Degradation of tetracycline by activated peroxodisulfate using CuFe2O4-loaded biochar

https://doi.org/10.1016/j.molliq.2022.120622Get rights and content

Highlights

  • BC-supported CuFe2O4/PDS catalytic system for TC removal from water is established.

  • Synergistic effect of Cu and Fe continuously generates SO4•− and •OH to degrade TC.

  • Free radical and non-radical mechanisms are involved in catalytic system.

  • A PDS activation mechanism and TC degradation pathway are proposed.

Abstract

CuFe2O4 is often loaded onto biochar (BC) to prepare activated peroxodisulfate (PDS) catalysts for the removal of persistent organic pollutants from water. However, the activation mechanism remains unclear. Therefore, in this study, a BC-loaded CuFe2O4 (CuFe2O4@BC) system that activates PDS was constructed and used to degrade the antibiotic tetracycline (TC). The synthesized materials were fully characterized, and the optimal reaction conditions of the system were determined to be a CuFe2O4@BC dosage and PDS concentration of 0.1 mg/L and 10 mM, respectively, under acidic conditions, achieving a TC removal of 30 mg/L. Under these conditions, the removal efficiency reaches 96.0 %. The synergistic effect of CuFe2O4 and BC greatly improves catalytic performance and avoids the agglomeration of the metallic nanoparticles. The PDS-activation mechanism was studied through electron paramagnetic resonance and quenching experiments, revealing that TC is degraded under the combined effects of sulfate radicals (SO4•−), hydroxyl radicals (•OH), and singlet oxygen (1O2). Two plausible degradation pathways of TC are proposed. Overall, this study provides a deeper understanding of metal-modified biochar activated PDS and provides a new strategy for the degradation of TC in this catalytic system.

Introduction

The antibiotic tetracycline (TC) is widely employed in the treatment and prevention of human and animal bacterial infections owing to its convenience of use and low cost [1], [2]. Accordingly, it is often present in medical and pharmaceutical wastewater [3]. Furthermore, it is not readily absorbed or metabolized by humans and animals and is thus excreted in feces and/or urine [4]. When such effluent is used as fertilizer in fields or discharged into natural waterways, it can lead to the development of antibiotic resistance, posing a threat to aquatic ecosystems and ultimately human health [5], [6], [7]. TC in water and soil not easily degraded naturally and thus can persist for a long time [8]. Accordingly, methods for the treatment of TC-containing wastewater are urgently required.

In recent years, advanced oxidation processes (AOPs) have emerged as a promising technology for the degradation of organic pollutants in sewage and soil remediation [9]. AOPs mainly remove pollutants by activating H2O2 to generate hydroxyl radicals (•OH) with strong oxidation capacity, which then mineralize pollutants into small compounds with low toxicity, even completely transforming them into CO2 and H2O [10], [11]. The most common used technology is Fenton oxidation [12]. Liu et al. have studied Fenton technology, in which activated H2O2 produces •OH by using the synergistic effect of Cu-Fe, and this technology can also be used to adsorb TC [13]. Sulfate-radical-AOPs (SR-AOPs), which is based on SO4•− produced by activated persulfate, have received extensive research attention as a means of pollutant removal [14]. The more common activation methods include thermal activation [15], ultraviolet irradiation [16], and transition metal activation [17] etc. Compared with other free radicals, SO4•− has a higher redox potential (2.5–3.1 V) [18], longer half-life (30–40 μs) [3], and wider pH range [19]. Furthermore, it has higher selectivity for organic compounds with unsaturated bonds and aromatic components [20]. In addition, non-radical singlet oxygen (1O2) is considered to be a main active species in AOPs. Accordingly, SR-AOPs has become a focus for water-treatment research in recent years.

