Tetracycline removal using flower-like Fe<sub>3</sub>O<sub>4</sub>/UiO-66/ZnO composite: Adsorption and white LED photocatalysis performances

Le Thi Thanh Nhi; Le Huy Thai; Trinh Ngoc Dat; Ta Ngoc Bach; Ly Diem Hang; Nguyen Van Son Lam; Le Van Thuan; Ha Thi Như Y; Le Hoang Sinh; Dinh Quang Khieu

doi:10.4491/eer.2025.101

Environ Eng Res > Volume 31(1); 2026 > Article

Nhi, Thai, Dat, Bach, Hang, Van Son Lam, Van Thuan, Y, Sinh, and Khieu: Tetracycline removal using flower-like Fe3O4/UiO-66/ZnO composite: Adsorption and white LED photocatalysis performances

Research

Environmental Engineering Research 2026; 31(1): 250101.

Published online: June 5, 2025

DOI: https://doi.org/10.4491/eer.2025.101

Tetracycline removal using flower-like Fe₃O₄/UiO-66/ZnO composite: Adsorption and white LED photocatalysis performances

Le Thi Thanh Nhi^1,², Le Huy Thai³, Trinh Ngoc Dat⁴, Ta Ngoc Bach⁵, Ly Diem Hang⁶, Nguyen Van Son Lam⁷, Le Van Thuan^1,², Ha Thi Như Y⁸, Le Hoang Sinh^3,^†

, Dinh Quang Khieu^8,^†

¹Center for Advanced Chemistry, Institute of Research and Development, Duy Tan University, 03 Quang Trung, Da Nang city, Vietnam

²Faculty of Natural Sciences, Duy Tan University, 03 Quang Trung, Da Nang city, Vietnam

³The University of Danang-VN-UK Institute for Research and Executive Education, Danang city, Viet Nam

⁴The University of Danang, University of Science and Education, Da Nang city, Vietnam

⁵Institute of Materials Science, Vietnam Academy of Science and Technology, Hanoi city, Vietnam

⁶College of Medicine and Pharmacy, Duy Tan University, 03 Quang Trung, Da Nang city, Vietnam

⁷Hospital 199, Son Tra Dist., Danang city, Viet Nam

⁸University of Sciences, Hue University, Hue city, Vietnam

^†Corresponding author: E-mail: sinh.le@vnuk.udn.vn (L.H.S.), dqkhieu@hueuni.edu.vn (D.Q.K.), Tel: 84-936119523 (L.H.S.), 84-368706850 (D.Q.K.), ORCID: 0000-0002-8540-9010 (L.H.S.), 0000-0003-3473-6377 (D.Q.K.)

Received February 21, 2025 Revised May 4, 2025 Accepted June 3, 2025

This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/3.0/) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract

The multifunctional magnetic material of Fe₃O₄/UiO-66/ZnO was synthesized with a three-layer structure: Fe₃O₄ core, an intermediate layer of octahedral UiO-66 nanoparticles, and an outer layer of flower-shaped ZnO nanoparticles. The materials were characterized using scanning electron microscopy, energy dispersive X-ray mapping, X-ray diffraction, Fourier transform infrared spectroscopy, and vibrating sample magnetometer techniques. The synthesized material showed excellent tetracycline (TC) adsorption capacity, reaching 302.99 mg/g, and good visible-light driven photocatalytic activity. The catalytic mechanism is driven by efficient electron transfer: electrons from Fe₃O₄ move sequentially through UiO-66 to ZnO, while photogenerated holes move in the opposite direction, contributing to preventing recombination and optimizing the catalytic efficiency. It is consistent with ion trapping experiments to determine the role of active species in the photocatalytic process in the order of influence: h⁺ > ^•OH > e⁻ > ^•O₂⁻. The material also exhibits strong enough magnetism to be recovered with an external magnet, facilitating its reuse. After five consecutive cycles of use, the material still maintains a stable TC removal efficiency under white LED light, confirming its sustainability and long-term effectiveness. With high adsorption capacity, stable photocatalytic activity, and outstanding reusability, Fe₃O₄/UiO-66/ZnO promises to be a potential material for practical wastewater treatment applications.

Keywords: Adsorption, Fe₃O₄, Photocatalytic activity, Tetracycline, UiO-66, ZnO

Graphical Abstract

Keywords: Adsorption, Fe₃O₄, Photocatalytic activity, Tetracycline, UiO-66, ZnO

1. Introduction

Tetracycline (TC) is one of the antibiotics used to treat human and animal diseases and is also one of the agricultural antibacterial agents [1]. Besides high biological toxicity, TC has a stable structure and high polarity because they have an aromatic ring [2]. The highly hydrophilic properties and low volatility of TC have led to its significant existence in aquatic environments and difficulty in disposal [3]. When TC accumulates in the ecosystem, it can significantly increase the antibiotic resistance of microorganisms [4,5]. Then, TC can enter the human body through water flows, leading to potentially harmful effects such as contributing to increased antibiotic resistance, forming stable complexes with calcium when absorbed into bones, thereby slowing bone development [6], and even having adverse effects on the developing fetus [7]. Therefore, developing effective treatment techniques and materials to eliminate TC antibiotics is necessary. Various methods for removing TC from water have been explored, including coagulation-flocculation, catalysis, photocatalysis, biodegradation, and advanced oxidation processes (AOPs) [8]. However, these methods have certain disadvantages, such as the formation of toxic byproducts in AOPs [9] and excessive sludge production during coagulation-flocculation [10]. Among them, adsorption and photocatalysis are promising and competitive techniques due to their high efficiency, low energy consumption, easy operation, and environmental sustainability [11,12].

