Three-dimensional electro-Fenton degradation of Rhodamine B with efficient Fe-Cu/kaolin particle electrodes: Electrodes optimization, kinetics, influencing factors and mechanism
Graphical abstract
Introduction
Huge amount of textile and dyeing wastewaters from industries are discharged annually, and the wastewaters can cause severe negative impacts on both aquatic ecosystems and human health, due to their various organic and inorganic pollutants with non-biodegradable, toxic and carcinogenic characteristics [1], [2], [3]. Therefore, effective treatment of textile and dyeing wastewaters is of great significance before their release into the environment.
So far, various methods have been employed to remove the pollutants and color from wastewater, including physical methods [4], chemical methods [5], biological processes [6], bioelectrochemical system-based methods [7], and advanced oxidation technologies (AOTs) [8], [9], [10]. Among these methods, the AOTs have been considered as the most efficient methods, and have been widely used to remove recalcitrant organics via generating highly powerful chemical oxidants such as hydroxyl radicals (OH) [8], [11], [12], [13]. As one of the most popular AOTs, Electro-Fenton (E-Fenton) technology has received considerable attention in recent years [14]. It can produce OH by continuous electrogeneration of H2O2 on the cathode fed with O2 and addition of Fe (II), as shown in Eqs. (1) and (2) [15], [16]. There are two major advantages for E-Fenton process: one is that in situ cathodic production of H2O2 can be achieved, which can avoid potential risks arisen from the transportation, storage, and handling of H2O2; and the other is that organic pollutants can be degraded very quickly and sludge production can be reduced, owing to the continuous electrochemical reduction of Fe3+ to Fe2+ at the cathode, as shown in Eq. (3) [15], [16].O2 + 2H+ + 2e− → H2O2 E0 = 0.67 VFe2+ + H2O2 → Fe3+ + OH + OH− K = 51 M−1 s−1Fe3+ + e− → Fe2+ E0 = 0.771 V
However, the aforementioned advantages of E-Fenton process could only be realized in acidic condition, which is harmful for the electrodes [17], and limits its widely application to various wastewaters with higher pH values. Also, due to the limited cathode surface area, the pollutants degradation efficiency could be low [18].
To overcome the drawbacks of conventional E-Fenton system, and to improve the wastewater treatment efficiency, many researchers have proposed to construct 3D E-Fenton (3D/EF) system by packing particles between the anode and cathode [16], [18], [19], [20]. When external voltage was applied to the system during electrochemical reaction, the particles can be easily polarized by external electric field to form charged microelectrodes, which can not only shorten the distance between reactants and electrodes and enhance mass transfer, but also enlarge electrode specific surface area and provide more active sites for pollutants adsorption and even catalytic reactions, resulting in higher removal efficiency [21], [22]. The pollutants degradation performance of the system is strongly conditioned by applied voltage, solution condition and electrodes (including the anode, cathode and particle electrode) properties [23], while electrodes play an essential role in the system [24]. For the anode, it has been reported that metal oxide (e.g. RuO2, IrO2), carbonaceous material (e.g. boron-doped diamond, BDD), as well as iron or stainless steel could be used as the anode in 3D E-Fenton system [24]. While the BDD electrode is an ideal anode for wastewater treatment because of its superior characteristics of high wide working potential range, long lifetimes, high stability and mechanical strength [25]. For the cathode, metals and carbon materials are most commonly used in 3D electrode cell [24]. It has been demonstrated that carbonaceous materials exhibit excellent performance for E-Fenton, giving 3–6 times higher degradation rates and 2–3 times higher mineralization yields than stainless steel [26], which is due to the high H2 evolution overpotential and low catalytic activity for H2O2 decomposition of carbon materials [27]. Recently, many studies focus on the fabrication of cost-effective particle electrodes with desired activity and stability [19]. Particle electrodes frequently used in 3D electrode electrochemical reactions are dominantly carbonaceous material (like granular activated carbon, GAT) [20], [24], [28] and metallic (including metal oxide) materials, such as foam nickel, steel slag derived material, and modified kaolin [16], [29], [30], [31]. Kaolin, an abundant raw material in the earth's upper crust, has been found to be effective in stabilizing metals and catalyzing oxidation reactions [31], thus could