In situ photoreduction of structural Fe(III) in a metal–organic framework for peroxydisulfate activation and efficient removal of antibiotics in real wastewater

Highlights

• Photogenerated electrons on MIL-100(Fe) in situ reduced structural Fe(III) to Fe(II).

• Fe(II) improved PDS activation to generate ·OH and O₂·⁻ for antibiotics removal.

• Photo-assisted PDS activation removed antibiotics efficiently in the real wastewater.

Abstract

Structural Fe(III) is widely found in various coordination complexes and inorganic compounds. In this work, a typical Fe-based metal organic framework (MOF) (viz. MIL-100(Fe)) was chosen as an example in the activation of peroxydisulfate (PDS) for the removal of antibiotic pollutants. Interestingly, an auto-acceleration effect was observed in the process of MIL-100(Fe) activating PDS aided by visible light irradiation. Compared to the processes with MIL-100(Fe)-activated PDS alone and the photo-activated PDS alone, the degradation efficiency of sulfamethoxazole (SMX) obtained in the visible light assisted PDS activation by MIL-100(Fe) process was enhanced by 2.1 and 5.6 times, respectively. Therein, the photogenerated electrons from MIL-100(Fe) carried out an in situ reduction of the surface structural Fe(III) to form Fe(II), which in turn significantly improved the PDS activation efficiency in the generation of ·OH and O₂⁻· radicals for the removal of SMX. The degradation pathways of SMX were deduced based on the experimental results and theoretical calculations. Acute toxicity estimation indicated the formation of less toxic products after the treatment of SMX. Additionally, degradation of five antibiotics in the real wastewater were investigated to further confirm the advantages of such in situ photoreduced structural Fe(III) in MOFs to activate the PDS process.

Introduction

Antibiotics are one of the most important inventions in the history of medicine and are widely used for the treatment of bacterial infection diseases in humans and animals (Gaffney Vde et al., 2016). Market sales data show that China is the largest producer and user of antibiotics in the world (Zhu et al., 2013). However, it is reported that more than 30 % of the total amount of consumed antibiotics are either directly or indirectly disposed into the environment, so that China currently faces the most severe antibiotic contamination of water, groundwater and soil in the world (Zhang et al., 2015). Thus, there is an urgent need to develop highly efficient treatment technologies for the complete removal of antibiotics from the environment.

Due to the rapid generation of various reactive oxidation species (ROS) such as hydroxyl radicals (OH), sulfate radicals (SO₄⁻), superoxide radicals (O₂⁻), singlet oxygen (¹O₂) and other nonradical oxidation species, peroxydisulfate (PDS)-based advanced oxidation processes (AOPs) have been proven to be some of the most efficient technologies for the removal of various organic contaminants (Avetta et al., 2015; Chu et al., 2019; Kang et al., 2019; Ke et al., 2019; Zheng et al., 2019). However, inactivated PDS shows limited oxidation ability toward pollutants decontamination, which motivates the wide development of activation technologies for realizing the high oxidation potential of PDS (Wacławek et al., 2017).

Among these activation processes, the use of transition metal ion Fe²⁺is particularly favorable for PDS activation (Anipsitakis and Dionysiou, 2004; Wacławek et al., 2017). For example, Fe²⁺ instantly reacts with PDS to rapidly release SO₄^−• as described by Eq. (1), followed by the release of other ROS (Eqs. (2), (3), (4), (5), (6), (7)).

