Fabrication of reactive flat-sheet ceramic membranes for oxidative degradation of ofloxacin by peroxymonosulfate

Highlights

• CoFe₂O4-decorated flat-sheet ceramic membrane was prepared via hydrothermal method.

• The functionalized ceramic membrane was used for catalytic filtration of ofloxacin.

• Nearly 100% removal of 40 μM ofloxacin could be achieved within 20 min.

• Sulfate radicals are predominant reactive radicals for ofloxacin oxidation.

• Electron transfer during catalytic oxidation was identified by DFT calculations.

Abstract

A novel CoFe₂O₄-decorated flat-sheet ceramic membrane (CFCM) was prepared via a facile one-step hydrothermal method and utilized for peroxymonosulfate (PMS) activation in the catalytic degradation of ofloxacin (OFX) in a dead-end filtration mode. Several characterization methods confirmed the successful deposition of a CoFe₂O₄ layer on the surface of a pristine Al₂O₃ ceramic membrane. The CFCM possessed a considerably smaller average pore size (45 nm) and contact angle (20°) than a pristine Al₂O₃ flat-sheet ceramic membrane (80 nm and 50°) after modification. The catalytic degradation results revealed that nearly 100% removal of 40 μM OFX could be achieved within 20 min at pH 6.0, with 2 mM PMS and 100 kPa of transmembrane pressure (TMP). Moreover, the CFCM suffered little interference from co-existing SO${}_4^{2-}$ and Cl⁻ in the water matrix, but was significantly hindered by HCO${}_3^{-}$ and humic acid. The low concentration of metal-leaching by CFCM will makes it reliable for catalytic degradation processes. Sulfate radicals were found to be the predominant reactive radicals that drove OFX degradation, and according to density functional theory (DFT) calculations they were generated via electron transfer from CoFe₂O₄ to chemisorbed PMS.

Introduction

Antibiotics have been overused both in humans and livestock to inactivate or eliminate pathogens since their discovery in 1928 [1]. Most antibiotics cannot be completely metabolized, and their residues are released into the environment. The presence of these antibiotic residues in water systems is an environmental problem, as they may induce adverse effects on human health by triggering the development of antibiotic-resistance in pathogens [2]. Therefore, effective management and treatment processes should be provided and adopted to remove antibiotic residues from all water and wastewater matrices [3].

Conventional biological treatment is not sufficient to degrade antibiotic residues, as the biological toxicity of antibiotics means such treatment would also generate domesticated resistant strains [4]. Adsorption processes can absorb antibiotics from aquatic environments, but the disposal of used adsorbents is a great challenge and may cause secondary pollution. Electrochemical methods are often adopted to produce strong oxidizing agents in situ for the degradation of antibiotics [5], but these require expensive facilities and large amounts of energy, thus limiting their practical application. Heterogeneous sulfate radical-based advanced oxidation processes (SR-AOPs) have recently attracted great attention for their potential in decomposing antibiotics, due to the high redox potential (2.5–3.1 V) and long life-time (30–40 μs) of sulfate radicals [6]. Nevertheless, it is hard to separate dispersed catalysts in this process from effluents.

Compared with the above methods, membrane filtration is a promising strategy for removing small molecules from wastewater. However, membranes are typically made from polymer composites with low mechanical strength, high thermal sensitivity, low chemical resistance and an invariable tendency to foul membranes. These drawbacks have limited the widespread utilization of organic membranes in wastewater treatment. Furthermore, nanofiltration (NF) organic membranes with relatively low flux are commonly utilized for antibiotics separation [7,8], which will limit the treatment capacity of antibiotic wastewater.

Currently, ceramic membranes are used in water treatment due to their high mechanical strength, high flux, chemical and thermal stability, great hydrophilicity, low energy requirement and long operating life. Despite the production cost of ceramic membranes is higher than that of organic membranes, the lower operation and maintenance cost, and longer lifespan of ceramic membranes render them cost-effective in wastewater treatment [9]. However, the large pore size of common microfiltration (MF) and ultrafiltration (UF) ceramic membranes means that they cannot effectively reject antibiotics [10]. To address this limitation, functionalized ceramic membranes with catalytic layers for PMS activation were developed [11,12], and these can realize the catalytic degradation of antibiotics. In brief, the high-flux ceramic membrane provides a stable platform to immobilize catalysts and separate them from effluent, and the catalysts react with oxidants to generate reactive radicals and subsequently destroy the antibiotics in the effluent. Under forced filtration, these catalytic ceramic membranes can enhance the contact between reactants, reducing the mass transfer resistance and thereby achieving a higher catalytic efficiency than heterogeneous catalysis using catalyst particles [13].

PMS could be activated by UV, heat, base and transition metals [14]. Among them, the activation of PMS by transition metals is easy to manipulate and requires no extra energy, so transition metals have been widely adopted as activator for PMS. Previous literature has reported that among all of the transition metals evaluated, Co²⁺ exhibited the highest reactivity for PMS activation [15]. But the aqueous toxicity of dissolved cobalt limits its application in wastewater treatment. The spinel structure with the general formula of A^IIB^III₂O₄ was investigated in our previous studies to immobilize metal ions [16,17], where divalent metal A occupies tetrahedral sites and trivalent metal B is in the octahedral environment. This structure possesses great stability with low leaching behavior [17], which could be considered for Co(II) immobilization. Therefore, the cobalt ferrite spinel (CoFe₂O₄) was selected to act as a stable catalyst for PMS activation. The two types of Co-O bond length in CoFe₂O₄ are 1.86 Å and 2.02 Å, shorter than those in CoO (2.17 Å), CoTiO₃ (2.17 Å and 2.05 Å) and Co(OH)₂ (2.10 Å) [18,19]. The shorter bond length of Co-O in CoFe₂O₄ means stronger Co-O bond energy, which could help immobilize Co(II) in spinel and may result in better water stability with reduced leaching concentration of cobalt than other cobalt compounds.

Existing functionalized ceramic membranes are typically synthesized by a modified sol-gel method accompanied by a dip-coating process. This requires a long operation time and combustion to form the coated catalytic layer, resulting in high energy consumption. Therefore, a facile and low-energy strategy should be developed to manufacture novel catalytic ceramic membranes.

To that end, we have fabricated a cobalt ferrite (CoFe₂O₄)- decorated flat-sheet ceramic membrane via a one-step hydrothermal method, and used the resulting functionalized membrane in a catalytic degradation process to eliminate ofloxacin (OFX) via activation of PMS. The degradation of OFX under different conditions was investigated to confirm the optimal conditions for this catalytic degradation. The stability and reusability of the membrane were also studied to evaluate its practicability and environmental friendliness. The mechanism of catalytic degradation was further probed by X-ray photoelectron spectroscopy (XPS) and density functional theory (DFT) calculations.