One-step fabrication of novel MIL-53(Fe, Al) for synergistic adsorption-photocatalytic degradation of tetracycline

Highlights

• MIL-53(Fe, Al) was applied to remove tetracycline for the first time.

• MIL-53(Fe, Al) exhibited a significant adsorption and photocatalytic synergy for tetracycline removal.

• A possible tetracycline degradation mechanism was proposed.

Abstract

Bimetallic MOFs (MIL-53 (Fe, Al)) were successfully fabricated via a facile one-step solvothermal method for the removal of tetracycline (TC) from aqueous solutions. Tetracycline adsorption and photocatalytic experiments indicate that the optimum bimetallic synthetic molar ratio is 3:2 (40%MIL-53(Fe, Al)). The adsorption data are well fitted by the Freundlich model and pseudo-second-order adsorption kinetics. 40%MIL-53(Fe, Al) has an adsorption capacity of up to 402.033 mg/g. After the dark adsorption phase, 10 mg of 40%MIL-53(Fe, Al) can remove 94.33% of the tetracycline in a 70 mL aqueous solution (20 mg/L) under 50 min irradiation, while only 71.39% and 81.82% of the tetracycline are removed by MIL-53(Fe) and MIL-53(Al) under the same conditions. In addition, 40%MIL-53(Fe, Al) exhibits a significant adsorption-photocatalytic synergy (under direct irradiation without a dark adsorption phase), in which the pseudo-first-order kinetic constant increases by a factor of 3.11. Quenching experiments and ESR characterization indicate that ·O₂⁻, ·OH, and h⁺ are the main active species in the photocatalytic process. Meanwhile, 40%MIL-53(Fe, Al) demonstrates good stability, with a tetracycline removal rate that still reaches 83.70% after 4 cycles. These results suggest that the prepared 40%MIL-53(Fe, Al) catalyst is a novel adsorption-photocatalytic material that can be used for the efficient treatment of tetracycline.

Introduction

In recent years, environmental problems have become increasingly prominent. Water pollution, especially pollution caused by pharmaceuticals and personal care products (PPCPs), has threatened the safety of water ecosystems and human health. This has caused widespread concern (Hena et al., 2021, Liu et al., 2021a). Tetracycline is a typical antibiotic that is widely used in medicine, animal husbandry, and farming applications. This inevitably leads to a large amount of incompletely removed tetracycline entering aquatic environments. Tetracycline pollution kills aquatic organisms that are not resistant to this drug. This pollution impacts ecological community structures and disrupts existing ecological balances, which in turn affects the overall food chain and human society (Rodriguez-Mozaz et al., 2015; Wang et al., 2018a). Therefore, exploring efficient methods for the removal of tetracycline from water environments is extremely important. Currently, water treatment methods can be divided into biological, physical, and chemical methods (Du et al., 2021). Among these methods, adsorption (Jin et al., 2019; Paula Fagundes et al., 2021) and photocatalysis (Zhu et al., 2013; Lei et al., 2020) have proven to be effective strategies for the removal of tetracycline. Adsorption has the advantages of simple operation, low energy consumption, and no secondary pollution, but adsorption methods cannot completely degrade and mineralize pollutants (Wang et al., 2020). However, photocatalysis can degrade pollutants that are difficult to completely remove from treated water by traditional biological methods. Furthermore, photocatalytic methods have many significant advantages, such as high efficiency, cleanliness, and no pollution generation (Du et al., 2021, Liu et al., 2021b). To effectively treat water, a combination of both adsorption and photocatalysis has been considered. In a dual treatment process, a catalyst reaches adsorption-desorption equilibrium. Pollutant degradation is then achieved by photocatalysis. This process results in self-cleaning material properties. At the same time, adsorption enhances the removal efficiency of photocatalysis due to the rapid reactions that occur between photo-active species and the pollutants adsorbed on the surface of the catalytic material (Kant et al., 2014). From this perspective, the combination of high adsorption capacity and excellent photocatalytic properties facilitates the efficient removal of tetracycline from water. Therefore, novel materials that combine high adsorption capacity and photocatalytic properties need to be urgently explored.

In recent years, metal–organic frameworks (MOFs) have attracted a significant amount of attention in the fields of adsorption (Shi et al., 2021; Uddin et al., 2021) and photocatalysis (Hu et al., 2017; Li et al., 2021; Ma et al., 2021). MIL-53, a typical MOF material in the MIL (MIL = Materials Institute Lavoisier) series, is a three-dimensional skeletal material with a rhombic pore structure formed by the self-assembly of the metal salt MO₄(OH)₂ (M = Al³⁺, Cr³⁺ or Fe³⁺) with an organic terephthalic acid ligand (Serre et al., 2002). Among the possible MIL-53 formulations, MIL-53(Fe) has the advantages of good structural stability, strong light absorption ability, and the presence of unsaturated metal coordination (Liang et al., 2015). However, its specific surface area is relatively low and its adsorption ability is weak. In contrast, MIL-53(Al) has a high specific surface area and unique pore respiration (Shi et al., 2021), demonstrating excellent potential for application in the field of adsorption. The main disadvantage of MIL-53(Al) is its poor light absorption ability. To enhance the adsorption/photocatalytic effect of these materials, most researchers have focused on doping modifications (Guo et al., 2015; Hu et al., 2017; Tang et al., 2019; Khodayari and Sohrabnezhad., 2021). In addition, some researchers have concentrated on modifying the MOF framework structure.

The performance of MOFs can be significantly improved by introducing bimetallic nodes, which lead to better crystallization properties and more active photosensitivity (Chen et al., 2020). For example, Wu et al. (2021) demonstrated that the enhanced synergy between different metal nodes in bimetallic MOFs (L-MIL-53(Fe, Mn)) significantly enhanced their ciprofloxacin removal efficiency. Hu et al. (2018) synthesized bimetallic MOFs (Co/Fe-MOFs), which had a rhodamine B degradation rate of up to 99.7% under visible light conditions. Wang et al. (2018b) synthesized ammonia-functionalized bimetallic MOFs (Fe/Ti-MOF-NH₂) for the enhanced visible light photocatalytic degradation of acid orange. The performance of these MOFs was attributed to Fe³⁺/Fe²⁺ and Ti⁴⁺/Ti³⁺ redox cycles, which resulted in efficient electronic cycling. Ideally, the introduction of a second metal in a MOF leads to retaining the beneficial features of the initial framework structure while changing the existing properties or introducing new favorable properties. Therefore, framework structure modifications enable the simultaneous enhancement of the adsorption and photocatalytic properties of these materials. However, the synergistic enhancement of the adsorption performance of MOFs with photocatalytic properties by introducing a second metal has rarely been reported.

In this work, MIL-53(Fe) was used as a substrate and Al³⁺ was used as a second metal dopant to combine the excellent light absorption characteristics of MIL-53(Fe) with the strong adsorption performance of MIL-53(Al). The adsorption and photocatalytic performance of the obtained adsorbent-photocatalyst MIL-53(Fe, Al) was tested by investigating the removal and degradation of tetracycline from water. The physical and chemical properties of the prepared samples were analyzed by a series of characterization techniques, including XRD, FT-IR, SEM, EDS, XPS, ICP-AES, BET, TG, UV–vis, PL, and ESR. Meanwhile, a possible adsorption-photocatalytic synergistic mechanism was also discussed. Finally, the reusability performance of the catalyst was evaluated by recycling experiments. The results and analysis provided herein offer a new approach for future environmental remediation.