Mechanistic insights into the efficient activation of peracetic acid by pyrite for the tetracycline abatement✰
Graphical abstract
Introduction
Peracetic acid (CH3C(O)OOH, PAA), as a peroxyacid oxidant, has been widely used for disinfection and oxidation in food processing, aquaculture, and other industries (Liu et al., 2017). Compared with the traditional chlorine disinfectants, application of PAA exhibits great advantages with little formation of toxic byproducts, such as halogenated compounds, acetaldehyde and nitrosamines (Dell'Erba et al., 2007). Considering the high redox potential (1.8 eV) of PAA, some special organic pollutants can be removed by the PAA alone (Chen et al., 2022; Zhang et al., 2017). For example, β-lactams antibiotics can be oxidized by PAA with the apparent second-order rate constants (kapp) of 15.56∼44.38 M−1 s−1 (Zhang et al., 2017). This provides a broad prospect for the practical application of PAA in the water and wastewater treatment. However, the high selectivity of PAA greatly results in the ineffectiveness in the abatement of various organic pollutants.
Recently, activation of PAA to produce strong oxidizing radicals for the degradation of organic micropollutants has attracted extensive interest in the advanced oxidation processes (AOPs) (Kim et al., 2019; Wang et al., 2020b). Previous studies have confirmed that organic carbon radicals (CH3C(O)O• and CH3C(O)OO•) and hydroxyl radicals (•OH) can be produced by activation of PAA with various methods (e.g., heat (Wang et al., 2020a), UV (Chen et al., 2019), and transition metal ions (Kim et al., 2020; Kim et al., 2019)). Among these methods, transition metal activation is widely studied because of its simplicity and high efficiency. PAA activation with homogeneous transition metal ions including Fe2+, Co2+, Cu2+, Mn2+ has been previously reported (Kim et al., 2020; Kim et al., 2019; Luukkonen et al., 2015; Rothbart et al., 2012). Among these various activators, Fe2+ is a good candidate because of its environmental friendliness and good activation performance. For example, Kim et al. has demonstrated that the micropollutants can be degraded in the Fe2+/PAA process with the kapp in the range of 1.10 × 105 to 1.56 × 104 M−1·s−1 at pH 3.0∼8.1. However, the slow conversion from Fe3+ to Fe2+ with the kapp of 2.72 M−1·s−1 greatly limits the PAA activation efficiency (Kim et al., 2019). At the same time, the addition of high concentrations of Fe2+ can also quench free radicals, thereby reducing the effectiveness of pollutant removal (Xiao et al., 2020). Therefore, iron-based heterogeneous catalysts can be an alternative to address the above problems.
Heterogeneous iron-based catalysts can not only reduce the quenching effect on radicals and the potential environmental risk, but also facilitate the conversion of Fe(III) to Fe(II) via the reducing substances on their surfaces. For example, Zhang et al. studied the activation of PAA by nano zero-valent iron, in which the released Fe2+ from nano zero-valent iron mainly contributed to the activation process and the reductive Fe0 was favorable for the conversion from Fe(III) to Fe(II) (Zhang et al., 2022). Wang et al. also found that activation of PAA by nano zero-valent iron under UV irradiation and multiple reuse of nano zero-valent iron exerted no effect on the removal of pollutants, which was mainly because of the reduction of Fe(III) by UV and Fe0 on the surface of nano zero-valent iron (Wang et al., 2021b). However, one major challenge for the application of nano zero-valent iron in the PAA activation is that the passivation of nano zero-valent iron on the surface inhibits the transformation of Fe(III) to Fe(II), thereby greatly limiting the activation of PAA under neutral and alkaline conditions. Recently, the reductive sulfur species have been introduced into iron-based catalysts to enhance the conversion of Fe(III) to Fe(II). For instance, Yang et al. found that sulfur conversion product (H2S) due to the significant iron and sulfur leaching from FeS promoted Fe(II) regeneration (Yang et al., 2022b). Compared with FeS, pyrite, as the typical iron-rich sulfur-bearing mineral, is advantageous owing to its recyclability and stability in the AOPs (Zhou et al., 2018). Based on the above assumptions, pyrite can be expected to achieve efficient activation of PAA regarding the efficient activation of PAA by iron and the important role of reducing sulfur species for the reduction of Fe(III).
Tetracycline (TC) is widely used as an antibiotic and frequently detected in the wastewater treatment effluents (2.2 mg/L), groundwater (0.1 µg/L) and surface water (4.5 µg/L) (Xu et al., 2020). Due to its potential threat to the ecosystem and human health, TC is selected as a model pollutant in this study. The objectives of this work are: (i) to evaluate the feasibility of the pyrite/PAA process towards TC abatement and distinguish the homogeneous and heterogeneous contribution to TC abatement; (ii) to determine the reactive radicals contributing to TC abatement and validate their generation pathways; (iii) to elucidate the PAA activation mechanism with pyrite, especially the synergy between homogeneous and heterogeneous phase; (iv) to identify the degradation intermediates and degradation pathways for TC abatement and elucidate the toxicity of the degradation products; (v) to analyze the effect of various factors on TC abatement and evaluate the application potential of the pyrite/PAA process in the treatment of real waters.
Section snippets
Chemicals
The detailed information of chemicals was shown in Text S1.
Characterization of pyrite
Scanning electron microscope (SEM, Tescan Mira4, Czech) and Transmission electron microscopy (TEM, FEI Tecnai G2 F20, USA) were employed to display the morphologies of pyrite. X-ray photoelectron spectra (XPS, Thermo Scientific K-Alpha, USA) was used to analyze the chemical valence of pyrite before and after reaction. XPSPEAK software was utilized to conduct the XPS spectral analysis with all the binding energies calibrated with C1s
Feasibility of PAA activation with pyrite for TC abatement
Fig. 1(a) displays TC abatement as a function of time in different processes. There is a negligible TC adsorption by pyrite. TC abatement efficiency in the pyrite/H2O2 process and the pyrite/PAA process is 22.5% and 100%, respectively. In contrast, H2O2 or PAA oxidation leads to 10.0% and 25.4% of TC abatement, suggesting the inconsequential contribution of H2O2 or PAA oxidation to TC abatement in the pyrite/PAA process. In addition, the pseudo first-order rate constants (kobs) for the pyrite/H2
Conclusions
Activation of PAA with pyrite can achieve efficient TC abatement under acidic to alkaline conditions, in which CH3C(O)OO• play a major role. The activation of PAA mainly originates from the Fe2+ leached from pyrite and the >Fe(II) sites on pyrite surface. The electron donating sulfur species on pyrite surface can enhance the reduction from Fe(III) to Fe(II) in the homogeneous and heterogeneous phase, resulting in the efficient PAA activation and concomitant TC abatement. The complexation of Fe2+
Notes
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.
Declaration of Competing Interest
None.
Acknowledgements
This work was funded by the National Natural Science Foundation of China (Grant No. 51808233). The Shiyanjia Lab (www.shiyanjia.com) was also acknowledged for providing DFT calculation.
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Post address: Department of Environmental Science & Engineering, College of Chemical Engineering, Huaqiao University, Xiamen, Fujian, 361021, China