Research PaperEfficient removal of PFOA with an In2O3/persulfate system under solar light via the combined process of surface radicals and photogenerated holes
Graphical Abstract
Introduction
Perfluorooctanoic acid (PFOA), a representative perfluorinated compound (PFC) with an extra-long half-life, is extremely recalcitrant in environment (0.09–0.92 mg/L in municipal wastewaters, 0.11–0.97 mg/L in natural rivers) (Wang et al., 2020b, Wang et al., 2020a; D’Agostino and Mabury, 2017; Padilla-Sanchez et al., 2017). It has been widely used in industrial and consumer applications, resulting in its frequent occurrence in groundwater (Loos et al., 2010), sediments (Bai et al., 2021) and human beings worldwide. Many studies have shown that the presence of PFOA is associated with birth defects, hepatocyte apoptosis, renal toxicity, metabolic syndrome and other diseases, and thus it poses a serious threat to human health (Li et al., 2018; Schroder and Meesters, 2005). Therefore, the development of efficient technologies for PFOA removal is imperative (Weon et al., 2021). Owing to the high energy of the C−F bond (631.5 kJ mol/L) and the strong electronegativity of the F atom, PFOA is very stable and difficult to degrade effectively, making it resistant to most conventional reduction and oxidation processes. In fact, it is even inert to hydroxyl radicals (•OH) because there are no C−H bonds available in PFOA molecule for H-atom abstraction (Wardman, 1989, Kwak et al., 2020).
In recent years, advanced oxidation processes (AOPs) involving the in-situ generation of strong oxidant species via diverse chemical or physical approaches (e.g. photochemical, electrochemical, sonochemical and thermochemical processes) have gained considerable attention for the degradation of organic matters (Huang et al., 2016, Pan et al., 2019, Liu et al., 2019, Roy et al., 2022). In particular, photochemical AOPs have been regarded as promising treatment techniques for the destructive removal of PFOA. Various photocatalysts have been developed for the degradation of PFOA under UV irradiation, including Fe−TiO2 (with 69% removal within 12 h of UV irradiation), a composite of TiO2 and reduced grapheme oxide (~ 93% of removal rate in 12 h) (Chen et al., 2015), and Pb−BiFeO3 (with a degradation efficiency of 69.6% in 8 h but a defluorination rate of only 37.6% in 8 h) (Shang et al., 2018). Recently, MXene-derived Ti3C2/TiO2photocatalyst were synthesized for the degradation of PFOA (providing a removal efficiency of almost 100% and defluorination efficiency of 49% after 16 h of UV irradiation) (Song et al., 2020). Our recent studies demonstrated that nitrogen dioxide radicals (•NO2) could mediate the efficient degradation of PFOA by UV photolysis of nitrate aqueous solution (UV/Nitrate) (Li et al., 2017) and further explored the comprehensive effect of the •NO2 and Fe3+/Fe2+ redox cycle for the rapid decomposition of PFOA (∼ 92% removal efficiency within only 0.5 h of UV irradiation) (Yuan et al., 2020b, Yuan et al., 2020a). These studies have laid a good foundation for the degradation of PFOA; however, they still face intrinsically difficult problems. For example, most studies have been carried out under UV irradiation (only accounting for 4% of solar energy) and have achieved unsatisfactory removal rates and defluorination rates. Kopinke et al. recently achieved the photochemical degradation of PFOA using Fe-zeolites under UV-A irradiation (300 nm < λ < 400 nM) instead of UV−C irradiation (Qian et al., 2020). Unfortunately, light with longer wavelengths has difficulty achieving the destructive removal of PFOA (Wang et al., 2020a, Wang et al., 2020b). Energy is central to the challenge of sustainability in all its dimensions: social, economic, and environmental. As one of the cleanest and most abundant energy sources, solar light energy has been regarded as a promising alternative to fossil fuels, especially for off-grid industrial applications. Therefore, it is important to explore inexpensive, safe, and green-based strategies for PFOA pollution removal using environmental−friendly solar light sources (Zhang et al., 2020b, Zhang et al., 2020a, Bi et al., 2020).
