High efficient removal of bisphenol A in a peroxymonosulfate/iron functionalized biochar system: Mechanistic elucidation and quantification of the contributors
Graphical abstract
The new and low-cost iron embedded carbon composites (Fe-BC-700) were prepared by a facile pyrolysis technology. The as-prepared material exhibited an extremely high removal efficiency of biphenol A in the presence of peroxymonosulfate (PMS). Besides the sulfate radicals (SO4−) from the PMS activation, the inherent persistent free radicals (PFR) in biochar also played an important role in the degradation of BPA.
Introduction
Reactive radicals, such as hydroxyl radical (HO·), O2·−, OOH·, and sulfate radical (SO4·−) have been recently reported as an emerging technology for the degradation of organic pollutants [1], [2], [3]. Among them, SO4·− has received increasing attention as an alternative to conventional Fenton processes in recent years [4]. Compared with HO·, SO4·− possesses a more positive reduction potential (2.5–3.1 V vs. 1.9–2.7 V of HO·), a longer lifetime (t1/2 = 30–40 μs, vs. 10−3 μs of HO·), and a higher oxidation selectivity [5]. Thus, using SO4·− to degrade pollutants has gained increasing attention [6], [7], [8].
In general, SO4·− is generated by the activation of peroxymonosulfate (PMS). Various activation methods including heating [9], base treating [10], UV [11], and ultrasound [12] have been proposed. For instance, Takdastan et al. developed a sulfate radical-based advanced oxidation process by using ferroferric oxide supported on carbon as a catalyst, and combining with UV light and ultrasound irradiation to efficiently activate PMS for the degradation of BPA. The parameters affecting the degradation efficiency were detailedly studied [13]. Jorfi et al. prepared TiO2-functionalized magnetic activated carbon for the activation of PMS under UV light, which showed an excellent efficiency for benzotriazole degradation [14]. Compared with those energy-based activation methods, using heterogeneous catalysts to activate PMS is less complex in reactor/system configuration and more economical [4]. The commonly used heterogeneous catalysts for the PMS activation to produce sulfate radicals are transition metals (M), such as Fe, Co, Cu and Mn (Eqs. (1), (2).
Among these metals, zero valent iron (ZVI) -based catalysts have been widely used for PMS activation due to its eco-friendliness, nontoxicity, and low-cost (Text S1). However, the ZVI suffers from poor stability due to its high surface energy [15]. To avoid such drawbacks, porous and stable supports are usually used. For instance, Wang et al. [16] synthesized a carbon encapsulated nano iron hybrid catalyst by the hydrothermal method and self-reduction process. Zeng et al. [17] prepared an Fe/Fe3C@N-doped porous carbon hybrid by pyrolysis of an Fe, N-containing nano-scale metal–organic framework (MOF).
Biochar is a byproduct of pyrolysis of waste lignocellulosic biomass to produce renewable fuels, which can be used as a low-cost carbonaceous support for metal-based catalysts [18], [19], [20]. Abundant functional groups on the biochar surface can quickly adsorb organic pollutants [21]. The high adsorption performance of biochar facilitates the mass transfer during the catalytic reactions, and can accelerate chain reactions. Moreover, in our previous research, we found that the Fe species could catalyze the formation of nanofibers in the fast pyrolysis process [22]. The nanofibers on the surface of biochar could activate PMS through a nonradical mechanism [23]. The carbon nanotubes can also accelerate the electron transfer occurred in the PMS activation. In addition, the persistent free radicals (PFR), which has been previously reported to exist in biochar, may react with PMS to generate SO4·−and HO· to decompose pollutants [24], [25]. Thus, Fe functionalized biochar may be an excellent activator for PMS to enhance the degradation of organic pollutants. However, little is known about the removal mechanism of pollutants in the PMS/Fe/biochar system. In addition, in previous studies, adsorption and reaction are simultaneously observed [26], but the exact portions of different interactions during the removal of pollutants have not been addressed.
The main objectives of this study are to prepare an Fe functionalized biochar composite that contain Fe0, porous carbon with abundant functional groups and nanofibers, demonstrate the novel application of the as-prepared composite on PMS activation, elucidate the removal mechanism of bisphenol A (BPA), and quantify the contributions of different factors, such as Fe species and carbon structure, radical and nonradical pathway, and degradation and adsorption. In the present study, BPA, a widely used chemical in packaging, plastic, and epoxy resin, has been ubiquitously found in wastewater and natural water systems [27], [28], [29]. Considering its endocrine disrupting property and widespread application, BPA was selected as a model organic pollutant in this study.
Section snippets
Materials
All the chemical reagents used in the experiments were analytical grade and purchased from Sinopharm Chemical Reagent Co., Ltd., China. The biomass used in this study was sawdust, which was obtained from a local timber treatment plant in Hefei, China. The sawdust was crushed and screened through a sieve to collect the particles of size smaller than 0.12 mm.
Synthesis of the Fe-BC
The Fe-BC was prepared by fast pyrolysis of Fe-preloaded sawdust. In detail, 10 g of sawdust was added into a flask containing 200 mL of
Characterizations of Fe-BCs
Different Fe-BCs were prepared by fast pyrolysis of FeCl3 preloaded sawdust under different conditions. The crystal structure and the surface morphology of the materials related to the activation of PMS and the degradation of BPA were analyzed by XRD, SEM, Raman spectroscopy, and the BET method to reveal the effects of the pyrolytic temperature and retention time.
The XRD analysis was conducted to determine the crystal structure of Fe-BCs prepared at different pyrolytic temperature with the
Conclusion
The Fe-BC-700 was facilely prepared by fast pyrolysis of FeCl3 preloaded sawdust at 700 °C, and exhibited high removal efficiency of BPA in the presence of PMS. Under optimal conditions (0.2 g/L PMS and 0.15 g/L Fe-BC-700–1 h), 20 mg/L of BPA were completely removed in 5 min. The results revealed that the removal of BPA depends not only on the activation of PMS, but also involves adsorption, electron transfer, and nonradical compounds. Fe-BC-700 can be reused by simple thermal regeneration. The
Acknowledgements
The authors gratefully acknowledge financial support from National Natural Science Foundation of China (21677138, 21876166), and the Key Special Program on the S&T for the Pollution Control, and Treatment of Water Bodies (No.2017ZX07603-003).
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