Environmental Technology
ISSN: 0959-3330 (Print) 1479-487X (Online) Journal homepage: http://www.tandfonline.com/loi/tent20
Three-dimensional nickel foam electrode for efficient electro-Fenton in a novel reactor Yingshi Zhu, Shan Qiu, Fengxia Deng, Fang Ma, Guojun Li & Yanshi Zheng To cite this article: Yingshi Zhu, Shan Qiu, Fengxia Deng, Fang Ma, Guojun Li & Yanshi Zheng (2018): Three-dimensional nickel foam electrode for efficient electro-Fenton in a novel reactor, Environmental Technology, DOI: 10.1080/09593330.2018.1509890 To link to this article: https://doi.org/10.1080/09593330.2018.1509890
View supplementary material
Accepted author version posted online: 30 Aug 2018.
Submit your article to this journal
View Crossmark data
Full Terms & Conditions of access and use can be found at http://www.tandfonline.com/action/journalInformation?journalCode=tent20
Publisher: Taylor & Francis & Informa UK Limited, trading as Taylor & Francis Group Journal: Environmental Technology DOI: 10.1080/09593330.2018.1509890
Three-dimensional nickel foam electrode for efficient electro-Fenton in a novel reactor Yingshi Zhu1, Shan Qiu1,*, Fengxia Deng1, Fang Ma1,2, Guojun Li1, Yanshi Zheng1 1. School of Environment, Harbin Institute of Technology, Harbin 150090, China 2. Skate Key Laboratory of Urban Water Resource and Environment, Harbin 150090, China
Abstract: One of the bottlenecks often encountered in electro-Fenton technology is its low ability to produce hydrogen peroxide (H2O2). Thus, the hunt of suitable electrodes and reactor are a must to be tackled in order to improve the efficiency of the system. In this study, three-dimensional nickel foam was selected as cathode for in situ generating H2O2 efficiently and graphite was the control group in an enhanced oxygen mass transfer reactor. The micro-structure and electrochemical performance of electrodes were tested by Scanning Electron Microscopy (SEM), X-ray diffraction (XRD), Cyclic Voltammetry (CV), Electro-chemical Impedance Spectroscopy (EIS) and Tafel polarization techniques, respectively. The concentration of H2O2 produced by nickel foam cathode was 780.63 μmol/L and the removal efficiency of rhodamine B (RhB) was reached to 92.5% in 60 min. SEM and Tafel results showed that both nickel foam and graphite electrodes were porous structure cathodes. Moreover, CV and EIS experimental results indicated nickel foam electrode was controlled by charge transfer process while had a better transfer than graphite electrode. Electron spin resonance (ESR) spectra results demonstrated that the main oxidant species involved was ·OH, accounting for RhB degradation in electro-Fenton progress. Therefore, in terms of pollutant degradation in the electro-Fenton process, nickel foam electrode together with novel reactor were a promising technique.
Keywords Nickel foam; Hydrogen peroxide; Electro-chemical performance; Electro-Fenton; Electron spin resonance
* Corresponding author. E-mail:
[email protected]
1. Introduction Rhodamine B (RhB), being as an azo dye, mainly comes from textile and dyeing industries [1, 2]. It is harmful to human beings and animals, causing irritation to the skin, eyes and respiratory tract, long-term exposing to RhB even mutagenic and carcinogenic [3]. Some have been developed to degrade RhB wastewater due to its inhibition of the growth of the biota and harmfulness to the health of the human beings [4], including adsorption [5], biological degradation processes [6] and advanced oxidation processes [7] (AOPs), etc. Electro-Fenton technique is one of the environmental advanced oxidation processes used in recalcitrant organic contaminants removal from industrial waste water [8-12]. H2O2 is in situ electrochemically produced through the cathodic reduction of dissolved molecular oxygen in the electro-Fenton system (Eq. 1). Then the electro-generated H2O2 and Fe2+ undergoes the classical Fenton’ s reaction (Eq. 2) to produce hydroxyl radical, which unselectively oxidize recalcitrant pollutants with the standard potential of 2.8 V [13-15]. O2 +2H+ +2e- →H2 O2 Fe2+ +H2 O2 →Fe3+ +∙OH+OH-
(1) (2)
H2O2 generation, determined by the characteristics of cathodes, plays a substantial role in electro-Fenton since it influences the production of hydroxyl radical responsible for pollutant degradation. For RhB degradation, several kinds of electrode materials were used as electro-Fenton cathode, such as
