Activation of peroxymonosulfate with magnetic Fe3O4–MnO2 core–shell nanocomposites for 4-chlorophenol degradation
Graphical abstract
Introduction
Excess organic compounds in the aqueous solutions should be eliminated due to their hazards to the environment and humans [1]. However, degradation of refractory organic pollutants can be hardly achieved using conventional wastewater treatment technology due to the stable structure of organic compounds. Advanced oxidation is a promising pathway to mineralize the refractory organic pollutants through the activation of the oxidants and thus generating highly oxidizing radical species [2], [3].
Compared to hydroxyl radicals (OH, 1.8–2.7 V), sulfate radicals (SO4−) based advanced oxidation technologies have attracted great attentions due to its high redox potential (2.5–3.1 V) at neutral pH [4] and more selective for oxidation [5]. Homogeneous catalysis of peroxymonosulfate (PMS) with transition metal ion is an efficient route for production of sulfate radicals to destroy organic matters in aqueous solution [6], [7], [8], [9], [10], [11], [12] because every single catalytic entity can act as a single active site [13]. However, the application of homogeneous catalysis was limited because of the harm to the environment of most dissolved metal ion.
Heterogeneous catalysis with metal oxides are increasingly replacing homogeneous catalysis of PMS to present the accumulation of soluble metal ion in aqueous solution. As effective PMS activators, cobalt or cobalt oxide can reduce the cobalt ion in the water significantly. However, leaching of potentially carcinogenic Co2+/Co3+ [14] is still inevitable. On the other hands, previous studies found magnetic spinel ferrites CuFe2O4 had high catalytic activity to PMS [15], [16]. Nevertheless, the complicated preparation method and high calcination temperature may hamper its wide application.
As an environmental friendly material, MnO2 can also efficiently activate PMS for degradation of pollutants [17], [18], [19] and is a promising alternative to Co3O4 in Oxone activation. Unfortunately, MnO2 tend to form superfine particles in aqueous solution, leading to the solid–liquid separation difficult to achieve [20]. Fe3O4 and Fe3O4-based materials not only have unique and novel properties but also could be easily separated and collected from water by the employment of magnetic process for reusability [21], [22], [23]. Moreover, iron ion leached from Fe3O4 can react with Oxone to produce sulfate radicals. Thus, hybrid materials containing MnO2 and Fe3O4 could be an ideal catalyst with great potential for Oxone activation, despite no report has been published.
In this study, Fe3O4–MnO2 core–shell nanocomposites were synthesized through a facile one pot method and characterized. The obtained magnetic nanocomposites were applied for catalytic activation of PMS for degradation of 4-chlorophenol (4-CP). To the best of our knowledge, this is the first report using Fe3O4–MnO2 core–shell nanocomposites as a PMS activator for the degradation of 4-CP. And then, the radical generation mechanism were proposed according to the results of ATR-FTIR and XPS analysis.
Section snippets
Materials
All chemicals used in the experiment were analytical grade and without further purification. MnCl2·4H2O and Poly (vinylpyrrolidone) (PVP, K-30) were purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). Ferrus sulfate (FeSO4·7H2O), potassium permanganate (KMnO4), sodium hydroxide (NaOH), ethanol and tert-butyl alcohol (TBA) were obtained from Xilong Chemicals, Ltd. (Shantou, China). 4-Chlorophenol (4-CP) and Oxone (PMS, KHSO5·0.5KHSO4·0.5K2SO4) were purchased from Aladdin
Synthesis and characterization of Fe3O4–MnO2 nanocomposites
During the synthesis process, five samples were prepared with initial Fe/Mn molar ratios of 6:1(sample 1), 5:1(sample 2), 4:1(sample 3), 3.3:1(sample 4) and 2.7:1(sample 5). After the dark green suspension formed, KMnO4 solution was added into the mixture, and then the dark brown solids were generated. The main reaction can be expressed as Eqs. (S1) and (S2). According to the Eq. (S2), the stoichiometric molar ration of Fe/Mn is 4.5. When the Fe2+ was excessive (sample 1 and sample 2), the
Conclusions
In summary, magnetic Fe3O4–MnO2 nanocomposites with low cost and little hazard were synthesized through a facile one pot method, and then used as PMS activators for the degradation of 4-CP. Sample 3 with a Fe/Mn molar ratio of 4:1 exhibited the much higher catalytic activity compared with other Fe3O4–MnO2 nanocomposites with various Fe/Mn molar ratio and pure MnO2. The catalyst showed stability in element valence, crystallinity and catalytic activity during the successive repeated reactions.
Acknowledgments
We gratefully acknowledge the financial support by the Funds for Creative Research Groups of China (Grant 51121062), State Key Laboratory of Urban Water Resource and Environment (Harbin Institute of Technology) (Grant 2013TS06), Scholarship Award for Excellent Doctoral Student granted by Ministry of Education of China (2012) and Fundamental Research Funds for the Central Universities (Grant 01508043).
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