In situ generation of H2O2 using MWCNT-Al/O2 system and possible application for glyphosate degradation
Graphical abstract
Introduction
Advanced oxidation processes (AOPs) that use hydroxyl radical (OH) with redox potential of 2.80 eV as principal active species for the degradation and mineralization of refractory pollutants are considered as promising methods of wastewater treatment (Wang and Xu, 2012; Wang and Bai, 2017). Hydrogen peroxide (H2O2) is used for OH generation in many AOPs, such as H2O2/UV, H2O2/Fe2+, H2O2/O3, etc. (Neyens and Baeyens, 2003; Wan and Wang, 2016; Tang and Wang, 2018). In these H2O2-based AOPs, the potential safety risks from the transportation, and storage process of commercially available and concentrated H2O2 have received increasing attention due to its chemical instability (Asghar et al., 2015; Zhang et al., 2018). Therefore, a technology that can generate H2O2 in situ would be highly desirable.
Among various alternative approaches for the in situ generation of H2O2, the reduction of O2 to H2O2 is the most attractive because O2 can be obtained conveniently from air. Many reducing agents have been used for the in situ generation of H2O2, such as hydrogen, hydrogen substituted organic compounds, photoelectron, cathode of electrochemical or microbial fuel cells, active metals, etc. (Asghar et al., 2015; Edwards et al., 2015; Fu et al., 2010; Annabi et al., 2016; Trovó et al., 2009; Luo et al., 2015; Yalfani et al., 2011; Liu et al., 2017). The direct reaction of hydrogen and oxygen to H2O2 is conceptually the most straightforward, however, there is potential problem of operational safety due to the wide explosive range of H2/O2 mixtures (Liu et al., 2018a). Hydrogen substituted organic compound such as hydrazine, formic acid and hydroxylamine has been suggested to overcome the shortcomings of direct method (Yalfani et al., 2011). However, incomplete conversion of organic compound may increase the toxicity of the treated wastewater and the decomposition of H2O2 over catalyst surface may decrease the process efficiency (Liu et al., 2018b). The photochemical reactions of semiconductors (ZnO, TiO2 etc) can produce electrons and holes. The electron as a reducing agent, helps to the production of H2O2 and the holes helps to the degradation of organic contaminant (Hu et al., 2018; Clarizia et al., 2017). However, it cannot be considered as an alternative for in situ H2O2 generation because of its intensive energy demand (Asghar et al., 2015). The electrochemical synthesis via a 2-electron oxygen reduction reaction (ORR) on suitable cathode was confirmed to be more effective in H2O2 generation rate and O2 utilization efficiency. The accumulation of H2O2 could reach 566 mg/L in 0.05 M Na2SO4 at a current density of 7.1 mA/cm2 and air flow rate of 0.5 L/min after 180 min (Yu et al., 2015). The in situ generation of H2O2 in microbial fuel cells can reduce the intense energy requirement by the production of electricity (Zhuang et al., 2010; Chen et al., 2014). However, a lot of inorganic electrolyte was required in electrochemical system and many nutrient substances must be provided for microbial fuel cells. Active metals such as zinc, aluminum and iron have been known as an attractive reducers for reducing oxygen into H2O2 because of its simple steps and no extra cost on material and energy (Keenan and Sedlak, 2008; Fan et al., 2015; Wen et al., 2014). However, the maximal concentration of H2O2 was only 180 μM in Al/O2 system at 250 min (Fan et al., 2015), which limited its application in the wastewater treatment.
The electrochemical corrosion theory implies that when the metals contact directly with other conducting material, with higher electrode potential, the galvanic-type corrosion cell will be formed, which will accelerate the corrosion rate of metal and most of the depolarizing agent reduction on the surface of metal will be transferred to cathode. Based on this theory, our group has successfully prepared zinc carbon nanotubes (Zn-CNTs) composite and used it to generate H2O2 in situ. The results showed that the maximum cumulative concentration of H2O2 in Zn-CNTs/O2 system was 293.51 mg/L under the initial pH 3.0, Zn-CNTs dosage 2 g/L and O2 flow rate 400 mL/min, which was 15 times of zinc alone (Gong et al., 2018). Aluminum has lower electrode potential and more abundant storage in the earth than zinc. The fresh aluminum compound produced by the corrosion of aluminum has strong flocculation properties and the ability of precipitation to remove phosphorus. Therefore, multi-walled carbon nanotubes-Al (MWCNT-Al) composite may have greater potential than Zn-CNTs composite for the in situ generation of H2O2 and for the removal of toxic contaminants from the wastewater.
Glyphosate (N‑(phosphonomethyl) glycine) is an organophosphorus compound, which has been extensively used as herbicide (Lan et al., 2016; Wang et al., 2016). Glyphosate has been classified as “probably carcinogenic in humans”, which can cause a variety of symptoms including eye and skin irritation, contact dermatitis, eczema, cardiac and respiratory diseases and allergic reactions (Hu et al., 2011).
In this paper, we synthetized MWCNT-Al composite for the first time by high-energy ball milling and sintering the mixture of aluminum and MWCNT directly. XRD, SEM, EDS and Raman were used to characterize the as-prepared MWCNT-Al composite. The catalytic performance of MWCNT-Al composite for the in situ generation of H2O2 under oxygen aeration was investigated. Meanwhile, the effects of operational factors, such as the initial pH in solution, operating temperature and MWCNT-Al composites dosage, on the in situ generation of H2O2 were also discussed. The in situ generated H2O2 was used to peroxone process (O3/H2O2 process) for the degradation of glyphosate in aqueous solution Glyphosate was selected as the model pollutant due to its high toxicity and extensive application (Koskinen et al., 2016; Ibáñez et al., 2005).
Section snippets
Materials
All chemicals and reagents in this study were of analytical reagent grade or better and commercially available. Polyethylene glycol (4000) and aluminum powder were provided by the National Medicines Corporation Ltd., China. Hydroxylated multiwalled carbon nanotube (MWCNT) was purchased from Chengdu Organic Chemicals Co. Ltd., Chinese Academy of Sciences. Deionized (DI) water was used in all experiments. Glyphosate [N‑(phosphonomethyl) glycine] (C3H8NO5P) was a sigma Aldrich product (≥99.0%
Effect of sintering temperature and ratio of aluminum to MWCNT
In the synthetic process of MWCNT-Al composite, the sintering treatment for the ball milled mixture of aluminum and MWCNT (denoted as milled MWCNT/Al mixture) could make the melted aluminum fully contact with MWCNT by decomposing the dispersant of polyethylene glycol (Gong et al., 2018). The sintering temperature might affect the microstructure of MWCNT-Al composite (Kwon et al., 2013) and its catalytic performance for the in situ generation of H2O2. As demonstrated in Fig. 1a, the
Conclusions
MWCNT-Al composite was synthesized and applied for in situ generation of H2O2 by the reaction between MWCNT-Al and O2. In the MWCNT-Al/O2 system, the in situ generation of a high concentration H2O2 was achieved through the formation of numerous corrosion cells, in which MWCNT was acted as cathode due to the existence of many defects in MWCNT. In the MWCNT-Al/O2 system, the accumulation concentration of H2O2 was 947.27 mg/L in 60 min under MWCNT-Al dosage of 8.0 g/L, initial pH of 9.0, O2 flow
Acknowledgement
The research was financially supported by the National Natural Science Foundation of China (No. 51878427) and the Key Laboratory of Special Waste Water Treatment, Sichuan Province Higher Education System (No. SWWT2016-1).
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