Fe@Fe2O3/ACF, graphite felt and graphene with removal efficiency were 46.9%, 30 ± 1.4% and 100%, respectively [16-18]. Considering wastewater treatment by electro-Fenton, as a carbonaceous material, graphite cathodes has been commonly studied, while using nickel foam as a cathode can be related to these reference [19-23]. In recent years, Bocos et al. [24] demonstrated nickel foam cathode was able to remove Poly R-478 in the electro-Fenton process with a better performance in contrast with other carbonaceous materials, like carbon fiber and graphite sheet. Song et al. [25] studied H2O2 accumulation at nickel foam cathode was 58.8 mg/L, which was 5 times higher than that of graphite-based gas diffusion electrode (11.3 mg/L), suggesting the superior advantage of the three-dimensional porous structure of nickel foam. Liu et al. [26] confirmed that -
oxygen can only accept one electron and generate ∙O2 on the surface of nickel foam and then produce more H2O2 in the system to form more ·OH for RhB degradation. However, in terms of electro-generating H2O2 and pollutant degradation, deep research in electrochemical performance of nickel foam was few. The purpose of this work was to establish an efficient electro-Fenton system. A new type reactor for high efficiency mass transfer was designed. Several electrochemical tests, including SEM, XRD, CV, EIS and Tafel polarization were used to understand the interface reaction between solution and electrodes in the electro-Fenton system. RhB was selected as a target pollutant and electron spin resonance was carried out to figure out the radicals formed in the system.
2. Materials and methods 1.1. Materials Sodium sulfate (Na2SO4), potassium permanganate (KMnO4), hydrogen peroxide (H2O2, 30%) sodium oxalate (Na2C2O4, ≥99.8%) iron sulfate heptahydrate (FeSO4·7H2O) and RhB (C28H31CIN2O3) were obtained from Tianjin Kemiou Chemical Reagent Co., Ltd. Potassium phthalate monobasic (C8H5KO4, ≥99.8%), potassium iodide (KI) and ammonium molybdate ((NH4)6Mo7O24·4H2O) were purchased from Sinopharm
Chemical Reagent Co., Ltd. All chemicals were of analytical grade. 5,5-dimethyl-1-pyrroline-N-oxide (DMPO) was acquired by Sigma-Aldrich (China). Nickel foam was purchased from Suzhou Longde Foam Material CO., Ltd. Its thickness, aperture and surface density were 1 mm, 110 ± 10 PPI and 350 ± 20 g/m2, respectively. High purity graphite (≥ 99.95%) was provided by Harbin Institute of Technology. Millipore-Q water was used throughout the investigation. Unless stated otherwise, all the experiments were carried out at room temperature (25°C) and 1.0 atm pressure.
2.2. Analysis methods The microstructure was carried by SEM (EVO18, Carl ZEISS, Germany). The electrochemical performance was measured with a three-electrode system using an electrochemical workstation (CHI 660E Shanghai Chenhua Instrument Co., Ltd., China) with nickel foam or graphite as working electrode, saturated calomel electrode (SCE) as reference electrode and Pt sheet as reference electrode. The XRD measurements were collected on a D8 ADVANCE X-ray diffractometer with Cu Kα radiation (40 kV, 40 mA over the 2θ range, 10−90°C). The three-dimensional spectra were measured by the luminescence spectrometry (F-2700 spectrophotometer, Hitachi, Japan). Total organic carbon (TOC) was determined with a Jena TOC-Vcsh instrument (Germany) at 680 °C. CV measurement scan rate was from 5 mV/s to 50 mV/s and the third cycle was recorded. EIS was carried out from 1 × 105 to 1 × 10−1 Hz at the over potential with an amplitude of 5 mV. Tafel polarization experiments were performed with a scan rate of 0.5 mV/s. The concentration of H2O2 was measured spectrophotometrically using the iodide method (Lowest test limit of ≈ 10 -6 mol/L) at wavelength of λ=352 nm [27]. The formula for the current efficiency (CE) of H 2O2 generation is as follows [28]: t
CE= nFcH2 O2 V⁄∫0 Idt ×100
(3)
Where cH2O2 is the concentration of H2O2 (mol/L), F is the Faraday constant (96,486 C/mol), n is the number of electrons transferred for dissolved oxygen to produce H2O2, V is the volume of electrolyte (L), I is the applied current (A), and t is electrolysis time (s). ESR spectra of DMPO-·OH was obtained by using a Brucker
A200 S-9.5/12 spectrometer (Germany). The settings were as follows: microwave frequency = 9.72 GHz, microwave power = 19.57 mW, center field = 3470 G, sweep width = 100 G, sweep time = 20 s, modulation frequency = 100 kHz.
