Effect of pyrolysis conditions on environmentally persistent free radicals (EPFRs) in biochar from co-pyrolysis of urea and cellulose
Graphical abstract
Introduction
Biochar, otherwise known as char or black char, is the carboniferous by-product of the pyrolysis of biomass at temperatures above 250 °C under oxygen limited conditions (Odinga et al., 2020). Biochar can be used for adsorbing organic and inorganic pollutants, soil fertilization (Yang et al., 2019), remediation of contaminated soil (Omondi et al., 2016), carbon sequestration (El-Naggar et al., 2018, El-Naggar et al., 2018), etc.
Nitrogen-rich biochar can be obtained from pyrolysis of biomass under nitrogen-rich conditions. Nitrogen can be divided into exogenous nitrogen and endogenous nitrogen (Wan et al., 2020). Some types of biomasses, such as algae, have high endogenous nitrogen contents (Yu et al., 2018). However, most agricultural and forestry biomass types have low nitrogen contents, so they require the introduction of exogenous nitrogen, such as via pyrolysis in an NH3 atmosphere (Chen et al., 2018) or co-pyrolysis with substances containing high nitrogen content to serve as nitrogen carriers (Zheng et al., 2020). The first method, i.e., pyrolysis in an NH3 atmosphere, can be dangerous, while co-pyrolysis is safe because it uses stable nitrogen carriers such as urea as stable nitrogen carrier. Therefore, this experiment selected the co-pyrolysis method using urea as the exogenous nitrogen source.
Nitrogen-rich biochar has a wide range of applications. Plaza et al. (2011) obtained nitrogen-rich biochar using a high-temperature NH3 modification method and found that the surface of the biochar contained a large number of basic nitrogen-containing groups which had a good CO2 adsorption ability, with an adsorption capacity as high as 0.82 mol/kg. Furthermore, Yin et al. obtained nitrogen-containing biochar by modifying aquatic waste and found that it had strong adsorption capacity for groundwater pollutants (2, 4-dichlorophenol (2,4-DCP) and Ni (II)) (Yin et al., 2019). Recently, Wan et al. (2021) found that N-doped biochar had a good phenol removal performance from waste water due to the effect of nitrogen vacancy-based carbocatalysis. Huang et al. (2021) used mantis shrimp shells to prepare nitrogen-doped biochar with a high specific capacitance and noted that this biochar has the potential to serve as an electrode material for supercapacitors.
However, recent studies have identified a new substance in biochar called environmentally persistent free radicals (EPFRs) (Ruan et al., 2019). Different from common free radicals, EPFRs have long lives (days to months) and can exist in various environmental media such as atmospheric particulate matter (Gehling and Dellinger, 2013; Gehling et al., 2014), activated carbon (Gong et al., 2016), petroleum coke (Kiruri et al., 2013), and biochar. EPFRs can play both positive and negative roles in the various biochar applications. For example, EPFRs can transfer electrons to molecular oxygen to form super oxide radical ions, which then induce reactions to produce ·OH in the presence of CuO/SiO2 particles in the process of removing organic pollutants (such as diethyl phthalate, 1,3-dichloroprope, p-Nitrophenol, etc.) from biochar (Fang et al., 2014; Fang et al., 2017; Qin et al., 2017). There have been reports that the generation of reactive oxygen species (ROS) induced by EPFRs inhibited the germination of crop seeds and the growth of bud (Liao et al., 2014). EPFRs can induce stress in the human body via oxidation, producing reactive oxygen species (ROS), which leads to the oxidation of molecules in tissues and damages DNA (Burn and Varner, 2015; Chuang et al., 2017). While biochar can be used to remove pollutants, it can also be a source of new pollutants. Therefore, it is very important to understand mechanisms regulating EPFRs in biochar.
The EPFRs in biochar depend, to a certain degree, on the pyrolysis conditions. Although some studies have examined the effects of biomass pyrolysis at different temperatures on EPFRs (Fang et al., 2013, Fang et al., 2014, Fang et al., 2015a, Fang et al., 2017, Qin et al., 2017, Yang et al., 2017), few have considered the effects of pyrolysis conditions on EPFRs in nitrogen-rich biochar, especially under rapid pyrolysis conditions. Therefore, this paper explores the influence of urea mass ratio, pyrolysis temperature, and residence time on EPFRs in nitrogen-rich biochar. We propose that high pyrolysis temperature and the introduction of N element will reduce the concentration of EPFRs in biochar, which is conducive to reducing the adverse effects of EPFRs in application of biochar in environmental field.
Section snippets
Chemicals
Urea (AR) was purchased from Tianjin Bodi Chemical Co., Ltd. 2,2,6,6,-Tetramethylpiperidine 1-oxyl free radical (TEMPO, 98%) and α-cellulose (AR) were obtained from Shanghai Aladdin Co., Ltd.
Biochar preparation
Biomass contains a variety of components, such as cellulose, hemicellulose, lignin, and alkali metals, and they all influence the pyrolysis process and products. In order to better identify the causes of EPFRs generation in nitrogen-rich biochar, the number of potential influencing factors was reduced by
Physical and chemical properties of nitrogen-rich biochar
Agroforestry biomass pyrolytic char has a rich pore structure and surface morphology. The pores are formed because volatile substances in the biomass escape from the interior (Sun et al., 2016). According to the SEM results (Fig. S1), C400, C500, and C600 had no obvious pore structures, but flocculent structures had formed on their surfaces due to the expansion of urea mixed with cellulose during the pyrolysis process. The higher the pyrolysis temperature was, the more obvious the expansion
Conclusion
In this study, the effects of various pyrolysis conditions on the biochar EPFRs of cellulose-urea mixtures were investigated. The concentrations of EPFRs in nitrogen-rich biochars prepared at 400, 450, 500, 550, and 600 °C increased at first and then decreased, peaking at 2.132 × 1018 spin·g−1 at C500. The formation of EPFRs was related to the substitution of aromatic compounds. When the pyrolysis temperature exceeded 500 °C, the concentrations of EPFRs decreased because the substituted
CRediT authorship contribution statement
Dongmei Bi: Funding acquisition. Fupeng Huang: Writing – original draft. Mei Jiang: Formal analysis. Zhisen He: Data curation. Xiaona Lin: Writing – review & editing.
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
The authors would like to thank Prof. Xueyuan Bai (School of Agricultural Engineering and Food Science, Shandong University of Technology, Zibo 255000, China.) for the guiding of the experiment. The authors would like to thank the seven anonymous reviewers for useful suggestions and comments. The authors would also like to thank Ackley L (Senior Editor University of Oregon Agricultural Engineering) for correcting the grammar of this pepper. This research was sponsored by National Natural
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