of the Zn-BTC MOF’s growth mechanism.
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Figure 4. N2 adsorption–desorption isotherms of Cu3(BTC)2, Zn3(BTC)2, Fe3O4/Cu3(BTC)2, and Fe3O4/Zn3(BTC)2.
Figure 4. N2 adsorption–desorption isotherms of Cu3(BTC)2, Zn3(BTC)2, Fe3O4/Cu3(BTC)2, and Fe3O4/Zn3(BTC)2.
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Figure 5. VSM analysis of Zn3(BTC)2 MOF, Fe3O4, Fe3O4/Cu3(BTC)2 MMOF, and Fe3O4/Zn3(BTC)2 MMOF.
Figure 5. VSM analysis of Zn3(BTC)2 MOF, Fe3O4, Fe3O4/Cu3(BTC)2 MMOF, and Fe3O4/Zn3(BTC)2 MMOF.
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Figure 6. The effect of oscillation time on (A) the removal rate and (B) the adsorption quantity of CIP by Cu3(BTC)2, Zn3(BTC)2, Fe3O4/Cu3(BTC)2, and Fe3O4/Zn3(BTC)2 (pH of solution was 3.84, the concentration of CIP was 20 mg/L, and the dosage of MOFs adsorbent was 40 mg).
Figure 6. The effect of oscillation time on (A) the removal rate and (B) the adsorption quantity of CIP by Cu3(BTC)2, Zn3(BTC)2, Fe3O4/Cu3(BTC)2, and Fe3O4/Zn3(BTC)2 (pH of solution was 3.84, the concentration of CIP was 20 mg/L, and the dosage of MOFs adsorbent was 40 mg).
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Figure 7. The effect of pH on (A) the removal rate and (B) the adsorption quantity of CIP by Fe3O4/Zn3(BTC)2 (the solution concentration was 20 mg/L, and the dosage of Fe3O4/Zn3(BTC)2 was 40 mg).
Figure 7. The effect of pH on (A) the removal rate and (B) the adsorption quantity of CIP by Fe3O4/Zn3(BTC)2 (the solution concentration was 20 mg/L, and the dosage of Fe3O4/Zn3(BTC)2 was 40 mg).
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Figure 8. Zeta potentials of Fe3O4/Zn3(BTC)2 MMOF at different pH values.
Figure 8. Zeta potentials of Fe3O4/Zn3(BTC)2 MMOF at different pH values.
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Figure 9. (A) The regeneration and (B) the reusability efficiency of Fe3O4/Zn3(BTC)2 and Fe3O4/Cu3(BTC)2 for CIP adsorption during five successive adsorption–regeneration processes. Experimental conditions: the initial concentration of CIP was 20 mg/L; the pH of CIP solution was 3.84; the amount of magnetic MOFs was 0.05 g; and the oscillation time was 1 h.
Figure 9. (A) The regeneration and (B) the reusability efficiency of Fe3O4/Zn3(BTC)2 and Fe3O4/Cu3(BTC)2 for CIP adsorption during five successive adsorption–regeneration processes. Experimental conditions: the initial concentration of CIP was 20 mg/L; the pH of CIP solution was 3.84; the amount of magnetic MOFs was 0.05 g; and the oscillation time was 1 h.
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Figure 10. The SEM images of (A) Fe3O4/Zn3(BTC)2 and (B) Fe3O4/Zn3(BTC)2 before and after adsorption.
Figure 10. The SEM images of (A) Fe3O4/Zn3(BTC)2 and (B) Fe3O4/Zn3(BTC)2 before and after adsorption.
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Figure 11. The leaching of Zn ions during CIP adsorption by Fe3O4/Zn3(BTC)2. Experimental conditions: the initial concentration of CIP was 20 mg/L; the pH of CIP solution was 3.84; the amount of Fe3O4/Zn3(BTC)2 MMOF was 0.05 g.
Figure 11. The leaching of Zn ions during CIP adsorption by Fe3O4/Zn3(BTC)2. Experimental conditions: the initial concentration of CIP was 20 mg/L; the pH of CIP solution was 3.84; the amount of Fe3O4/Zn3(BTC)2 MMOF was 0.05 g.
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Figure 12. Schematic diagram of the H-bond between the H atom of BTC linker and O atom of CIP.
Figure 12. Schematic diagram of the H-bond between the H atom of BTC linker and O atom of CIP.
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Table
Table 1. BET data of Cu3(BTC)2, Zn3(BTC)2, Fe3O4/Cu3(BTC)2, and Fe3O4/Zn3(BTC)2.
Table 1. BET data of Cu3(BTC)2, Zn3(BTC)2, Fe3O4/Cu3(BTC)2, and Fe3O4/Zn3(BTC)2.
AdsorbentSBET (m2/g) Pore Volume (cm3/g)Average Particle Size (nm)
Cu3(BTC)21054.360.973.68
Zn3(BTC)213.810.039.60
Fe3O4/Cu3(BTC)2619.130.533.45
Fe3O4/Zn3(BTC)227.350.1318.31
Table
Table 2. Kinetic parameters for the kinetic model for CIP adsorption by Fe3O4/Zn3(BTC)2.
Table 2. Kinetic parameters for the kinetic model for CIP adsorption by Fe3O4/Zn3(BTC)2.
Adsorbentqexp (mg/g)Pseudo-First-Order ModelPseudo-Second-Order Model
qe (mg/g)k1r2qek2r2
Fe3O4/Zn3(BTC)25.194.890.2050.9525.120.0650.971
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Yu, B.; Chang, H.; Wei, W.; Yu, H.; Chen, Z.; Cheng, X.; Chen, D.; Jin, Y.; Han, D.; Xu, W. Highly Effective Removal of Ciprofloxacin Antibiotic from Water by Magnetic Metal–Organic Framework. Water 2023, 15, 2531. https://doi.org/10.3390/w15142531

AMA Style

Yu B, Chang H, Wei W, Yu H, Chen Z, Cheng X, Chen D, Jin Y, Han D, Xu W. Highly Effective Removal of Ciprofloxacin Antibiotic from Water by Magnetic Metal–Organic Framework. Water. 2023; 15(14):2531. https://doi.org/10.3390/w15142531

Chicago/Turabian Style

Yu, Binbin, Hongchao Chang, Wenwan Wei, Hua Yu, Zhangxin Chen, Xiaoye Cheng, Dan Chen, Yanxian Jin, Deman Han, and Wei Xu. 2023. "Highly Effective Removal of Ciprofloxacin Antibiotic from Water by Magnetic Metal–Organic Framework" Water 15, no. 14: 2531. https://doi.org/10.3390/w15142531

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