In recent years, metal-organic frameworks (MOFs) have been a porous material formed from the link between organic matter and metal ions, which attracted many studies for adsorbent and catalyst in removing organic pollutants in aqueous solution [13]. Among them, UiO-66 is one of the most common MOFs with the advantage of highly high foam, large surface area, high chemical and high thermal stability, adjustable size/shape of holes, and efficient function [14]. It has been applied in adsorbents and catalysts to effectively eliminate TC antibiotic residues [15,16]. Specifically, Chen et al. published the research results on UiO-66 material for TC adsorption with a maximum adsorption capacity of 23.1 mg/g [17]; this result can be improved when the surface of UiO-66 material is modified through the combination with metals, metal oxides or metal sulfides, and so on. at the nanoscale [18]. Wenbo et al. developed In₂S₃/UiO-66 material to treat TC in an aqueous environment with a maximum capacity of 106.3 mg/g [19], similar to Shuangqin et al. used Co-modified UiO-66 surface to adsorb TC with very high efficiency, the maximum adsorption capacity is up to 224 mg/g [20]. It can be seen that the porous material based on UiO-66 modification combined with metal oxide shows good TC treatment efficiency through the enhancement of active adsorption sites from the adsorption capacity of UiO-66 and metal oxide. Besides the porous structure and the number of adsorption sites, which are the main characteristics affecting the adsorption capacity of the material, the selectivity and regeneration ability are also considered important factors in evaluating the adsorption efficiency of the material. Among metal oxides, zinc oxide (ZnO) is a semiconductor with high photocatalytic efficiency for most organic compounds with the advantages of being easy to synthesize, low cost, and relatively stable to oxidizing agents in aqueous environments [21,22]. ZnO only works well in the ultraviolet (UV) light region due to its wide band gap energy of about 3.2 eV [23]. The combination of metal oxides and ZnO has recently increased its catalytic activity in the visible region. The study of Jin-ChungSin [24] published a spherical microporous photocatalytic material based on Fe₃O₄/ZnO for phenol treatment. The research results showed that integrating Fe₃O₄ into the structure of ZnO significantly increased the catalytic ability of ZnO in both the ultraviolet and visible regions.

For the above reasons, we use UiO-66 as a carrier to attach metal oxide semiconductors to form Fe₃O₄/UiO-66/ZnO composite materials for photocatalytic application to treat TC in water under visible (indoor) light (LED light). Each component in the composite above has photocatalytic activity. Thanks to the resonance effect, the composite formed from this combination will possess superior catalytic activity compared to each element. In addition, as mentioned above, in addition to its role as a carrier to fix photocatalytic particles, UiO-66 also has a high adsorption capacity to increase the ability to remove polluting organic compounds. The presence of Fe₃O₄ in the composite, on the one hand, increases the catalytic activity of ZnO, on the other hand, increases the ability to recover materials thanks to magnetism, thereby allowing the material to be reused many times, helping to reduce processing costs and improve the ability to apply in practice.

2. Experimental

2.1. Material

Chemicals used in the study including benzene-1,4-dicarboxylic acid (H₂BDC), Tetracycline (TC), ZrCl₄ were obtained from Sigma-Aldrich; N, N′-dimethylformamide (DMF), ethanol (EtOH), isopropyl alcohol (IPA), 3, 5 - dihydroxybenzoic acid (DHB), L-Lysine hydrochloride (Lys), NH₄OH, FeSO₄· 7H₂O, FeCl₃· 6H₂O, ZnCl₂, KI, KBrO₃, HCl, NaOH, NaCl, NaNO₃, NaHCO₃, and Na₂SO₄ were purchased from the Meck company (Germany) and used without further purification.

2.2 Synthesis of Materials

2.2.1. Synthesis of Fe₃O₄

A mixture of 0.65 g FeCl₃.6H₂O and 0.37 g FeSO₄. 4H₂O was dispersed in 60 mL of DMF solvent for 2 h at 60°C. Then NH₄OH solution was added until pH reached 11 before being transferred to heat-resistant Teflon, and heated at 160°C for 18 h. The resulting solid was washed before drying at 60°C for 12 h to obtain Fe₃O₄.

2.2.2. Synthesis of Fe₃O₄/UiO-66 and UiO-66

Weigh 0.312 g ZrCl₄, 0.264 g H₂BDC, and 0.4 g Fe₃O₄ dissolved in 75 mL DMF under ultrasonication to obtain a clear solution (add 5 mL CH₃COOH acid until clear). The mixture was then transferred to a Teflon bottle and heated in a drying oven at 140°C for 18 h. After cooling to room temperature, the black solid was magnetically separated, washed, and dried at 80°C for 12 h to obtain Fe₃O₄/UiO-66. The same procedure, without the addition of Fe₃O₄, gave pure UiO-66.

2.2.3. Synthesis of ZnO, UiO-66/ZnO and Fe₃O₄/UiO-66/ZnO

The synthesis involved soaking 0.4 g of Fe₃O₄/UiO-66 and 1.088 g of ZnCl₂ in 60 mL of DMF under magnetic stirring for 1 h. After adding 10 mL of 0.1M NaOH and 0.146 g of Lys, the mixture was stirred at 50°C for 2 h before being transferred to a Teflon vessel and heated at 140°C for 18 h. The consequent solid was then isolated by filtration, washed, and air-dried at 60°C for 4 h to obtain Fe₃O₄/UiO-66/ZnO. Similar to the above process, replaced Fe₃O₄/UiO-66 with UiO-66 to get UiO-66/ZnO, and did not add Fe₃O₄/UiO-66 to get ZnO.