Fe²⁺ + S2O8²⁻ → Fe³⁺ + SO₄^∙− + SO₄²⁻

Fe²⁺ + SO₄^∙− → Fe³⁺ + SO₄²⁻

SO₄^∙− + H₂O → SO₄²⁻ + ∙OH + H⁺

Fe³⁺ + S₂O₈²⁻ → Fe²⁺ + S₂O₈^∙−

S₂O₈²⁻ + 2OH⁻ → 2SO₄²⁻ + HO₂⁻ + H⁺

S₂O₈²⁻ + HO₂⁻ → SO₄^∙− + SO₄²⁻ + O₂^−∙ + H⁺

Fe³⁺ + O₂^−∙ → Fe²⁺+ O₂

However, similar to the Fenton reaction systems, the Fe²⁺/PDS system has several intrinsic drawbacks. Due to the slow reduction of Fe³⁺ to Fe²⁺, the rate of Fe²⁺ regeneration is much lower than the rate of Fe²⁺ consumption, leading to the accumulation of Fe³⁺ (Li et al., 2019a; Zeng et al., 2019). The accumulation of Fe³⁺ prevents ROS generation and induces the formation of ferric oxide sludge that significantly restricts the wide application of the Fe²⁺/PDS reaction for the high cost of iron sludge disposal (Li et al., 2019b; Zeng et al., 2019). Thus, the initiation of PDS oxidation processes by heterogeneous reactions has been an attractive research direction because this approach avoids iron sludge formation due to wide working range of pH, excellent stability, and easy recovery properties of these reaction systems (Duan et al., 2015; Park et al., 2018). However, for most of the Fe-based coordination complexes or inorganic compounds, Fe(III) content is the main structural component with high stability (Tang and Wang, 2018). Unfortunately, Fe(III) is the less-effective component for the activation of Fenton-like reactions (Wang et al., 2019a). Therefore, it is highly necessary to explore a facile route to transform the Fe(III) in the structure to Fe(II) in order to realize a highly efficient yet stable Fenton-like catalyst.

In recent years, Fe-based metal organic frameworks (Fe-MOFs) have emerged as a class of multifunctional porous catalysts and have attracted intense attention (Ren et al., 2019; Tang and Wang, 2018; Wang and Li, 2016). These materials are commonly designed to have highly uniform nanopores with controllable size by coordinating metal ions with poly-functional organic ligands, endowing these materials with the characteristics of abundant nanopores, large specific surface area and thermal stability (Férey et al., 2007; Wang et al., 2015; Zhao et al., 2016; Zheng et al., 2017). Due to these intriguing characteristics, Fe-MOFs have been shown to be the efficient catalysts in heterogeneous Fenton-like reactions for antibiotics’ removal due to their strong adsorption and active metal sites (Tang and Wang, 2018). However, many previous studies have focused on exploiting a single particular advantageous property of the Fe-MOFs and a method for simultaneously exploiting all of their advantageous features still requires further development.

For example, Fe-MOFs are also good organic semiconductors that can exhibit high photo-catalytic efficiency in pollutant degradation (Laurier et al., 2013, Wang et al. 2015) and water splitting (Wen et al., 2019). When an Fe-MOF is excited by light irradiation, electrons and holes are generated in its conduction band (CB) and its valence band (VB), respectively. It is well-known that these photogenerated electrons have a strong reductive ability, and may in situ reduce the structural Fe(III) on the surface of Fe-MOFs to Fe(II). Unfortunately, few researchers have considered this phenomenon in the activation of Fenton-like reactions such as PDS activation. Herein, taking the representative MIL-100(Fe) as an example, heterogeneous PDS activation by MIL-100(Fe) under visible light (VL) irradiation was used to remove antibiotic pollutants. This PDS activation is expected to include the following reactions. (1) Photocatalytic process: PDS serves as the electron acceptor to prevent the recombination of charges, enhancing the photocatalytic process to produce a greater amount of reactive radicals. (2) Activation process: (i) MIL-100 (Fe) activates PDS to produce strong reactive oxidation species for pollutants removal; (ii) the photogenerated electrons are trapped by PDS for improving the efficiency of activation; and (iii) structural Fe(III) was in situ reduced to Fe(II), greatly accelerating the activation process. These synergistic promotion effects show great potential for future applications in wastewater treatment and water purification.

As a proof-of-concept application, sulfamethoxazole (SMX), a persistent pollutant and high-priority common pharmaceutical, was chosen as the target pollutant because of its strong anti-bacterial effect and pronounced ecological toxicity and potential carcinogenicity that posed a large threat to the environmental sustainability and water safety (Xu et al., 2019; Yin et al., 2018b). The synergistic effects of VL and MIL-100(Fe) on the activation of PDS were comprehensively investigated and explained. The synergistic mechanism of the photo-assisted PDS activation (PPA) process for SMX degradation was analyzed in detail. Moreover, the oxidative ability, recyclability and suitability of the PPA process were evaluated in real wastewater (pharmaceutical wastewater, hospital wastewater and rain water). Additionally, the degradation pathways of SMX in the PPA process were deduced and the acute toxicities of SMX and its products were predicted to evaluate the risks posed by the treated wastewater to the environment.