Insight into a highly efficient electrolysis-ozone process for N,N-dimethylacetamide degradation: Quantitative analysis of the role of catalytic ozonation, fenton-like and peroxone reactions
Graphical abstract
Introduction
N,N-Dimethylacetamide (DMAC) is an important organic polar solvent widely used in the chemical industry, especially in the manufacture of synthetic leather, polymer dissolution and fiber industry, and also as a reagent in the production of medical intermediates in the pharmaceutical industry (Li et al., 2009; Ghazali and Inayat-Hussain, 2014; Jia et al., 2017). Due to the wide use of DMAC in industry, a large amount of industrial wastewater with different concentrations of DMAC is produced. In 2000, the production of DMAC worldwide was estimated to be from 50, 000 to 60, 000 tons per year (Ghazali and Inayat-Hussain, 2014). The ecotoxicological testing results indicate that DMAC is not acutely toxic to aquatic organisms but very toxic to earthworms (Oechtering et al., 2006; Ghazali and Inayat-Hussain, 2014). DMAC also has been listed as a chemical to cause reproductive toxicity (Ghazali and Inayat-Hussain, 2014). Potential human exposure to DMAC through inhalation of vapors or skin contact with the liquid substance in the workplace may cause skin irritation, headache, inappetence, fatigue, and hepatic damage (Ge et al., 2012; Ghazali and Inayat-Hussain, 2014; Chen et al., 2017). The current permissible exposure limit is 10 ppm assigned by the US Occupational Safety and Health Administration (Princivalle et al., 2010; Ghazali and Inayat-Hussain, 2014). In view of its wide presence, toxicity and recalcitrance, DMAC has an adverse effect on the environment and public health if the DMAC wastewater is directly discharged into the environment (Princivalle et al., 2010; Ge et al., 2012).
With the aggravation of environmental pollution, many countermeasures have been developed to remove DMAC contamination from the industrial effluents. The technologies mainly include biodegradation (Chen et al., 2017), sorptive microextraction (Li et al., 2009), photocatalytic oxidation (Ge et al., 2012), adsorption (Takatsuji and Yoshida, 1998) and internal micro-electrolysis (Liu et al., 2012). For instance, a 500 mL of 50 mg/L DMAC aqueous solution was degraded by internal micro-electrolysis. The degradation efficiency of DMAC was decreased with the elevation of initial solution pH from 3.0 to 8.0 (Liu et al., 2012). But conventional processes are not efficient for complete DMAC removal from industrial wastewater due to the highly resistant to biological and chemical degradation of DMAC. Nowadays, advanced oxidation processes (AOPs) have been developed as the effective alternative methods for the degradation of refractory pollutants. These AOPs are based on the in situ generation of hydroxyl radicals (•OH), which is a very powerful and nonselective oxidizing agent and can promote the degradation of a wide range of contaminants (Barrera-Díaz et al., 2012; Akbari et al., 2016). Thereinto, combining with electrochemistry and ozone (e.g., electro-peroxone or photoelectron-peroxone) is one of the efficient processes due to its simplicity, strong oxidation potential, environment-friendly and do not require addition reagent (Asaithambi et al., 2012; Li et al., 2014). The electro-peroxone process can drive the peroxone reaction (the reaction between O3 and H2O2) via sparged O3 and O2 mixed gas into a reactor that has a carbon polytetrafluorethylene (carbon-PTFE) cathode to in situ electro-generated H2O2 (Eq. (1)). The generated H2O2 then reacts with the O3 to produce •OH (Eq. (2)), which nonselective oxidize organic pollutants (Bakheet et al., 2013; Wang et al., 2015a).
However, the materials of electrode (e.g., Pt, boron-doped diamond or carbon-PTFE) in electro-peroxone systems are too expensive to limit the practical application. Therefore, electrolysis (EC) using iron electrode combined with O3 (electrolysis-ozone) was developed as a progressive process. When O3 is aerated in the EC system, Fe2+ acts as a catalyst for the ozone decomposition to generate •OH. Meanwhile, catalytic reaction of Fe2+ with O3 generates the intermediate FeO2+, a species that evolves to •OH (Eqs. (3), (4)) (Song et al., 2008; Akbari et al., 2016).
Although previous studies suggest that O3 assisted EC process was more effective than single EC and ozonation for wastewater treatment due to the synergistic effect of released Fe2+ and O3 (Song et al., 2008; Orescanin et al., 2011; García-García et al., 2014). However, these studies just only focused on the homogenous catalytic ozonation by released Fe2+. The possible formation of other reactive oxygen species (ROS) such as superoxide ion (O2•−) and/or H2O2 in O3 assisted EC process had not been reported. The generation of H2O2 can induce the reactions of Fenton-like and peroxone in the presence of Fe2+ and O3, respectively. Thus, explore the feasibility of H2O2 generation in O3 assisted EC system is urgent to preferably understand generating pathway of •OH. Besides, the relative contributions of catalytic ozonation, Fenton-like and peroxone reactions toward DMAC degradation in O3 assisted EC system has not been investigated yet to the best of our knowledge. Therefore, it is of great importance to quantitatively analyze the role of above involved reactions and the generating pathway of •OH in O3 assisted EC system.
Herein, O3 gas was aerated into EC system with iron plate electrode to construct a highly efficient electrolysis-ozone (ECO) system for DMAC treatment in aqueous solution. The main objectives of this study were to (i) investigate the effect of key operational parameters on removal efficiency of DMAC by ECO process, (ii) explore the feasibility of O2•− and H2O2 generation pathway in ECO system, (iii) quantitatively evaluate the contribution of catalytic ozonation, Fenton-like and peroxone reactions toward DMAC degradation by analyzing the generation of ROS (e.g., H2O2, •OH and/or O2•−), (iv) determine the degradation intermediates to propose a possible DMAC oxidation pathway in ECO system.
Section snippets
Materials
N,N-dimethylacetamide (DMAC) and its five degradation products (N-methyl-N-hydroxymethylacetamide, N-methylacetamide, N-hydroxymethylacetamide, acetic acid, acetamide), Na2SO4, H2SO4, HCl, NaOH, tert-butanol (TBA), and isopropanol (IPA) were purchased from Chengdu Kelong chemical reagent factory. Superoxide dismutase (SOD) and catalase (CAT) were purchased from Shanghai Kayon biological technology Co. All chemicals used in the experiment were of analytical grade and used as received without
Kinetics for DMAC degradation under different initial pH by different processes
The solution pH can significantly affect the degradation efficiency of pollutants in all chemical oxidation processes. Herein, three levels of pH (i.e., 3.0, 6.6 (without adjustment) and 10.0) were selected to evaluate the effect of initial pH on DMAC degradation by different processes. Fig. 1 shows the degradation curves of DMAC under different initial pH by EC, ozonation, and ECO processes. As shown in Fig. 1(a), it can be observed that less than 5% and 20% of DMAC was degraded after 60 min
Conclusion
In summary, DMAC can be effectively degraded by the ECO process under a variety of tested reaction conditions. The extraordinary efficiency was mainly caused by the more generation of •OH derived from multiple oxidation reactions including catalytic ozonation, Fenton-like and peroxone in ECO system. Comparing the ROS generated in ECO system, the H2O2 derived from O2•− could be involved in Fenton-like and peroxone reactions with the released Fe2+ and the aerated O3, respectively. Importantly,
Acknowledgement
The authors would like to acknowledge the financial support from National Natural Science Foundation of China (No. 21207094), and Fundamental Research Funds for the Central Universities (No. 2015SCU04A09).
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