2.3 Characterization Methods

The materials were characterized using advanced techniques: scanning electron microscopy (SEM) for surface morphology and microstructure, energy-dispersive X-ray analysis (EDX) for elemental composition, and X-ray diffraction (XRD) with CuKα radiation (λ = 0.15406 nm) for crystal structure analysis. Fourier transform infrared spectroscopy (FTIR) identified surface functional groups, while vibrating sample magnetometer (VSM) measured magnetic properties. The adsorption and photocatalytic performance for TC removal were evaluated using UV-Vis spectroscopy by observing concentration changes in the solution. Chemical oxidation demand (COD) was used to assess the amount of organic matter through carbon remaining in the solution after TC is photocatalytically decomposed. The COD was recorded according to the reference [25].

2.4. Adsorption Experiments

The adsorption kinetics and isotherm experiments were conducted by continuously stirring 20 mg of the material in 50 mL of TC solution for 5 h. The adsorption isotherm was determined by changing the initial TC concentration from 15 to 120 mg/L. The effect of pH on the adsorption efficiency was evaluated at values of 4, 7, and 10, by adjusting with HCl or NaOH with TC concentration measured by UV-Vis spectroscopy at 355 nm. The adsorption capacity of TC is calculated using the following equation (Eq. (1)):

(1)

q_{t} = (C_{o} - C_{t}) V / m

where C_o, C_t (mg/L) represents the initial concentration and the concentration at time t of TC solution, q_t (mg/g) is the adsorption capacity at time t, V (L) describes the volume of the solution, and m (mg) stands for the mass of the adsorbent.

2.5. Photocatalytic Experiments

The photocatalytic degradation of TC under white LED irradiation (30 W LED lamp for a distance of 10 cm to the reaction surface) was performed by adding 0.02 g of catalyst to a conical flask with ground-glass stopper containing 50 mL of TC solution with concentrations ranging from 15 mg/L to 120 mg/L as shown in Scheme 1. The distance between the lamp and the conical flask is about 100 mm. The Lab shaker contains 10 conical flasks. The sample was stirred and allowed to reach adsorption equilibrium for 5 h. After equilibration, the TC concentration in the solution was measured by UV-Vis spectroscopy at 355 nm, and this value was recorded as the initial concentration (C₀, mg/L) for the photocatalytic degradation experiments. Next, the solution was illuminated with a white LED light for 2.5 h to monitor the TC degradation process. Each photocatalytic degradation experiment was repeated three times to ensure the accuracy and repeatability of the results.

3. Results and Discussion

3.1 Characterization of Materials

Initially, before immobilizing UiO-66, the Fe₃O₄ particles were spherical with diameters ranging from 6 to 8 micrometers, as shown in SEM images in Fig. 1a. Then, the small octahedral UiO-66 particles were evenly coated on the Fe₃O₄ surface, forming a characteristic porous structure (Fig. 1b). When continuing to attach ZnO nanoparticles to Fe₃O₄/UiO-66, the final material Fe₃O₄/UiO-66/ZnO had a special morphology, with flower-shaped ZnO clusters covering the surface (Fig. 1c). To confirm the formation of Fe₃O₄/UiO-66/ZnO, EDX analysis was performed to examine the distribution of elements in the composites. Fig. 1d presents the key elements, including O, Zr, Zn, C, and Fe, and their respective proportions in the material. Meanwhile, Fig. 1e–i demonstrates the uniform distribution of these essential elements within the same region. The results of the XRD and FTIR analysis also further support this conclusion. Based on the XRD patterns in Fig. 2a, diffraction peaks at 31.8°, 34.4°, 36.3°, 47.4°, 56.2°, 62.6°, 66.3°, 68.1°, and 69.1° can be observed, corresponding to the (100), (002), (101), (102), (110), (103), (200), (112) and (201) planes, respectively, confirming the formation of ZnO nanoparticle phase, in agreement with the JCPDS No. 36-1451 data [26–28]. Similarly, peaks appear at 2θ= 30.4°, 35.4°, 43.3°, 56.2°, and 62.8°, corresponding to the (200), (311), (400), (511), and (440) planes, respectively, indicating the successful synthesis of spherical Fe₃O₄, based on the JCPDS No. 89-2355 data [29]. This consequence agrees with the previous study by Dong et al. [30]. The XRD pattern of pure UiO-66 also shows similarities with previously published studies [31], with characteristic peaks at 2θ= 25.5°, 32.8°, 35.5°, and 43.4°, corresponding to the (600), (731), (820) and (664) planes, respectively, in agreement with the JCPDS No. 733-458 data of UiO-66 [32]. For the Fe₃O₄/UiO-66/ZnO composites, the XRD pattern still recorded some intense diffraction peaks, reflecting the existence of the UiO-66 crystalline phase as well as Fe₃O₄ and ZnO oxides. It confirms the successful synthesis of the material from three main components, including Fe₃O₄, UiO-66, and ZnO. In addition to XRD analysis, FTIR spectra of Fe₃O₄, ZnO, UiO-66, and Fe₃O₄/UiO-66/ZnO were also recorded to identify the surface functional groups, with the results shown in Fig. 2b. In the FTIR spectra of Fe₃O₄ and Fe₃O₄/UiO-66/ZnO, a characteristic absorption band at 548 cm⁻¹ was recorded, corresponding to the stretching vibration of the Fe–O–Fe bond, confirming the presence of Fe₃O₄ in the material structure [33,34]. For UiO-66, the signals at 748 cm⁻¹ and 668 cm⁻¹ are related to the vibration of the O–Zr–O bond and the vibration of the C–H group in the structural framework of UiO-66, respectively [35,36]. In addition, the two peaks at 1593 and 1377 cm⁻¹ correspond to the vibrations of the C=O and C–O groups, respectively, reflecting the presence of carboxyl groups [35]. These functional groups play a role in the binding of UiO-66 to the Fe₃O₄ surface through electrostatic interactions and complexation [37,38]. In addition, in the FTIR spectrum of the ZnO and Fe₃O₄/UiO-66/ZnO samples, a characteristic absorption band appeared at 499 cm⁻¹, which was identified as the bending vibration of the Zn–O bond, confirming the presence of ZnO in the composites [39,40]. Overall, from the investigated characteristics, the results demonstrate the successful combination of the three components Fe₃O₄, UiO-66, and ZnO. The resulting material structure consists of spherical Fe₃O₄, covered by a nano-sized UiO-66 layer, on which flower-shaped ZnO nanoparticles are tightly bound, forming a special hybrid structure. The presence of Fe₃O₄ particles in the composite structure plays an important role, making the material easily recoverable by an external magnetic field. VSM measurements were performed to evaluate the magnetic properties, and the material’s magnetization curve is shown in Fig. 2c. The saturation magnetization value obtained from this curve is 2.3 emu/g, accompanied by a characteristic hysteresis loop. Although significantly lower than that of previously reported magnetite nanoparticles (72.5 emu/g for Fe₃O₄ nanoparticles [41]), this result is near to that of GO–Fe₃O₄ (4.62 emu/g [42]), MoS₂@Fe₃O₄ (4.01 emu/g [43]) and better than MoS₂@Fe₃O₄@Cu₂O (0.5 emu/g [43]), the material responds well to magnetic separation. It is demonstrated when the material is dispersed in water, it can be quickly recovered with a magnet.