2.3. Experimental procedures The experiment was conducted in an undivided reactor (500 mL) with 0.05 mol/L Na2SO4 solution as supporting electrode, the bottom of the reactor is a sand core filter plate with a pore of 4.5-9 μm and the aeration pump is aerated from the bottom of the reactor as shown in Fig. 1. The carbon rod was worked as anode (2 cm diameter × 10.3 cm) and nickel foam (30 cm × 10 cm × 1 mm) or graphite ring (9.5 cm diameter × 10 cm × 3 mm) was worked as cathode. The electrolysis experiment was controlled by a three-electrode system with a Pt sheet (1 cm × 2 cm) as a counter electrode, nickel foam or graphite electrode (1 cm × 2 cm) as working electrodes and the SCE served as reference electrode in a 50 mL beaker. Distance between the cathode and anode was kept at 2 cm. Initial pH was adjusted to 3 with 0.5 mol/L H2SO4 and 0.5 mol/L NaOH prior to electrolysis, which was reported that the optimum pH in Fenton reaction is around 3 [29, 30], while current density (1 mA/cm2) was chosen according to previous report [31]. At predetermined time intervals, 1mL sample was taken to measure the concentration of H2O2. The RhB degradation was carried out in the novel system with adding of additional 0.1 mmol/L Fe2+ and 20 mg/L RhB.
3. Results and discussion 3.1. H2O2 generation The generation of H2O2 at nickel electrodes and graphite cathode was compared in Fig.2 to figure out the ability of H2O2 production. As shown in Fig.2a nickel foam cathode (780.63 μmol/L) produced more H2O2 than that of graphite cathode (540.37 μmol/L) and nickel plate (129.98 μmol/L) in the first 60 min electrolysis.
The H2O2 production rate remarkably changed during the first 30 min, and then the H2O2 concentration became steady, indicating that high concentration of H2O2 may cause a self-decomposition reaction (Eq. 4) or might occur oxidation of H2O2 at the anode (Eq. 5) [32, 33]. 2H2 O2 →2H2 O+O2
(4)
H2 O2 →O2 +2H+ +2e-
(5)
Efficiency of electrochemical processes on H2O2 production was evaluated by CE (Fig. 2b). It can be seen that CE decreased significantly from 32.2% to 7.0% during the 60 min with nickel foam as cathode in the electrochemical system. CE varied slightly with graphite cathode and nickel plate as well as the value of CE was lower than that of nickel foam cathode, demonstrating that nickel foam was more suitable as a cathode for producing H2O2.The higher CE could be explained by that nickel foam itself can provide more active sites by high BET surface area of 3 m2/g, which have been studied in our group [34]. What’s more, nickel foam can also promote the formation of H2O2, following the one-electron reduction reactions of Eq. 6 and 7 [26, 35], aside from the conventional oxygen two-electron reduction reaction. -
Ni+2O2 →Ni2+ +2∙O2 -
∙O2 +2H+ +e- →H2 O2
(6) (7)
3.2. Characterization of the electrode The morphologies of nickel foam and graphite were studied by SEM and the typical results are shown in Fig. 3. It shows that the nickel foam has interconnected macroporous open structure with the pore size of approximately 200 μm and smooth tetrapod-like melamine polymer skeletons with the thickness of about 25-35 µm (Fig. 3a and Fig. 3b), while Fig. 3c and Fig. 3d shows that the surface of the graphite is rough with porous and sheet-like structure. Both the interconnected macroporous open structure of nickel foam and sheet-like structure graphite suggested the good transfer of oxygen, favouring the oxygen reduction at cathode surface.
The crystal structure of electrode materials was characterized by XRD technique and the results are shown in Fig. 4. It shows from Fig. 4 that the (1 1 1), (2 0 0) and (2 2 0) reflections at 44.5°, 51.8° and 76.3° are assigned to cubic Nickel (JCPDS No.04-0850) with Fm-3m group and the only one (002) peak of graphite at 2θ = 26.5° is assigned to Hexagonal graphite (JCPDS No.08-0415) with P63/mmc group [36, 37]. The results demonstrate that both nickel foam and graphite are pure species.
