Catalytic degradation of tetracycline using peroxymonosulfate activated by cobalt and iron co-loaded pomelo peel biochar nanocomposite: Characterization, performance and reaction mechanism

https://doi.org/10.1016/j.seppur.2022.120533Get rights and content

Highlights

  • Co-Fe@PPBC catalyst can effectively activate peroxymonosulfate to remove TC.

  • Co-Fe@PPBC catalyst exhibited excellent stability, adaptability, and reusability.

  • Active sites for catalytic oxidation of TC were provided in Co-Fe@PPBC.

  • Synergistic effects between Fe and Co species facilitated the production of ROS.

  • Three possible degradation pathway of TC was proposed.

Abstract

Tetracycline (TC) is a refractory pollutant that widely exists in water environments. Therefore, effective TC treatment methods must be developed. In this work, a cobalt and iron coloaded pomelo peel biochar composite (Co-Fe@PPBC) was successfully synthesized and used to activate peroxymonosulfate (PMS) for TC removal. The crystal structure, surface morphology, pore structure, and elemental valence of the synthesized Co-Fe@PPBC catalyst were investigated by BET, SEM, XRD, FTIR, XPS, TGA, Raman, and VSM. The effects of Co-Fe@PPBC dosage, PMS concentration, initial pH, initial TC concentration, and coexisting anions on TC removal were studied. In comparison with the oxidation (13.6%) by PMS alone and adsorption (9.6%) by Co-Fe@PPBC alone, the removal efficiency of TC was increased to 86.2% in the Co-Fe@PPBC/PMS system, indicating that Co-Fe@PPBC can successfully activate PMS. Remarkably, the results of scavenging examination and EPR analysis demonstrated the synergistic effect of free radical and non-free radical pathways in TC degradation. The cycling test showed that Co-Fe@PPBC has favourable stability and recyclability during repeated activation processes of PMS. Finally, three possible TC degradation methods were presented through the analysis of eight intermediates. Overall, Co-Fe@PPBC could be an economic and high-efficiency catalyst for PMS activation to remove organic pollutants in wastewater.

Introduction

With the development of industrialization and urbanization, increasing attention has been given to the widespread use and abuse of antibiotics, which may pose a severe threat to aquatic environments, agriculture, human health, the livestock industry and so on [1], [2]. Among various antibiotics, tetracycline (TC) has been widely used in animal and human medicine worldwide, and is not biodegradable under natural conditions because it has a stable benzene ring structure [3], [4]. TC is often detected in wastewater and soil and poses a serious threat to human well-being and ecology. High level of TC has been detected in different environmental matrices: surface waters (0.07–1.34 μg/L), soils (86.2–198.7 μg/kg) and liquid manures (0.05–5.36 μg/kg) [5]. Therefore, it is imperative to eliminate these pollutants and minimize the environmental risks they pose. Conventional treatment technologies fail to treat TC effectively because of its chemical stability, low biodegradability, and antibacterial nature [5], [6], [7]. Towards the removal of TC in wastewaters and soils, many economical and effective methods have been developed, such as biodegradation, adsorption, membrane separation, electrolysis, and membrane filtration [8], [9], [10]. However, the purification efficiency of the abovementioned systems still does not meet people’s expectations. Compared to these approaches, advanced oxidation processes (AOPs), such as ozone (O3) oxidation, hydrogen peroxide (H2O2) oxidation, photocatalytic oxidation, and Fenton reagents have received extensive attention because of their environmental friendliness, high efficiency, and cost-effectiveness in the degradation of TC [11], [12], [13].

Recently, persulfate (PS)-based AOPs have been studied widely and thoroughly because of their high stability, long lifetime, low cost, and wide pH adaptability [14], [15], [16]. PS needs to be activated through various methods, including the use of transition metals, alkalinity, heat, carbon nanomaterials, ultrasound or UV light. In this process, hydroxyl radicals (∙OH), sulfate radicals (SO4), superoxide (O2–·), and singlet oxygen (1O2) are generated to oxidize contaminants and eventually mineralized into CO2, H2O, and nontoxic small molecules in a short period, as shown in Eqs. (1)-(5) [17]. Compared with other radicals, SO4 has attracted increasing attention due to its higher redox potential (E = 2.5–3.1 V), higher effectiveness in a wide pH range, and longer half-life (30–40 µs) [18], [19]. Among PS activation methods, transition metal ions and their oxides (such as Fe, Mn, Cu, and Co) have been broadly investigated for contaminant degradation because of their high efficiency and excellent reusability [20]. It has been reported that Fe2+ and Co2+ are excellent transition metal activators for activating PS to generate SO4. Furthermore, researchers found that bimetallic composite catalysts exhibited higher reactivity in activating PS than their corresponding monometallic oxide, which may be because the bimetallic oxide-based system enhances the effect of electron transfer [21], [22].S2O82- + heat/UV → SO4S2O82- +))) → 2SO42SO4 + H2O → H+ + 2SO42- + ∙OHS2O82- + e- → SO42- + SO4S2O82- + Mn+ → SO42- + SO4 + M(n+)+ (M = metal ions)

