Regular ArticleHigh performance of the A-Mn2O3 nanocatalyst for persulfate activation: Degradation process of organic contaminants via singlet oxygen
Graphical abstract
A-Mn2O3 nanocatalyst possess efficient catalytic properties for PS activation and exhibits remarkable performance for the degradation of organic contaminants via singlet oxygen.
Introduction
Over the last few decades, the discharge of wastewater from industries and human activities has severely affected human health and the ecosystem [1], [2], [3]. For instance, phenol has been listed as a priority pollutant for degradation due to its toxicity and low biodegradability [4], and antibiotics being emitting in municipal wastewater is an emerging environmental issues all across the world [5], [6], [7]. SCP is a broad spectrum sulfonamide used as an antibacterial agent. It mainly comes from domestic and medical wastewater, fertilizer livestock manure and waste dump leachates and is considered to be a common pollutant in surface water and groundwater [5], [6]. Various conventional treatment techniques have been applied to treat wastewater; however, they have some limitations, such as a slow reaction rate, difficult-to-remove organic contaminants, and complexity [8], [9]. Thus, it is imperative to develop or apply more cost-effective water treatment technologies to remove these contaminants from polluted effluents. Among these, advanced oxidation processes (AOPs) are an efficient technique, since they can generate ROS to degrade the organic contaminants in water [9], [10], [11]. Recently, several studies have reported that 1O2 could be produced in nonphotocatalytic AOPs, which have also shown a high selectivity toward electron-rich compounds [12], [13]. Moreover, persulfate (PS) is more suitable for in situ chemical oxidation (ISCO) compared to hydrogen peroxide (H2O2), ozone (O3), and peroxymonosulfate (PMS). Because it is more chemically stable, more convenient to transport and store, cheaper, and has a longer half-life, it has a profound efficacy compared to others, making it widely applicable for contaminant removal [14], [15]. PS can be activated by heat, UV-light, sonication, bases, and transition metal ions, specifically low-valent ions, such as zerovalent iron, ferrous ion (II), and silver (I) [16], [17]. All of these activation methods require intensive energy consumption, a specific type of equipment, a significant cost, and chemicals. Some organic compounds (e.g., quinones and phenoxides) were reported to be effective for activating PS [15], [16], [18]. However, these organic compounds cannot be injected into polluted water systems due to their toxicity and for technical reasons. On the other hand, the efficient redox properties of metal-based catalysts are believed to be significant in persulfate activation. For instance, vanadium oxides have shown an efficient activity for PS activation when generating ROS through the redox centers of V(III) ↔ V(IV) [19]. Magnetite nanoparticles were reported to activate PS and generated O2• − radicals [17]. Therefore, various heterogeneous metal-based catalysts have been studied for persulfate activation [15], [17], [18], [19], [20]. However, the sluggish efficiency of the catalysts, their long activation time, their noxious effects toward the environment, and the high PS dosage are still significant limitations for their industrial application. Thus, alternate materials are highly demanded to be cost-effective, less toxic and have a more efficient PS activation. Manganese oxides are commonly present in soils, and have such advantages as a low cost, low biotoxicity toward the environment, and multivalence states, enabling them for various applications in catalysis [21], [22]. Current studies have confirmed that peroxymonosulfate (PMS) could be efficiently activated by Mn oxides [23], [24], [25]. In an early example, an Mn2O3 catalyst with different shapes was reported [24]. For instance, Mn2O3-cubic, Mn2O3-octahedra, and Mn2O3-truncated catalysts can activate PMS effectively to generate sulfate (SO4•−) radicals, whereas Mn2O3-cubic showed the highest activity among them. Nonetheless, the efficient catalytic properties on Mn2O3 are still indistinct. In this regard, improving the catalytic properties within the catalyst would be a favorable approach for enhancing the catalytic performance. The efficient performance of manganese oxides is fundamentally dependent on the surface morphology, oxidative states, weaker/longer Mn-O bonds between edge-sharing MnO6 octahedra and the mobility of oxygen atoms within the disordered lattice [4], [23], [25], [26], [27], [28]. Some recent studies have discovered that the high content of the –OH group on the surface of the catalyst plays a significant role in persulfate activation [29], [30]. Additionally, the presence of Mn species in a catalyst exhibited redox properties, which can improve the electron transfer between Mn species and the cleavage of the OO bond in persulfate [25], [30], [31]. The reports above mentioned that the PMS was efficiently activated by Mn-based heterogeneous catalyst. However, there are still occurrences of problems retarding further developments of high activity in Mn-based catalysts in the PS system. For example, (i) low efficiency and (ii) uncleared catalytic mechanism of Mn-based catalysts in activating PS. Thus, it is desired to achieve a high performance and stable heterogeneous catalyst without the addition of a cometal, thus an improved redox cycle of MIII ↔ MIV and catalytic activity for PS activation can be fully manifested. However, a few investigations have been mentioned on catalytic PS activation by Mn oxides catalysts.