3.3. CV test CV curves at the nickel foam electrode in 0.05 mol/L Na2SO4 (pH = 3) with different scanning rates are shown in Fig. 5. As shown in Fig.5, the redox peak current increases and the reduction peak potential shifts to more negative potential (Fig. 5a and c) as the increase of scan rates. It can be seen from Fig. 5a, the CV curves of the nickel foam have a weak reduction peak which corresponds to the production of H 2O and an obvious reduction peak which corresponds to the production of H2O2. Based on Fig. 5c, the CV curves of the graphite only have a reduction peak (-0.5 V) corresponds to oxygen reduction. The linear relationship between the reduction peak current (Ip) versus the scan rate (ν) in the range of 5 – 50 mV/s is shown in Fig. 5b. The corresponding Equation is expressed as Ip=0.0006+0.1408v (R2=0.9988). It suggests that it is an adsorption-controlled process when nickel foam as cathode [38]. In addition, the reduction peak current versus the square root of the scan rate (ν1/2) in the range of 5 – 50 mV/s exhibits a linear relationship (Fig. 5d), suggesting a diffusion-controlled process using graphite as cathode [39]. The linear regression Equation is Ip=0.0009+0.0483v1/2 (R2=0.9951).
3.4. EIS test Nyquist plots show the frequency response of nickel foam or graphite electrode/electrolyte system and are a plot of the negative value of the imaginary component (-Z") of the impedance against the real component (Z'). The Nyquist plots consist of two parts: (i) the intersection of the curve and the horizontal axis represents the
internal or equivalent series resistance [40]; (ii) a straight slopping line at low frequency range demonstrated diffusion control, and here means the pure capacitance and the electrochemical capacitance were high for the electrode materials [41]. The impedance spectra of electrolytic cell could be simply explained by an equivalent circuit shown in the inset of Fig. 6, and the best-fit estimates of the different parameters obtained from the impedance measurements are shown in Table1. The Rs represents bulk resistance of the cell, representing the overall ohmic resistance of the electrolyte and working electrodes; CPE means constant phase element, using in a model in place of a capacitance to compensate for rough or porous surface [42]; Rct and CPE are the charge transfer resistance and its related double-layer capacitance between the electrolyte and cathode. The simulated results fit well with the experimental data, supporting by the small errors obtained by the fitting. As shown in low-frequency region, the bending upward curve of nickel foam extends steeper than that of graphite, indicating a capacitance behaviour and a weaker impact controlled by diffusion process. It implied a better mass transfer process through nickel foam [43] than graphite electrode. Table 1 shows that the nickel foam exhibited lower charge transfer resistance, favouring the oxygen reduction process at nickel foam surface [44]. In addition, as observed in Fig. 7, the phase angle of the nickel foam electrode in the low frequency region is -62.25°, which is higher than that of the graphite electrode (-39.95°). The three-dimensional structure of nickel foam, favoring ion diffusion and then reducing its diffusion resistance, could explain the higher phase angle, which was in agreement of research from Kim [45]. It is an excellent technique to use impedance spectroscopy for studying the electrochemical properties of the bulk and interface. As shown in Fig. 8, according to Nyquist plots and the following Equation (Eq. 8), it could obtain the relationship between the capacitor and the frequency [45, 46]. C(f)= -1⁄2πf×Z"(f)
(8)
Where ƒ is the frequency (Hz) and Z″ (ƒ) is the imaginary part of the impedance. The number of electrolyte ions, which have reached the pore surfaces of the electrode at a specific frequency, could be obtained from the capacitance-frequency dependence [45, 46]. It is shown that the capacitance decreased as frequency increasing. At low frequencies, the electrolyte ions penetrated deeply into the electrode’s pores, approaching more electrode surface and thereby leading to a high capacitance value. At high frequencies, the electrolyte ions could only reach the surface of electrode, instead of accessing to the deeper pores, causing a sharp drop in the capacitance [47]. Obviously, due to three-dimensional geometry, the nickel foam shows more outstanding capacitance than the graphite. These results obtained from EIS support the evaluation in terms of performance obtained from the CV.