3.2. Adsorption of TC

The reaction solution’s pH influences the adsorption capacity of Fe₃O₄/UiO-66/ZnO, as it involves both surface charge distribution and the ionic states of TC. As shown in Fig. 2d, the pH_PZC of the material determined by the pH drift method is 6.8 [44]. When the pH exceeds 6.8, the surface acquires a negative charge, whereas it becomes positively charged at pH below 6.8. Fig. 3a shows the adsorption capacity of TC by Fe₃O₄/UiO-66/ZnO material at pH 4, 7, and 10. TC adsorption capacity tends to reach a fast adsorption rate within the first 90 minutes and increases slowly thereafter. Specifically, at pH 4 and 7, the adsorption trend of TC by the material is almost the same until the end of the reaction at 5 hours, with a difference from the adsorption value when reaching equilibrium at 5 hours. It can be explained as follows: according to previous studies on TC, with the presence of functional groups such as -OH, -C=O, -N(CH₃)₃, and -CONH₂ [45], TC can exist in the cationic form H₄TC₊ at pH <3.3 (when combined with H⁺), amphoteric in the form H₂TC^o with a positively charged dimethylamino group and a negatively charged phenol hydroxyl group at pH around 3.3 to 7.7 [46], and anionic in the form HTCat pH from 7.7 to 9 and HTC₂⁻ when pH>9 [47]. Therefore, TC exists in an amphiphilic form in the investigated pH range of 4 and 7. It can electrostatically interact with the negatively charged surface (pH 7 > pH_PZC) and the positively charged surface (pH 4 < pH_PZC), so it is possible that at two pH values of 4 and 7, the removal of TC seems to be not dominated by the electrostatic interaction of the analyte and the material [8] but may mainly occur due to the π-π interaction through the presence of benzene ring on TCN and UiO-66 in the material [48]. Previous studies also showed that TC adsorption capacity is based on π-π interactions [49,50]. At pH 10, there is a repulsive interaction between the negatively charged surface and the anionic form of TC, so the adsorption capacity at pH 10 is the lowest. Based on the results obtained from observing pH on the TC adsorption capacity of Fe₃O₄/UiO-66/ZnO, pH at 7 was the optimal pH for the following experiments due to its superiority in good adsorption capacity, ease of implementation, and environmental friendliness.

At pH 7, the adsorption process of TC was studied with different materials over 5 hours at 30 mg/L, with the results in Fig. 3b. The TC adsorption efficiency of the material progressively increases with time and reaches equilibrium near 5 h. It is attributed to the gradual occupation of available adsorption sites by TC molecules, eventually resulting in saturation. Among the studied materials, ZnO and Fe₃O₄ nanoparticles showed the lowest TC adsorption capacity. The presence of UiO-66 significantly increases the adsorption efficiency of TC for UiO-66/ZnO and Fe₃O₄/UiO-66, possibly due to the π-π interaction between the benzene rings in the structure of TC and the organic linker of UiO-66 [15,20]. In addition, UiO-66, with outstanding advantages such as a large surface area, characteristic porous structure, and the presence of many oxygen-containing functional groups on the surface, enhances the ability to interact with organic molecules such as TC [17]. The adsorption efficiency of the UiO-66/ZnO is higher than that Fe₃O₄/UiO-66. Both these binary composites show lower values than those of pure UiO-66. It can be explained by the addition of Fe₃O₄ and ZnO, which partially occupied the inherent adsorption sites of UiO-66. However, flower-shaped nanostructured ZnO contributed to improved TC adsorption efficiency in the Fe₃O₄/UiO-66/ZnO system compared to UiO-66/ZnO or Fe₃O₄/UiO-66. It may be because flower-shaped nanostructured ZnO increases the surface area and creates more effective adsorption sites, thereby compensating for the reduction in efficiency due to the occupied space of Fe₃O₄. On the other hand, the addition of Fe₃O₄ in these material systems brings essential benefits to reusability due to the magnetic properties of Fe₃O₄, which facilitate the recovery of the material by magnets, thereby reducing the risk of secondary pollution to the environment.