However, problems such as metal ion recovery difficulty and metal leakage still develop when only metal oxides are used. To overcome this disadvantage, numerous carbon-based materials have been used as support materials to evenly distribute metal oxides to improve their stability and reusability. These materials were also attractive candidates for PS activation, such as graphene, porous carbon, and activated carbon [17], [23]. Biochar (BC), a sustainable and environmentally friendly material obtained from various biomass pyrolysis processes under conditions of limited oxygen, high temperature, and anaerobic conditions, has PMS activation performance because of its stability, porous structure, and abundant surface functional groups (such as aromatic structures, carboxyl groups, and hydroxyl groups) [24]. Recently, various biochar materials and biochar composites have been extensively studied as PS activators to degrade various harmful organic pollutants. Li et al. modified wheat straw by Fe/N doping and obtained a strong catalytic performance on PS [25].

Pomelo is one of the characteristic fruits of China, with an annual production of approximately 3 million tons (pomelo peel, which accounts for nearly 45% of the total weight). In the current study, pomelo peel was used as the raw material for biochar preparation. On this basis, a cobalt and iron codoped pomelo peel biochar composite (Co-Fe@PPBC) was prepared by the impregnation-coprecipitation method and used as a peroxymonosulfate (PMS) activator for the removal of TC from wastewater. The physical and chemical characteristics of the synthesized Co-Fe@PPBC were investigated using a series of characterization techniques. The effects of key factors (Co-Fe@PPBC dosage, PMS concentration, TC concentration, pH values, coexisting anions) on TC removal were studied to determine the best combination of conditions for material application. Moreover, the catalytic mechanism in the Co-Fe@PPBC/PMS system was discussed. Finally, the possible degradation routes of TC were proposed based on the identification of possible intermediate products.

Section snippets

Chemicals

Iron nitrate nonahydrate (Fe(NO3)3·9H2O, 98.5%), cobalt nitrate hexahydrate (Co(NO3)2·6H2O, 98.5%), tetracycline (TC, 98%), potassium monopersulfate (KHSO5, 99%), furfuryl alcohol (FFA, 98%), ethanol (EtOH, 99%), p-benzoquinone (p-BQ, 99%), and tertbutyl alcohol (TBA, 99%) were provided by Aladdin Industrial Corporation. Hydrochloric acid (HCl), humic acid (HA, 90%), potassium nitrate (KNO3, 99%), sodium chloride (NaCl, 99.5%), sodium bicarbonate (NaHCO3, 99%), sodium carbonate (Na2CO3, 99.8%)

SEM

The microstructures of PPBC, Co-Fe@PPBC and used Co-Fe@PPBC were observed by SEM (Fig. 1). Co-Fe@PPBC has a completely different surface from PPBC. The raw PPBC (Fig. 1a) exhibited a plate-like morphology and possessed a relatively smooth external surface, which was consistent with the characteristics of the carbon structure. In contrast, the surface of Co-Fe@PPBC (Fig. 1b) exhibited a well-developed, rough and more porous structure. SEM images showed that many fine particles or powders covered

Conclusions

In summary, a Co-Fe@PPBC catalyst was successfully fabricated by the impregnation-coprecipitation method and used to activate PMS to remove TC. The main properties of the catalyst were analysed by using BET, SEM, XRD, FTIR, XPS, TGA, Raman and VSM techniques. It was demonstrated that Fe and Co are uniformly dispersed in PPBC and have a high surface area, excellent graphitization degree, optimized structural defects, and good magnetic and thermal stability. Co-Fe@PPBC exhibited excellent PMS

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.

Acknowledgements

This work was supported by the Heilongjiang Provincial Natural Science Foundation in China(LH2019D002).

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