As a common semiconductor material, indium oxide (In2O3) has received increasing attention in recent years owing to its low toxicity, stable physicochemical properties, and high visible light utilization efficiency with a narrow bandgap (2.8 eV; conduction band (CB) located at ca. −0.62 V and valence band (VB) at ca. 2.18 Vvs. NHE) (Sun et al., 2020, Xu et al., 2017, Li et al., 2012). Several studies have reported the significant photoactivity of In2O3 for PFOA decomposition under UV-C irradiation (Lopes da Silva et al., 2017). It has been demonstrated that the terminal carboxylate group in the PFOA molecule can tightly coordinate to the surface of In2O3 via a bidentate or bridging configuration, which is beneficial for the direct decomposition of PFOA by the photogenerated holes of In2O3 (Li et al., 2012, Li et al., 2012). However, the photoactivity of In2O3 under solar light is almost ineffective for the destructive removal of PFOA. Meanwhile, alternative persulfate (PS, S2O82−) −AOPs have been shown to be more attractive than traditional •OH−based AOPs to abate a wide range of organic pollutants in water treatment because of the produced sulfate radicals (SO4•−) with a higher achievable radical formation yield, wider working pH range (pH ranging from 2 to 8), longer lifetime (30–40 μs), and higher selectivity. Generally, PS can be activated by photocatalysis, UV light, thermolysis, transition-metals, and sonolysis (Hori et al., 2005, Hori et al., 2008, Hori et al., 2012, Guo et al., 2021) to produce SO4•− via an energy / electron transfer reaction. Qian et al. reported that 85.6% of PFOA could be destroyed using UV/PS for 8 h (Hori et al., 2005). Notably, the negative conduction band potential of In2O3 (−0.62 V versus NHE) may induce PS activation to produce SO4•− via electron transfer ((1), (2)). By trapping the photogenerated conduction band electrons (Grilla et al., 2019, Zhao et al., 2021), PS may exert a dual−role: it can behave as a scavenger to restrain the photogenerated e−/h+ pair recombination, thus greatly enhancing the photoactivity of In2O3; It can also trigger PS−AOPs with the radical species generated in situ (e.g., SO4•− and·OH, via (3), (4)), and the photogenerated holes of In2O3 also synergistically contribute to PFOA removal. Therefore, it is highly likely that efficient PFOA degradation can be realized in an environmental−friendly and cost−effective manner by integrating In2O3 and PS (In2O3/PS) under natural solar light; however, this approach remains unexplored. To the best of our knowledge, there have been no reports on the application of the In2O3/PS system for the degradation of PFOA under natural solar light.In2O3 + hv → h+VB + e−CBS2O82− +e−CB → SO4•−+ SO42−SO4•−+OH−→SO42−+•OHSO4•−+H2O→ΗSO4−+•OH
Herein, we explored the simultaneous application of a heterogeneous In2O3 photocatalyst and homogeneous PS oxidation for PFOA degradation under solar light. The comprehensive effects of the photogenerated holes and the generated radicals, especially the surface radicals, were observed in detail for the destructive removal of PFOA. Response surface methodology (RSM) (Jiang et al., 2020, Zhang et al., 2020b) was applied to regulate the operation parameters for PFOA defluorination, including the concentration of In2O3, PS, and the pH value. Furthermore, the liberation of fluoride ions and generated intermediates were further investigated. Accordingly, a plausible decomposition pathway was proposed.
Section snippets
Chemicals and materials
All of the chemicals used are listed in the Supplementary Information.
Photochemical experiments
The photochemical experiments were carried out in an 80–mL quartz beaker with a xenon lamp (simulated sunlight, 320–780 nm, 320 W, PL−X300DF, Beijing, China), which was placed about 5 cm from the quartz beaker. In a typical photocatalytic experiment, 0.05 g/L of In2O3 dispersion containing 5 ppm PFOA solution (50 mL) was first stirred magnetically for 60 min in the dark to achieve the adsorption–desorption equilibrium between
PFOA defluorination performance in different systems
The photocatalytic degradation of PFOA was observed using sole In2O3, PS, and a combined system of In2O3/PS (Fig. 1a) under simulated solar light irradiation. It was found that the defluorination ratio of PFOA only reached ~ 6.0% with In2O3 or PS alone under solar light irradiation for 4 h. Although In2O3 possesses a certain solar-light absorption ability (ranging from 300 to 780 nm), as revealed by its UV–vis absorption spectra (Fig. S1a), the fast recombination rate of photogenerated e−/h+
Conclusion
This study demonstrates an attractive strategy for the efficient and complete mineralization of PFOA in a combined system of In2O3/PS under natural solar light irradiation. A high PFOA degradation efficiency (98.6%) and near-stoichiometric equivalents of fluorides release were achieved under solar light illumination for 10 h, confirming the outstanding capability of the system for the complete destructive removal of PFOA. The synergistic effect of the photogenerated holes and the in-situ
CRediT authorship contribution statement
Yijin Yuan: Investigation, Data curation, Conceptualization, Methodology, Writing – original draft. Lizhen Feng: Investigation, Software, Formal analysis. Xianqin He: Validation, Investigation. Xiufan Liu: Theoretical calculation. Ning Xie: Validation, Investigation. Zhihui Ai: Methodology, Writing – review & editing. Lizhi Zhang: Methodology, Writing – review & editing. Jingming Gong: Conceptualization, Methodology, Writing – review & editing, Project administration, Funding acquisition,
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
This work was supported by National Science Foundation of China (Grant No. 22076057, 21777052), National Key R&D Program of China (Grant 2018YFC1802003), the Project for Application Foundation Frontier for Wuhan (Grant 2019020701011486), and The Program of Introducing Talents of Discipline to Universities of China (111 program, B17019).
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