Pseudo-first-order and pseudo-second-order adsorption kinetic models of TC onto Fe₃O₄, UiO-66, ZnO, UiO-66/ZnO, Fe₃O₄/UiO-66, and Fe₃O₄/UiO-66/ZnO materials based on experimental data collected for 5 h were described in Fig. 3c and Table 1. The findings showed that both models exhibit high linear determination coefficients (R²), demonstrating a close agreement with the experimental data. However, the pseudo-first-order kinetic model provides a better fit than the other model, which is evident from the close agreement between the calculated adsorption capacity q_e(cal) and the experimentally determined q_e(exp). Specifically, for UiO-66 material, the calculated and experimental adsorption amounts are 68.06 mg/g and 68.05 mg/g, respectively; for Fe₃O₄, they are 23.14 mg/g and 20.86 mg/g; for ZnO, they are 22.01 mg/g and 22.49 mg/g; for UiO-66/ZnO, they are 38.09 mg/g and 36.76 mg/g; for Fe₃O₄/UiO-66, they are 27.86 mg/g and 28.51 mg/g; and for Fe₃O₄/UiO-66/ZnO, they were 44.88 mg/g and 44.94 mg/g. This agreement showed that the pseudo-first-order kinetic model better describes the adsorption processes on the surfaces of these nanomaterials. In particular, the Fe₃O₄/UiO-66/ZnO material achieves the highest pseudo-first-order adsorption rate constant value k₁ = 1.249 h⁻¹ among the studied materials. This result reflected that the adsorption rate of this material was proportional to the difference between the adsorption amount at time t and the equilibrium value [44]. As the adsorption process approaches equilibrium, the rate gradually decreases, consistent with the kinetic mechanism of the pseudo-first-order model. The superiority of Fe₃O₄UiO-66/ZnO can be explained based on the nanostructural properties. Fe₃O₄ plays an essential role in providing magnetic properties, which enables efficient material reuse. UiO-66, with its porous metal-organic framework structure and large surface area, offers many adsorption sites and good interaction with TC molecules through hydrogen bonding, Van der Waals forces, and the π-π interaction [51]. Flower-shaped nano-ZnO expands the effective surface area and adds active sites due to its strong surface interaction with TC [52]. In addition, to have a deeper assessment of the distribution of adsorbed molecules between the solid phase and the water phase under the equilibrium condition of TC onto the material [53], the Langmuir and Freundlich adsorption isotherm model from Fig. 3d with the calculated parameters shown in Table 1 was built from the experimental results of the adsorption process of TC at concentrations from 15 mg/L to 120 mg/L for 5 hours at room temperature. Comparing the R² values of the two isotherm models, the results showed a high correlation coefficient for both. However, the Freundlich model (R² ~ 0.989) was higher than that of the Langmuir model (R² ~ 0.972), indicating that the Freundlich model best performed the adsorption process. This model provides information on TC adsorption by materials occurring in multiple layers on heterogeneous surfaces [54]. The n value of 1.69 falls in the range of 1 to 10, indicating that TC adsorption is favorable [55], with the maximum capacity q_m calculated from the Langmuir equation recorded as 302.99 mg/g. By integrating these attributes, Fe₃O₄/UiO-66/ZnO composite emerges as a highly effective material for TC adsorption, demonstrating exceptional performance in both rate and efficiency. It highlights its significant potential for applications in mitigating antibiotic contamination in aquatic environments.

3.3. Photocatalytic of TC

3.3.1. Photocatalytic kinetics

Various materials, including UiO-66, Fe₃O₄, ZnO, Fe₃O₄/UiO-66, UiO-66/ZnO and Fe₃O₄/UiO-66/ZnO was studied for their catalytic ability toward TC under white LED irradiation with a concentration of 30 mg/L after reaching adsorption equilibrium for 5 hours by comparing the catalytic efficiency with the recorded results shown in Fig. 4a. Without catalyst, the TC degradation almost does not occur within 2.5 hours, with the TC concentration not changing significantly indicating that TC is very stable under LED. In the presence of a catalyst, the catalytic process occurred with increasing TC degradation in the order of ZnO<Fe₃O₄ <UiO-66 < Fe₃O₄/UiO-66 <UiO-66/ZnO < Fe₃O₄/UiO-66/ZnO. Notably, the catalytic activity of each substance, Fe₃O₄, ZnO, or UiO-66, is low, especially ZnO, which has almost no activity. For Fe₃O₄, the photocatalytic ability was mainly based on the photochemical effect from the Fe³⁺/Fe²⁺ pair. However, the rapid recombination of electron-hole pairs reduces the catalytic efficiency of this material [56]. The energy of LED light in the visible region cannot stimulate the electron transition from the valence band to the conduction band of UiO-66 or ZnO, which requires the energy of UV light. Although not high, the photocatalytic activity in the visible-light region of UiO-66 and many other organic framework materials has been reported in many works [57–59]. The mechanism of this process is still unclear; we believe that it may involve electron transfer between ligand-ligand or ligand-metal for charge transfer, in which the ZnO units act as ZnO quantum dots, as in MOF-5 [60]. When Fe₃O₄ or ZnO and UiO-66 were combined, the photocatalytic efficiency of Fe₃O₄/UiO-66 or UiO-66/ZnO was significantly improved by taking advantage of the advantages of both materials. UiO-66 is essential in minimizing the rapid recombination of electron-hole pairs while providing a firm adsorption surface for TC concentration and supporting the uniform dispersion of ZnO or Fe₃O₄ [61]. It is noteworthy that the photocatalytic activity of Fe₃O₄/UiO-66/ZnO prepared by adding flower-like ZnO to Fe₃O₄/UiO-66 is significantly enhanced, up to 75%, the highest compared to the remaining catalysts. The ternary composites showed higher degradation rates than ZnO/UiO-66 or Fe₃O₄/UiO-66, which further suggests that the presence of ZnO supports the synergism of the three components. It also indicates similarity with previous reports demonstrating the synergism of ZnO in magnetic nanocomposites or MOFs-based materials [40,62–65] in which they suggested that ZnO not only acts as a photocatalyst in response to visible light but also facilitates charge separation when combined with other components. The effect of pH (4, 7, and 10) on the degradation of TC (30 mg/L concentration) using Fe₃O₄/UiO-66/ZnO catalyst is illustrated in Fig. 4b. The highest degradation efficiency was recorded at pH 4 and 7, which can be explained by the effective electrostatic interaction between Fe₃O₄/UiO-66/ZnO and TC in the amphoteric state, as analyzed in the adsorption study, together with the formation of oxidative radicals under LED white light irradiation due to electron and hole transfer of Fe₃O₄, UiO-66 and ZnO nanoparticle [66,67]. Furthermore, the acidic environment was favorable for the generation of hydroxyl radicals (^•OH) in the catalytic system, while at higher pH, the ability to form hydroxyl radicals was reduced, leading to a lower photodegradation efficiency [68]. In particular, at pH = 10, the presence of TC³⁺ cation and the lower ^•OH radical generation efficiency caused the TC degradation efficiency to decrease to the lowest level. The effect of initial TC concentration on the photocatalytic efficiency of Fe₃O₄/UiO-66/ZnO composites was investigated at pH 7 under white LED. The results in Fig. 4c displayed the relationship between photodegradation rate and illumination time with TC concentrations ranging from 30 to 120 mg/L. The degradation efficiency gradually decreased with increasing initial TC concentration, which can be explained by the saturation of catalytic sites occupied by TC ions at higher concentrations over time [69].