In this study, synthesized Mn-based nanocatalysts, such as α-MnO2, β-MnO2, γ-MnO2, δ-MnO2, Ɛ-MnO2, A-Mn2O3, B-Mn2O3, C-Mn2O3, A-Mn3O4, B-Mn3O4, MnO oxides and other metals oxides, were systematically tested in a PS system for pollutant degradation. A-Mn2O3 was superior to the other PS activators, and the catalytic performance was studied in detail. The present research objective was to establish an efficient heterogeneous catalyst in a PS system. More importantly, based on the characterization results, the efficient catalytic properties of A-Mn2O3 were explored for PS activation and compared to B-Mn2O3 and C-Mn2O3. The plausible mechanism of PS activation via A-Mn2O3 was studied. Additionally, the effects of some potential influential factors on pollutant removal efficiency were systematically investigated. This study gives an insight into the further development of Mn-based catalysts for AOPs.
Section snippets
Chemicals
Chemicals for the synthetic procedures are described in Text S1 of electronic supporting information (ESI).
Material synthesis
Synthesis of A-Mn2O3 [26]. MnCO3 (1 g) was added into 40 mL of a dilute HCl solution (0.1 M) and stirred at room temperature for 1 h. Afterward, the solution was transferred into 40 mL, a Teflon-lined stainless-steel autoclave, and kept by a thermal treatment in an oven at 150 °C for 10 h, then allowed to cool to room temperature. The resulting product was filtered, repeatedly washed with
Characterization of catalysts
The XRD patterns of the different samples are shown in Figs. 1, S1 and S2. As shown in Fig. 1A, it was observed that A-Mn2O3, B-Mn2O3, and C-Mn2O3 have the same diffraction characteristics as the cubic phase of the Mn2O3 structure in space group la-3 (JCPDS Card No. 41-1442) with peaks at the 2θ angles 23.1◦, 33.0◦, 38.2◦, 45.2◦, 49.3◦, 55.2◦ and 65.8◦, which can be indexed as the (2 1 1), (2 2 2), (4 0 0), (3 3 2), (4 3 1), (4 4 0) and (6 2 2) planes [4], [27]. It demonstrated obvious peaks, suggesting the
Conclusions
Catalytic degradation activities of various transition metal oxides were studied in a PS system, and a promising and highly effective catalytic system was established from Mn oxides. A-Mn2O3 showed the most remarkable efficiency for pollutants removal compared to the other PS activators. Additionally, A-Mn2O3 showed a better performance than B-Mn2O3 and C-Mn2O3, attributed to its unique structure, the longer bonds between edge-sharing MnO6 octahedra, and a high content of surface –OH groups and
CRediT authorship contribution statement
Aimal Khan: Conceptualization, Investigation, Formal analysis, Validation, Methodology, Writing - original draft. Kaikai Zhang: Software, Validation, Resources. Peng Sun: Software, Resources. Honghui Pan: Software, Formal analysis. Yong Cheng: Resources. Yanrong Zhang: Supervision, Project administration, Conceptualization, Resources, Writing - review & editing.
Declaration of Competing Interest
The authors declare no conflict of interest.
Acknowledgments
The present work was funded by the International Science & Technology Cooperation Program of China (Nos. 2013DFG50150 and 2016YFE0126300) and the Innovative and Interdisciplinary Team at HUST (2015ZDTD027). The authors also show gratitude to the Analytical and Testing Center of Huazhong University of Science and Technology for help in FTIR, Raman, XRD, HRTEM, and XPS analysis.
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