Efficient removal of emerging organic contaminants via photo-Fenton process over micron-sized Fe-MOF sheet

Highlights

• A micron-sized Fe-MOF sheet showed good heterogeneous photo-Fenton performance.

• The content of Fe³⁺/Fe²⁺ were detected by X-ray absorption fine structure.

• The hydroxyl radicals yield and H₂O₂ consumption at different pH were revealed.

• Density functional theory helped to explain the band gap and degradation mechanism.

• The surface Fe sites were proved to be important in photo-Fenton process.

Abstract

A 2D micron-sized MOF sheet, namely BUC-21(Fe), was synthesized hydrothermally from FeSO₄·7H₂O, 1,3‐dibenzyl‐2‐imidazolidone‐4,5‐dicarboxylic acid (H₂L) and 4,4′‐bipyridine (bpy), which exhibited efficient removal of chloroquine phosphate (CQ) as emerging organic pollutant model via photon-Fenton process at pH = 5.0. The generated ·OH concentration was 242.5 μmol L⁻¹ with H₂O₂ consumption efficiency of 83.2%. The hydroxyl radical (·OH) was the primary reactive specie at the whole process, whereas superoxide radical (·O₂⁻) could boost the Fenton-like reaction at neutral pH. The oxidation state and content of iron in BUC-21(Fe) was detected by X-ray absorption fine structure (XAFS), in which 62% and 45% Fe²⁺ could be observed in the fresh sample and the used sample after photo-Fenton reaction. The CQ removal efficiency decreased from 100% to 68.3% when Fe sites were locked by phosphate, directly confirming the important role of Fe in the photo-Fenton reaction. The mechanism was proposed with the help of electrochemistry and density functional theory (DFT) calculation, which revealed that the photoinduced election was excited from oxygen atoms to Fe ions. The CQ was degraded into small molecules with low toxicity, and the degradation pathway was proposed. BUC-21(Fe) was water stable and showed excellent performance for photo-Fenton degradation of various organic contaminants (sulfamethoxazole (SMX) and bisphenol A (BPA)).

Introduction

Organic pollutants in water environment might exert great potential threat to environment due to their persistence and biotoxicity. Unfortunately, these organic pollutants, such as pharmaceutical and personal care products (PPCPs) and antibiotic, are difficult to efficiently remove via traditional process [1], [2]. Advanced oxidation processes (AOPs) like photocatalysis [3], [4], [5], [6], persulfate activation [7], [8], Fenton-like and so on [9], [10] have been proved to be highly efficient and eco-friendly, which are widely used in organic pollutants degradation.

Recently, increasing researchers focus on heterogeneous Fenton-like reaction for degrading organic contaminants into less toxic molecules and even mineralizing them into CO₂ and H₂O [8], [11]. It is expected to overcome the typical drawbacks of traditional homogeneous Fenton reaction, including but not limited to operation in the narrow pH range (<3) and producing huge amount of sludge. However, the application of heterogenous Fenton reaction is hampered by slow Fe²⁺ generation rate from Fe³⁺ and high H₂O₂ consumption [12]. Hou et al. introduced a reductive agent NH₂OH to goethite for surface Fenton reaction, which significantly accelerated Fe²⁺ generation from Fe³⁺ [13]. Zhu et al. demonstrated that Fe³⁺/Fe²⁺ cycle could be greatly promoted by combining ferrihydrite with carbon nanotubes (CNTs), since CNTs not only accelerated the electron transfer, but also decreased Fe³⁺/Fe²⁺ redox potential [14]. As well, coupling technologies like photo-Fenton [15], electro-Fenton [16] and ultrasound-Fenton [17] are efficient approaches to promote Fe³⁺/Fe²⁺ cycles and decomposition of H₂O₂ [18], [19], [20]. Xing et al. demonstrated that the metal sulfide (like MoS₂ [10], [21] and WS₂ [22]) cocatalyst greatly promoted Fe³⁺/Fe²⁺ conversion, which resulted in high H₂O₂ decomposition efficiency.

Metal-organic frameworks (MOFs) have attracted increasing attentions due to their diverse structures and versatile potential applications in separation [23], [24], catalysis [25], [26], photocatalysis [11], [27], and so on [28], [29], [30], [31], [32]. Fe-MOFs, their composites as well as derivatives exhibited huge potential in environmental remediation, especially for heterogenous Fenton-like reaction [33], [34], [35]. MIL-88A, a 3D Fe-MOF constructed by fumaric acid and Fe³⁺, is considered as an eco-friendly catalyst, which have received wide interest in photo-Fenton process [36]. For example, our group demonstrated that 100% bisphenol A (BPA) was degraded over MIL-88A within 80 min under the illuminance of visible light [37], whereas the complete BPA removal could be accomplished within 30 min over polyaniline@MIL-88A composite under white light [38]. Actually, the photoinduced charges transfer rate of MIL-88A is slow because of its low conductivity. In comparison, 2D MOFs possess higher aspect ratio, more exposed activated sites, faster mass transfer rate and quicker photoinduced charges transfer rate, leading to higher photocatalytic activities [39], [40], [41], [42]. At this point, 2D MOF nanosheets have been studied in the field of photocatalysis. Ding et al. reported in-situ growth of 2D Ni-MOF nanosheet on nickel foam (Ni-MOF/NF) without extra metal source, linked by 2-methylimidazole [43]. The as-prepared Ni-MOF/NF possessed abundant active sites and outstanding light absorption activity, which exhibited excellent photocatalytic volatile organic compounds (VOCs) degradation under visible light. Xiao et al prepared a 2D Mn-MOF (Mn-TBAPy, TBAPy = 1,3,6,8-tetrakis(p-benzoic acid)pyrene) nanosheet with average thickness of 9.5 nm by sonication exfoliation [44]. The H₂ evolution rate on the nanosheet was 11 times higher than that on bulk Mn-MOF, which resulted from short charge transfer distance and fast charge separation on Mn-MOF nanosheet. However, the water stability of 2D MOFs nanosheets should be further concerned [45]. Accordingly, it is desired to develop 2D micron-sized MOF sheets with high catalytic activity, good water stability and low biotoxicity for environmental remediation.

Herein, a micron-sized MOF sheet, namely BUC-21(Fe), was synthesized from Fe²⁺ nodes and two ligands (1,3‐dibenzyl‐2‐imidazolidone‐4,5‐dicarboxylic acid (H₂L) and 4,4′‐bipyridine (bpy)) with low toxicity (Table S1). Within this paper, chloroquine phosphate (CQ) was selected as the target emerging organic pollutant model to test the photo-Fenton performance of BUC-21(Fe). It was deemed that CQ, one of important drugs for treating malaria [46], might exert toxic effects to the non-target organisms, especially in higher pH conditions [47], [48]. Within this paper, the influences of H₂O₂ dosage, initial pH and co-existing inorganic anions on photo-Fenton degradation of CQ were investigated, and the role of the surface Fe ions in the reaction was studied. The oxidation state and content of Fe were determined by X-ray absorption fine structure (XAFS). The concentration of generated ·OH and the H₂O₂ consumption under different pH were monitored. Density functional theory (DFT) was introduced to clarify the band gap of BUC-21(Fe) and the transfer pathway of the charge carriers. As well, different organic contaminants including sulfamethoxazole (SMX) and bisphenol A (BPA) were selected to test the university of BUC-21(Fe) as photo-Fenton catalyst for organic pollutants degradation.