Table 2 compares different materials regarding adsorption and catalytic efficiency in removing TC under the corresponding reaction conditions. The results show that the synthesized materials in this study demonstrated multifunctionality when they can both adsorb and decompose TC effectively in a neutral environment (pH 7), with an adsorption capacity of up to 302.9 mg/g. Meanwhile, most of the compared materials only work effectively in low pH environments or can only perform one of the two functions. It shows the superiority of the synthesized materials in treating TC in environmentally friendly and safer conditions for health. Fig. 4e shows the variation of COD during the illumination time. The COD value decreases with increasing illumination time. It decreased from 88.95 mg/L for the initial sample to 29.32 mg/L for the sample irradiated after 150 minutes. This result shows that the TC decomposition process does not lead to intermediates but to complete mineralization to form CO₂.

3.3.2. Recyclability of prepared composite

To evaluate the reusability of the material, the sample was recovered after each reaction cycle and washed with water until the characteristic TC adsorption peak at 355 nm was no longer detected. The sample was then dried and reused for the next cycle. In the TC treatment experiment, five cycles were conducted using 0.02 g Fe₃O₄/UiO-66/ZnO each time for a TC solution of 30 mg/L concentration under LED light after reaching adsorption equilibrium. Fig. 4d illustrates the TC treatment efficiency based on both adsorption and catalysis mechanisms. Specifically, after five cycles, while the material still maintained a high adsorption capacity, the C/C_o ratio also showed a decrease in TC degradation efficiency of about 7% compared to the first cycle. This slight decrease in catalytic performance may be due to the loss of active sites during washing and the bound intermediates’ possible blocking of reaction sites [75,76]. This result confirms that the obtained composites have high reusability, promising long-term applications in pollutant treatment processes.

3.3.3. Interfering Ion and Radical Trapping Experiments

In real polluted water environments, anions can hinder the degradation of pollutants. In this study, we selected the anions Cl⁻, NO₃⁻, HCO₃⁻, and SO₄²⁻ to evaluate their effects on treating TC. Fig. 5a shows the combined TC removal efficiency (including adsorption and photocatalytic processes) under LED white light irradiation in the presence of NaCl, NaNO₃, NaHCO₃, Na₂SO₄ at concentrations of 0.01 M and no-interference. The results showed that the TC removal efficiency decreased in the order of nointerference > NaNO₃ > NaCl > Na₂SO₄ > NaHCO₃ (from 93.3%, 89.4%, 87.8%, 87.1% to 85.1%), in which the catalytic degradation efficiency under irradiation decreased from no-interference > NaCl> NaNO₃ > NaHCO₃ > Na₂SO₄ (from 50.4%, 47.0%, 46.6%, 45.1% and 44.2%, respectively). The presence of anions in solution may compete with TC for interaction with active sites on the catalyst surface, either through electrostatic interactions or chemical bonding. It is consistent with the observation that more negatively charged anions form stronger bonds with the adsorbent, reducing the TC adsorption capacity [20,77]. In addition, TC degradation was significantly inhibited in the presence of Na₂SO₄ and NaHCO₃. It may be explained by the ability of SO₄²⁻ and HCO₃⁻ anions to act as effective radical scavengers for ^•OH, reducing the number of hydroxyl radicals involved in the degradation process [74]. At the same time, HCO₃⁻ also can trap h⁺ radicals, reducing reactive radicals required in the photocatalytic process and reducing the TC treatment efficiency [78]. Therefore, Fe₃O₄/UiO-66/ZnO still showed good TC removal ability in an aqueous solution despite interfering ions with near-equilibrium performance between adsorption and catalysis under white LED irradiation. In addition to investigating the influence of interfering ions, trapping experiments of active species such as ^•OH, h⁺, ^•O₂⁻, and e⁻ were conducted to study the TC degradation of the material under white LED irradiation. Different quenching agents such as isopropyl alcohol (IPA; ^•OH scavenger) [13], 3, 4 dihydroxybenzoic acid (DHB; ^•O₂⁻ scavenger) [79], KI (h⁺ scavenger) [66], KBrO₃ (e⁻ scavenger) [80] were used to trap the corresponding active radicals during the catalytic reaction. Fig. 5b shows that the degradation efficiency from 90.4% (without ion trapping agent) after adding ion trapping agents was 79.0% (DHB), 77.9% (KBrO₃), 74.0% (IPA) and 67.0% (KI), respectively, demonstrating that these active radical groups all contributed to the catalytic process and followed the order of existence and action h⁺ > ^•OH > e⁻ > ^•O₂⁻ under white LED irradiation.