Biochar (BC) is often used as catalyst support by virtue of its large specific surface area, unique porous structure, and good adsorption performance [21]. Furthermore, the surface of BC is rich in oxygen-containing functional groups [22]. However, this would depend on pyrolysis temperature. With the increase of carbonization temperature, the number of oxygen-containing functional groups of biochar decreased, acid groups decreased and basic groups increased. Compared with other adsorption materials, the raw materials for BC are more readily available and cheaper [23]. For instance, waste raw materials such as straw [17], peanut shells [22], and municipal sludge [24] can be transformed into BC through high-temperature pyrolysis. Ngigi et al. employed BC to adsorb antibiotics from pig manure [25]. Sheng et al. took biogas residue from a sewage-treatment plant as a raw material and modified it with citric acid. They then studied the adsorption capacity and mechanism of the new material for TC in sewage [26]. Combined with the advanced oxidation technology mentioned above, BC is often used as a carrier to load metal ions that activate PS, and the most common metals used are Fe(III), Cu(II), Co(II), and Mn(II) [27]. For instance, Huang et al. degraded TC using Mn-doped magnetic BC (MMBC), and improved the catalytic performance of the constructed system by exploiting the surface defects of MMBC and the active sites provided by Fe and Mn oxide [28]. Among the metals mentioned above, Co is considered to be the most effective catalyst [29]. However, metal-ion leaching in the catalytic process and the toxicity of Co itself limit its application in water treatment and environmental remediation [30]. Spinel ferrite MFe2O4 (M = Fe2+, Mg2+, Co2+, Mn2+, Cu2+, etc.) has magnetic properties and good chemical stability, so it presents advantages in terms of catalyst recovery and reuse [31]. For instance, Ding et al. prepared CuFe2O4 magnetic nanoparticles (MNPs) as a multiphase catalyst to activate peroxymonosulfate (PMS) and degrade tetrabromobisphenol A. Furthermore, the catalytic efficiency of CuFe2O4 MNPs was shown to be much higher than those of the single metal or its oxides [30]. Nevertheless, CuFe2O4 MNPs are prone to agglomeration, and there are few studies on loading it onto BC to activate PS.

Accordingly, the purpose of the experiment lies in the following aspects: (i) Spinel ferrite CuFe2O4 was loaded onto peanut-shell BC by a sol–gel method to prepare a CuFe2O4@BC multiphase catalytic system. (ii) In addition, the removal of TC by activated PDS in the system and the influences of catalyst dose, pH, and inorganic anions on the TC degradation rate were also studied, allowing the optimal conditions for the adsorption and degradation of TC to be established. (iii) Finally, the activation mechanism and TC degradation pathway were studied by quenching experiments. Our work shows that CuFe2O4@BC + PDS as a novel heterogeneous catalyst system has considerable development potential for environmental remediation.

Section snippets

Materials

Tetracycline (C22H24N2O8) was purchased from Shanghai Macleans Biochemical Technology Co., Ltd. Copper sulfate pentahydrate (CuSO4·5H2O), ferric chloride hexahydrate (FeCl3·6H2O), citric acid (C6H8O7), potassium persulfate (K2S2O8), hydrochloric acid (HCl), sodium hydroxide (NaOH), methanol (MeOH), tert-butanol (TBA; C4H10O), and furfuryl alcohol (FFA; C5H6O2) were purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai). Sodium chloride (NaCl), sodium bicarbonate (NaHCO3), and potassium

Characterization of the catalysts

The SEM images in Fig. 1a-c show the surface characteristics and elemental compositions of the BC, CuFe2O4, and CuFe2O4@BC. The BC has a porous structure and a rough surface. The CuFe2O4 particles are agglomerated. However, in the composite material, CuFe2O4 is uniformly loaded onto the surface and in the pores of the BC, indicating the successful preparation of the CuFe2O4@BC catalyst. The EDS images shown in Fig. S1, show that C, O, S, Fe, Cu and other elements appear in the synthesized

Conclusions

In this study, a sol–gel method was used to successfully prepare CuFe2O4@BC, which was used to activate PDS and consequently remove TC from water. Under the optimal conditions, the removal rate of TC was 93.3 %, and it was found to be correlated with PDS concentration and CuFe2O4@BC dosage in the ranges 0.5–10 mM and 0.05–0.1 g/L, respectively. Increasing TC concentration has the opposite effect. In this system, SO4•−, •OH, and 1O2 jointly promote the degradation of TC. Furthermore, the valence

CRediT authorship contribution statement

Chenyue Zhang: Writing – review & editing. Zheng Wang: Conceptualization, Methodology, Supervision. Fulin Li: Validation. Jiahao Wang: Writing – original draft, Methodology. Nannan Xu: Data curation. Yannan Jia: Conceptualization, Methodology, Supervision. Shiwei Gao: Conceptualization. Tian Tian: Data curation. Wei Shen: Validation.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

The authors express their sincere gratitude to the National Key Research and Development Program of China (No.2018YFC0408000, 2018YFC0408004).

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