3.5. Tafel test Tafel polarization graphs on Pt/Nickel foam and on Pt/Graphite in the electrochemical system are shown in Fig. 9. From this figure, it shows that Pt/Nickel foam and on Pt/Graphite suggest different electrochemical performances. The graph for Pt/Nickel foam has a linear region for the potential from E = −0.474 V to E = −0.534 V range. The graph corresponding to the Pt / Graphite has a linear region for the potential from E = −0.448 V to E = −0.508 V range. b is the slope of the linear portion of an over potential (η) versus log current density (j) curve, which indicating the increase of the η required in order to raise the j by 10-fold [48]. If expressed by a formula the Tafel equation is η = blog(j/j0) [48]. Table 2 presents the j0 and b values gotten from the Tafel plots. The slope of a Tafel plot depends on the essence of the rate limiting charge transfer in the reduction mechanism [49]. The b value for nickel foam electrode (-129.68 mV/dec) is lower than that of graphite electrode (-221.32 mV/dec), which is expected due to the superior electro-catalytic activity of nickel foam in the electrochemical system [50]. As mentioned earlier in SEM analysis, the result may be attributed to the three-dimensional structure of nickel foam which can provide the large specific surface area and then supply
abundant active sites for charge transfer.
3.6. RhB degradation The decolorization of RhB and TOC removal by nickel foam and graphite in novel reactor was shown in Fig. 10. The figure reveals that when graphite working as cathode, the decolorization efficiency and TOC removal was only 63.6% and 28.9% in 60 min, respectively. On the other hand, the decolorization efficiency and TOC removal reached 92.5% and 41.5% by using nickel foam. The oxidation of RhB could be explained by two kinds of oxidant: H2O2 and ·OH, since the standard redox potential of Eo (H2O2/H2O) =1.77 V/SHE [14] and Eo (·OH/H2O) =2.80 V/SHE [51]. For different electrodes and processes, the catalytic ability for RhB degradation can be compared by RhB removal efficiency. As shown in Table 3, the low-cost electrode, such as graphite was used to degrade RhB, but have long electrolysis time. Some modified electrodes, such as ACF+Fe2+-chitosan and Fe@Fe2O3/CNT are also need long reaction time even the removal efficiencies are not high. In order to reveal the discoloration process of RhB, the products of RhB were investigated with excitation-emission matrix (EEM) fluorescence spectroscopy (Fig. 11). Fig. 11 shows that the major peak was located at 550/580 nm (Ex/Em), while other three weak peaks were observed at the Ex/Em of 400/580 nm, 350/580 nm and 260/580 nm, respectively. It is obvious that with the reaction time increasing, the fluorescence intensity of the peaks decreased more significantly with nickel foam electrode than using graphite electrode. In addition, the whole peaks were disappeared after reacting 40 min when nickel foam was working as cathode. However, only the weakest one peak at 400/580 nm was disappeared after reacting 60 min with graphite cathode. Considering that no new fluorescence peaks and the uncomplete TOC removal rate, indicating that some small molecular weight organics formatted with the break-up of the large molecules during RhB removal process [56, 57]. The production of H2O2 has been measured in the previous study and the formation of ·OH was verified by
ESR. As Fig. 12 shows a typical spectrum of DMPO-·OH composed by a characteristic 1:2:2:1 quartet with hyperfine couplings αN=αβH = 1.50 mT appeared in both systems and no other peaks like the peaks of DMPO-·O2- was detected. Moreover, the higher intensity of DMPO-·OH spectra in nickel foam electrode system compared to graphite electrode system means that more ·OH was produced, which indicated that nickel foam electrode system enhanced the formation of ·OH by generating more H 2O2 (Eq. 2). The structural stability of the material before and after the RhB degradation reaction was carried by SEM test (see Fig. S1). It can be seen from the figure that the nickel foam retains a good three-dimensional structure in addition to the precipitated residue and the surface of graphite is still sheet-like after the reaction. It shows that both cathode materials have good structural stability.