3.4. Possible Mechanism of Degradation TC

Based on the ion trap experiment results, the role of active species groups in TC degradation is determined in decreasing order: h⁺ > ^•OH > e⁻ > ^•O₂⁻. The holes (h⁺) show a strong inhibition effect, indicating that they dominate the TC degradation mechanism. Fig. 6 illustrates the proposed degradation mechanism. Initially, TC molecules will be adsorbed onto the material surface thanks to van de Waals’ interactions and hydrogen bonding on the surface. Upon LED irradiation, Fe₃O₄, with a narrow band gap of approximately 1.8 eV, becomes excited, facilitating electron transfer to UiO-66.

UiO-66 is composed of a zirconium-based cluster [Zr₆O₄]⁶⁺, surrounded by 1,4-benzene dicarboxylic acid to form a cubic microporous [81]. This unique UiO-66 structure enhances effective charge transfer and favors electron-hole charge separation. The reason UiO-66 remains active in the visible region is still unclear; we suggest that the organic framework acts as a photon absorber, mediating the transfer of electrons from the valence band to the conduction band [82]. Excited electrons transferred from UiO-66 (VB) to UiO-66 (CB) is illustrated an arrow in Fig. 6.

Simultaneously, photogenerated holes (h⁺) in the valence band (VB) of Fe₃O₄ migrate into the valence band of UiO-66. This process enhances charge separation while minimizing electron-hole recombination, improving the overall reaction efficiency. Next, the electrons from UiO-66 will transfer to ZnO. Photo-induced electrons accumulated on the ZnO surface interact with O₂ to create ^•O₂⁻ radicals, as the conduction band (CB) of ZnO is more negative than the standard redox potential of the O₂/^•O₂⁻ pair (−0.33 eV) [83,84]. At the same time, the holes from UiO-66 possibly migrate to ZnO, which has a sufficiently high positive potential to oxidize OH⁻ and H₂O molecules to ^•OH free radicals. Specifically, the oxidation potential of these holes exceeds the thresholds of the OH⁻/^•OH pairs (+1.99 eV) and H₂O/^•OH (+2.27 eV) [66,67,84]. The accompanying mechanism diagram, as Fig. 6, illustrates that the entire TC decomposition process proceeds through consecutive reactions (Eq. (2) – Eq. (11)).

(2)

{Fe}_{3} O_{4} / UiO - 66 / ZnO + TC \to {Fe}_{3} O_{4} / UiO - 66 / ZnO (TC)

(3)

{Fe}_{3} O_{4} + h υ \to h + + e -

(4)

UiO - 66 + h υ \to h + + e -

(5)

UiO - 66 + e - ({Fe}_{3} O_{4}) \to UiO - 66 (e -)

(6)

UiO - 66 + h + ({Fe}_{3} O_{4}) \to UiO - 66 (h +)

(7)

ZnO + UiO - 66 (e -) \to ZnO (e -)

(8)

ZnO + UiO - 66 (h +) \to ZnO (h +)

(9)

ZnO (e -) + O_{2} \to ZnO (• O_{2} -)

(10)

H_{2} O \to H + + OH - + h + \to • OH

(11)

TC + • OH + • O_{2} - \to Products

4. Conclusion

In this study, we successfully synthesized a three-layered magnetic material consisting of a Fe₃O₄ core surrounded by a shell of octahedral UiO-66 nanoparticles and an outer layer of flower-shaped ZnO nanoparticles. The synthesized material exhibited an impressive TC adsorption capacity, reaching 302.99 mg/g according to the Langmuir isotherm model and following first-order kinetics. Under white LED light irradiation, the material exhibited good catalytic activity due to the effective coordination between the constituent components. After 5 reuse cycles, the material maintained a high TC removal efficiency under white LED light and a strong adsorption capacity. These results demonstrate that Fe₃O₄/UiO-66/ZnO is an effective adsorbent and a strong photocatalyst with outstanding reusability. With its low synthesis cost and convenient recycling process, this material is a potential candidate for practical wastewater treatment applications.

Notes

Conflicts of Interest Statement

The authors declare that they have no conflict of interest.

Author Contributions

L.T.T.N. (PhD) wrote the original draft, reviewed and edited the manuscript, validated the data, and contributed to methodology, investigation, formal analysis, data curation, and conceptualization. L.H.T. (Student) and H.T.N.Y (Master) contributed to writing the original draft, resources, methodology, investigation, formal analysis, and data curation. T.N.D. (PhD), T.N.B. (Master) and L.D.H. (Student) conducted the investigation, formal analysis, and data curation. N.V.S.L. (Medical Doctor) contributed to software, resources, formal analysis, and data curation. L.V.T. (PhD) contributed to software, resources, methodology, investigation, and data curation. L.H.S. (PhD) wrote the original draft, reviewed and edited the manuscript, validated the results, supervised the work, and contributed to resources, methodology, data curation, and conceptualization. D.Q.K. (Professor) wrote and reviewed the manuscript, supervised the study, managed the project, and contributed to software, formal analysis, data curation, and conceptualization.