be a great option for manufacturing particle electrodes. While using the above mentioned particle electrodes and nonferrous anode, iron should be added to the system to trigger Fenton reaction, which makes the operational process more complicated. To simplify the process, researchers proposed to load catalysts on particle electrodes [24], [32]. The catalysts on the particle electrodes can undergo Fenton and Fenton-like reactions, which are the most important ones among the heterogeneous reaction in the 3D electrochemical system [24]. Heterogeneous Fenton catalyst can address the issue of iron sludge disposal, saving the treatment cost by eliminating additional separation steps [33]. Our previous result showed that the Pd-Fe/Ni foam could be efficient particle electrodes for dimetridazole degradation [34]. Yet, Pd is noble metal and the Ni foam is also expensive; besides, this required acidic condition, which limited the practical application. Hence, more cost-effective catalysts and supporting materials, which can exhibit great performance under a wider pH range, should be developed. Apart from Fe, other metals can also exhibit catalytic activity for Fenton-like reaction, such as copper (Cu), manganese (Mn) and cobalt (Co) [24], [35]. Interestingly, it was found that when using the Cu or modified Fe-carbon as the heterogeneous catalysts, good performance for organic wastewater treatment could be achieved under neutral or even alkaline pH conditions [35], [36]. The intrinsic lewis acidic property of Cu, Mn and Co can provide acidic micro-environment when using as a Fenton catalyst even at higher pH conditions [37]. Though some progress has been achieved for developing efficient, robust and cost-effective particle electrodes, more research should be carried out.
Considering the superior catalytic performance of Fe and the lewis acidic characteristic of Cu, using Cu to modified Fe could be a desired heterogeneous Fenton catalyst; and combining the obtained Fe-Cu catalyst with the easily available kaolin, favorable particle electrodes should be expected. Therefore, the objective of this study was to prepare efficient Fe-Cu/kaolin particle electrodes for the 3D/EF system and evaluate the particle electrodes performance on RhB removals. More specifically, the optimization of Fe-Cu/kaolin preparation, system operational parameters for effective RhB degradation would be conducted. Besides, comparison of different systems for RhB removal was performed, and RhB degradation mechanism was discussed.
Section snippets
Preparation of the Fe-Cu/kaolin particle electrodes
All chemicals, mainly including FeSO4·7H2O, Cu(NO3)2·7H2O, Na2SO4, NaOH, and RhB, were analytical grade and used as received. Kaolin was purchased from China Kaolin Clay Co., LTD. Deionized water was used in all experiments.
Fe-Cu/kaolin particle electrodes were prepared by a four-step procedure as follows: (1) Grinding--the purchased kaolin was grinded into powder with a ball mill and a mortar, then the powder was screened with a 100 mesh molecular sieve to obtain the fine powder, and then
Characterization of the as-prepared Fe-Cu/kaolin particles
The photograph of Fe-Cu/kaolin particle electrodes is shown in Fig. S2. The particle electrodes are black ball particles with diameters of 3–5 mm, which are suitable for 3D/EF system. The SEM images of the kaolin and Fe-Cu/kaolin are shown in Fig. 1a-c. Compared with bare kaolin, a layer of porous spherical particles (as can be seen from the inset) were coated on the surface of the Fe-Cu/kaolin, which enlarged the specific surface area (22.70 m2/g vs. 19.93 m2/g; Fe-Cu/kaolin vs. kaolin) of the
Conclusions
In summary, efficient Fe-Cu/kaolin particle electrodes were prepared and optimized for 3D/EF system to treat RhB-containing wastewater. The as-synthesized Fe-Cu/kaolin particle electrodes were characterized by XRD, SEM, EDS and BET. The effects of various factors on RhB removal in the 3D/EF system with Fe-Cu/kaolin particle electrodes were determined. The Fe-Cu/kaolin particle electrodes exhibited better performance than pristine kaolin particle electrodes and activated carbon particle
Acknowledgements
This work was financially supported by the National Science Foundation of China (Nos. 21367002, 51707021), Guangxi Natural Science Foundation (Nos. 2016GXNSFAA380212, 2017GXNSFBA198186), Ability Promotion Project of Education Department of Guangxi Province for the University Middle Age and Youth Teachers (No. 2017KY0031), Open Fund of Guangxi Key Laboratory of Clean Pulp & Papermaking and Pollution Control (KF201723). China Postdoctoral Science Foundation Grant (No. 2018M633295), Young Teachers
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Boge Zhang and Yanping Hou are co-first authors.