4. Conclusions In summary, nickel foam electrode displayed a superior electro-catalytic activity than that of graphite electrode in the newly designed electro-Fenton reactor by the analysis of CV, EIS and Tafel tests. Moreover, nickel foam exhibited excellent performances in producing hydrogen peroxide in the solution pH of 3: 780.63 μmol/L H2O2 produced at nickel foam electrode after 60 min reaction, and the removal efficiency of RhB was reached to 92.5% in 60 min. The oxygen reduction reactions were adsorption-controlled process when nickel foam as cathode and diffusion-controlled process when graphite as cathode, respectively. The three-dimensional open structure of nickel foam electrode provided abundant active sites for charge transfer, and then promoted the rapid ion diffusion in the electrolyte and ion adsorption onto the electrode surface, exhibiting lower charge transfer resistance than graphite electrode. Hydroxyl radical was working as the main oxygen free radical to remove RhB. Further investigations are under analysis in order to study the optimal operating conditions and calculate of the increase in the utilization of oxygen in the novel reactor.
Acknowledgments The research was supported by the National Key Research and Development Plan of China (No. 2016YFC0401102).
References [1] K. Shen and M. A. Gondal, Removal of hazardous Rhodamine dye from water by adsorption onto exhausted coffee ground, J. Saudi Chem Soc. 21 (2017), pp. S120-S127.doi:10.1016/j.jscs.2013.11.005 [2] X. Su, X. Li, J. Li, M. Liu, F. Lei, X. Tan, P. Li, and W. Luo, Synthesis and characterization of core–shell magnetic molecularly imprinted polymers for solid-phase extraction and determination of Rhodamine B in food, Food Chem. 171 (2015), pp. 292-297.doi:10.1016/j.foodchem.2014.09.024 [3] J. Chen and X. Zhu, Magnetic solid phase extraction using ionic liquid-coated core–shell magnetic nanoparticles followed by high-performance liquid chromatography for determination of Rhodamine B in food samples, Food Chem. 200 (2016), pp. 10-15.doi:10.1016/j.foodchem.2016.01.002 [4] S. Su, W. Guo, Y. Leng, C. Yi, and Z. Ma, Heterogeneous activation of Oxone by CoxFe3 − xO4 nanocatalysts
for
degradation
of
rhodamine
B,
J.
Hazard.
Mater.
244-245
(2013),
pp.
736-742.doi:10.1016/j.jhazmat.2012.11.005 [5] X. Jiang and J. Huang, Adsorption of Rhodamine B on two novel polar-modified post-cross-linked resins: Equilibrium and kinetics, J. Colloid Interf. Sci. 467 (2016), pp. 230-238.doi:10.1016/j.jcis.2016.01.031 [6] D. J. Ju, I. G. Byun, J. J. Park, C. H. Lee, G. H. Ahn, and T. J. Park, Biosorption of a reactive dye (Rhodamine-B) from an aqueous solution using dried biomass of activated sludge, Bioresour. Technol. 99 (2008), pp. 7971-7975.doi:10.1016/j.biortech.2008.03.061 [7] J. Kumar and A. Bansal, A Comparative Study of Immobilization Techniques for Photocatalytic Degradation of Rhodamine B using Nanoparticles of Titanium Dioxide, Water, Air, Soil Pollut. 224 (2013), doi:10.1007/s11270-013-1452-1
[8] R. Davarnejad and J. Azizi, Alcoholic wastewater treatment using electro-Fenton technique modified by Fe2O3 nanoparticles, J. Environ. Chem. Eng. 4 (2016), pp. 2342-2349. [9]T. X. H. Le, T. V. Nguyen, Z. Amadou Yacouba, L. Zoungrana, F. Avril, D. L. Nguyen, E. Petit, J. Mendret, V. Bonniol, M. Bechelany, S. Lacour, G. Lesage, and M. Cretin, Correlation between degradation pathway and toxicity of acetaminophen and its by-products by using the electro-Fenton process in aqueous media., Chemosphere. 