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Fig. 1

The SEM images of Fe₃O₄ (a), Fe₃O₄/UiO-66 (b), Fe₃O₄/UiO-66/ZnO (c), EDX spectrum of Fe₃O₄/UiO-66/ZnO (d); and element mapping of Fe₃O₄/UiO-66/ZnO (e–i).

Fig. 2

The XRD (a), FTIR (b) patterns of as-prepared materials, VSM profiles (c) and pH_PZC (d) of Fe₃O₄/UiO-66/ZnO

Fig. 3

The effect of pH on the TC adsorption capacity of Fe₃O₄/UiO-66/ZnO (a); the adsorption capacity (b) and kinetic adsorption (c) of TC with the presence of various materials; the Langmuir and Freundlich isotherm models of Fe₃O₄/UiO-66/ZnO composite (d). Experimental conditions: dose of adsorbent: 0.5 g/L; temperature: 25°C; initial concentration of TC: 30 mg/L for (a, b, c) and from 15 mg/L to 180 mg/L for (d) experiments.

Fig. 4

The catalytic degradation (a) with the presence of Fe₃O₄, UiO-66, ZnO, UiO-66/ZnO, Fe₃O₄-UiO-66, Fe₃O₄/UiO-66/ZnO and without the catalyst (TC) under white Led light; the effect of pH (b), initial concentration (c) on the degradation of TC, and recyclability of the Fe₃O₄/UiO-66/ZnO catalyst (d), and The dependence of COD on the illumination time (e). Experimental conditions: dose of catalyst: 0.5 g/L; temperature: 25°C; initial concentration of TC: 30 mg/L for (a, b, d) and from 30 mg/L to 120 mg/L for (c) experiments.

Fig. 5

The effect of interference ion on adsorption and degradation of TC (a) and effect of scavengers (b) on degradation of TC by Fe₃O₄/UiO-66/ZnO. Experimental conditions: dose of material: 0.5 g/L; temperature: 25°C; initial concentration of TC: 30 mg/L.

Fig. 6

The proposed mechanism for degradation of TC by Fe₃O₄/UiO-66/ZnO

Scheme 1

Illustration of equipment used for photocatalytic reaction

Table 1

The kinetic parameters of UiO-66, Fe₃O₄, ZnO, UiO-66/ZnO, Fe₃O₄/UiO-66, Fe₃O₄/UiO-66/ZnO

Composite	R²	k₁ (h⁻¹)	Q_e(cal) (mg/g)	Q_m(exp) (mg/g)
Pseudo-first order kinetic model
UiO-66	0.966	0.981	68.06	68.05
Fe₃O₄	0.934	0.429	23.14	20.86
ZnO	0.963	0.929	22.01	22.49
UiO-66/ZnO	0.992	0.697	38.09	36.76
Fe₃O₄/UiO-66	0.960	0.931	27.86	28.51
Fe₃O₄/UiO-66/ZnO	0.995	1.249	44.88	44.94
Pseudo-second order kinetic model
UiO-66	0.983	0.014	82.21	68.05
Fe₃O₄	0.999	0.010	33.35	20.86
ZnO	0.992	0.040	26.65	22.49
UiO-66/ZnO	0.988	0.014	49.15	36.76
Fe₃O₄/UiO-66	0.985	0.031	34.00	28.51
Fe₃O₄/UiO-66/ZnO	0.969	0.032	51.87	44.94

Table 2

Comparison of the performance removal of TC with other adsorbents/catalyst

Adsorbent/catalyst	Q_m (mg/g)	K (min⁻¹)	Reaction condition	Ref
Br-MIL-53(Fe)	309.6		Dose= 0.2 g/L; C = 20 mg/L; pH=7; 24h	[8]
Fe₃O₄/ZIF – 8-G	238.7		Dose=0.02 g/L; C=200 mg/L; pH=5–6; 10h	[70]
In₂S₃/UiO-66	106.3		Dose=0.03 g/L; C=40 mg/L; 60 mins	[19]
Pistachio shells coated with ZnO nanoparticles	95.1		Dose=0.8 g/L; C=100 mg/L; pH=5; 2h	[71]
Soft foam-like UiO-66/Polydopamine/Bacterial cellulose	184.0		Dose=0.5 g/L; C=100 mg/L; pH=3–4; 24h	[72]
SnSPs@UiO-66		0.024	Visible light; Dose=1 g/L; C=20 mg/L; 75 mins	[73]
In₂S₃/UiO-66		0.017	Visible light; Dose=0.3 g/L; C=40 mg/L; 60 mins	[19]
AgBr/Bi₂WO₆		0.028	Visible light; Dose=1 g/L; C=20 M; 60 mins	[74]
ZnO		0.007	Visible light; Dose=1 g/L; C=50 mg/L; 100 mins	[40]
MGO/ZnO		0.014	Visible light; Dose=1 g/L; C=50 mg/L; 100 mins	[40]
Fe₃O₄/UiO-66/ZnO	302.99		Dose=0.5 g/L; C=30 mg/L; pH=7; 4h	This study
Fe₃O₄/UiO-66/ZnO		0.132	LED light; Dose=0.5 g/L; C=30 mg/L; 150 mins	This study

Tetracycline removal using flower-like Fe3O4/UiO-66/ZnO composite: Adsorption and white LED photocatalysis performances