3D porous carbon-embedded nZVI@Fe2O3 nanoarchitectures enable prominent performance and recyclability in antibiotic removal (2023)

The abuse of antibiotics in livestock industry and human therapy has aroused serious environmental pollution, and the long-term accumulation of antibiotics in the environment would also induce the spread of antibiotic resistant bacteria and antibiotic resistant genes, further endangering human health and ecosystems (Huang et al., 2021b; Tan et al., 2022; Wen et al., 2022). The most frequently detected antibiotics in sewage include sulfonamides, ciprofloxacin (CIP) and tetracycline (TC), finding extensive use as antibacterial agents and veterinary medicines (Huang et al., 2021a; Zhang et al., 2021; Hu et al., 2022a; Michelon et al., 2022; Wen et al., 2022; Zhao et al., 2022a). With hydrophilic property, stable structure and non-biodegradability, these antibiotic residues cannot be sufficiently eliminated by conventional wastewater treatment processes or biological remediation strategies, and their residual levels in aqueous environments are as high as mg L⁻¹ (Zhang et al., 2021; Hu et al., 2022a). Considering adverse impacts caused by the long-term accumulation of antibiotics, it is imperative to develop novel advanced technologies to reduce their harmful effects.

Recently, various technologies have been developed to treat antibiotics, including advanced oxidation process, electrochemical degradation, photocatalysis, membrane separation, microwave catalysis, and adsorption (Hena et al., 2021; Liu et al., 2022a; Wang et al., 2022b; Yang et al., 2022; Yu et al., 2022; Zhang et al., 2022d; Li et al., 2023; Lin et al., 2023). By comparing the aforecited methods, adsorption with nanotechnology has becoming a promising approach for practical application on account of its high effectivity, easy operation, low installation costs, and no involvement of toxic intermediates. Further, adsorption process is preferred in this field because it exhibits high efficiency for removing low-concentration antibiotic residues in natural waterbodies (Hao et al., 2021). To date, various nano-based adsorbents have been developed and applied for efficient remediation of antibiotic pollutants in water, including carbon materials (active carbon, carbon nanotubes, biochar, and graphene-based materials) (Chen et al., 2015; Xu et al., 2021; Yao et al., 2021; Tian et al., 2022; Zhang et al., 2022c), metal oxides (Yang et al., 2021), polymer materials (Hu et al., 2022b; Saya et al., 2022), and organic porous materials (metal organic frameworks and covalent organic frameworks) (Liu et al., 2022b; Zhang et al., 2022b). Among them, the presence of abundant suitable functional groups on the surface coupled with high surface area and low cost characteristics makes porous carbon nanomaterials one of the brightest adsorbents for antibiotic removal (Mao et al., 2019; Lei et al., 2021b). The main challenge associated with carbon nanomaterials, however, is their uneasy recyclability caused by their light weight and small size, which severely limit their practical applications.

To address the above-mentioned issues, various strategies have been explored to improve the separation performance of carbon nanomaterials, among which the controllable integration of magnetic units and carbon nanomaterials is the most widely used in sewage treatment (Jaswal et al., 2021; Liu et al., 2021a; Feng et al., 2022). As one of the most charming Fe-based materials, environment-friendly nanoscale zero-valence iron (nZVI) has many prominent advantages such as good in-situ reactivity, strong adsorption property and magnetic separation, which has been used for the treatment of various pollutants in recent decades (Fu et al., 2014; Kao et al., 2021; Li et al., 2021a; Shanableh et al., 2021; Liang et al., 2022). Encapsulating nZVI nanoparticles and further imbedding them in supporting matrixes such as porous carbon would prevent them from excessive aggregation as well as introduce apparent modification (e.g., introduction of defects, functional groups) in composites, thereby promoting the overall removal performance of fabricated composites (Gong et al., 2022; Yang et al., 2022). Compared to original nZVI and carbon materials, nZVI-supported carbon-based materials have more adsorption sites, better stability and reactivity, as well as outstanding convenience of separation. Wang et al. (2020) used a cellulose hydrogel as supporting material to immobilize nFe⁰ particles, and this composite structure displayed much higher Cr(VI) adsorption capability and reducibility than nFe⁰ and cellulose hydrogel. Li et al. (2021b) immobilized nZVI particles within a 3D carbon foam, which gave higher Pb²⁺ adsorption capacity. Rashtbari et al. (2022) showed that the adsorption of furfural by activated carbon loaded with nZVI was significantly improved. Despite substantial progress in nZVI-incorporated architectures, there are still several unsolved limitations such as narrow operating pH range, instability and poor recyclability (Tang et al., 2021), which not only restrict the scope of application and shorten service life, but also increase the cost of materials.

The development of nZVI/oxide core-shell structure would change this situation (Panda et al., 2020; Prasannamedha et al., 2022). Typical oxide shells including FeO, Fe₂O₃, Fe₃O₄ and iron hydroxides (FeOOH) can act as a barrier for preventing undesired corrosion of iron core in aquatic environments thereby in favor of long-lasting storage and recovery (Mu et al., 2017). Therefore, incorporating nZVI/oxide core-shell particles within supporting matrixes would give exciting materials with excellent comprehensive properties for environmental remediation, catalysis, energy storage, drug delivery, and so on (Mehdipour et al., 2021; Chandrashekhar et al., 2022; Park et al., 2022; Zhang et al., 2022a). Currently, the most common synthetic route of above-mentioned supported nZVI/oxide architecture composites is to synthesize nZVI/oxide structure first, incorporate it within a supporting matrix, followed by a reduction process of annealing. However, this synthesis route is fairly complicated or not eco-friendly (Li et al., 2009; Wang et al., 2018; Yang et al., 2018). Furthermore, because the properties of supported nZVI/oxide composites depend strongly on morphology and porous structure, the poor dispersion of magnetic units and the difficulty in tuning the morphology of the composite resulting from this preparation process are not conducive to improving the whole adsorption performance. Therefore, it remains a great challenge to obtain new magnetic @ carbon composites in a precisely controlled, convenient and economical way to inherit the merits of both units as much as possible.

Herein, we report a novel strategy to synthesize [emailprotected]₂O₃ core-shell architectures in-situ imbedded in a 3D-interconnected porous carbon network (denoted as [emailprotected]₂O₃/PC), which is very different from the traditional synthesis method of supported nZVI composites. Sodium alginate (SA), a natural, nontoxic, biodegradable, and porous polymer, were chosen as the precursor of porous carbon. Ferric chloride was used as the iron source as well as the binder for assembling alginate chains into 3D hydrogel matrix, which was later directly transformed into a catkin-like [emailprotected]₂O₃/PC nanoarchitecture just via a simple carbonation process. This one-pot synthesis route can not only create a very strong interaction between the 3D porous carbon network and the embedded [emailprotected]₂O₃ nanoparticles, but also inherit the porosity and 3D-interconnected network of alginate hydrogel. All of these are very important for enhancing the stability and capture property of composites. [emailprotected]₂O₃/PC combines the advantages of high porosity of carbon materials, high stability of core-shell materials as well as strong magnetism of magnetic nanomaterials. As a result, [emailprotected]₂O₃/PC can work steadily over a wide pH range (2–8) during the adsorptive removal of antibiotic (sulfamethoxazole (SMX), CIP, and TC, etc.), exhibiting a high removal ability (>99%) just within 10 min, and can be easily recycled by magnetic separation. The facile production and high cyclic stability of [emailprotected]₂O₃/PC reinforce its potential in the practical remediation of antibiotics. Importantly, this work would provide a novel solution for the design and green production of new iron-based composites with stable structure and excellent comprehensive properties.