172 (2017), pp. 1-9.doi:10.1016/j.chemosphere.2016.12.060 [10] H. Lin and H. Zhang, Treatment of Organic Pollutants Using Electro-Fenton and Electro-Fenton-Like Process in Aqueous Solution, Prog. Chem. 27 (2015), pp. 1123-1132. [11] M. S. Yahya, N. Beqqal, A. Guessous, M. R. D. Arhoutane, and K. El Kacemi, Degradation and mineralization of moxifloxacin antibiotic in aqueous medium by electro-Fenton process: Kinetic assessment and oxidation products, Cogent Chem. 3 (2017), doi:10.1080/23312009.2017.1290021 [12] B. Zhang, J. Sun, Q. Wang, N. Fan, J. Ni, W. Li, Y. Gao, Y. Y. Li, and C. Xu, Electro-Fenton oxidation of coking wastewater: optimization using the combination of central composite design and convex optimization method, Environ. Technol. 38 (2017), p. 2456.doi:10.1080/09593330.2016.1265591 [13] Z. G. Aguilar, E. Brillas, M. Salazar, J. L. Nava, and I. Sirés, Evidence of Fenton-like reaction with active chlorine during the electrocatalytic oxidation of Acid Yellow 36 azo dye with Ir-Sn-Sb oxide anode in the presence of iron ion, Appl. Catal. B: Environ. 206 (2017), pp. 44-52.doi:10.1016/j.apcatb.2017.01.006 [14] F. C. Moreira, R. A. R. Boaventura, E. Brillas, and V. J. P. Vilar, Electrochemical advanced oxidation processes: A review on their application to synthetic and real wastewaters, Appl. Catal. B: Environ. 202 (2017), pp. 217-261.doi:10.1016/j.apcatb.2016.08.037 [15] J. Yan, L. Qian, W. Gao, Y. Chen, D. Ouyang, and M. Chen, Enhanced Fenton-like Degradation of Trichloroethylene by Hydrogen Peroxide Activated with Nanoscale Zero Valent Iron Loaded on Biochar, Sci. Rep.-UK. 7 (2017), p. 43051.doi:10.1038/srep43051
[16] X. Ding, Z. Ai and L. Zhang, A dual-cell wastewater treatment system with combining anodic visible light driven photoelectro-catalytic oxidation and cathodic electro-Fenton oxidation, Sep. Purif. Technol. 125 (2014), pp. 103-110.doi:10.1016/j.seppur.2014.01.046 [17]M. Sun, X. Ru and L. Zhai, In-situ fabrication of supported iron oxides from synthetic acid mine drainage: High catalytic activities and good stabilities towards electro-Fenton reaction, A Appl. Catal. B: Environ. 165 (2015), pp. 103-110.doi:10.1016/j.apcatb.2014.09.077 [18] X. Zhao, S. Liu and Y. Huang, Removing organic contaminants by an electro-Fenton system constructed with
graphene
cathode,
Toxicol.
Environ.
Chem.
98
(2015),
pp.
530-539.doi:10.1080/02772248.2015.1123495 [19] B. Bouzayani, J. Meijide, M. Pazos, S. C. Elaoud, and M. A. Sanroman, Removal of polyvinylamine sulfonate anthrapyridone dye by application of heterogeneous electro-Fenton process, Environ. Sci. Pollut. R. 24 (2017), pp. 18309-18319.doi:10.1007/s11356-017-9468-5 [20] Z. Ma, H. Meng, M. Wang, B. Tang, J. Li, and X. Wang, Porous Ni−Mo−S Nanowire Network Film Electrode as a High-Efficiency Bifunctional Electrocatalyst for Overall Water Splitting, ChemElectroChem. 5 (2018), pp. 335-342.doi:10.1002/celc.201700965 [21] Z. Ma, Q. Zhao, J. Li, B. Tang, Z. Zhang, and X. Wang, Three-dimensional well-mixed / highly-densed NiS-CoS nanorod arrays: An efficient and stable bifunctional electrocatalyst for hydrogen and oxygen evolution reactions, Electrochim. Acta. 260 (2018), pp. 82-91.doi:10.1016/j.electacta.2017.11.055 [22] H. Meng, W. Zhang, Z. Ma, F. Zhang, B. Tang, J. Li, and X. Wang, Self-Supported Ternary Ni–S–Se Nanorod Arrays as Highly Active Electrocatalyst for Hydrogen Generation in Both Acidic and Basic Media: Experimental Investigation and DFT Calculation, ACS Appl. Mater. Inter. 10 (2018), pp. 2430-2441.doi:10.1021/acsami.7b14506 [23] M. Wang, Z. Ma, J. Li, Z. Zhang, B. Tang, and X. Wang, Well-dispersed palladium nanoparticles on
nickel- phosphorus nanosheets as efficient three-dimensional platform for superior catalytic glucose electro-oxidation
and
non-enzymatic
sensing,
J.
Colloid
Interf.
Sci.
511
(2018),
pp.
355-364.doi:10.1016/j.jcis.2017.10.008 [24] E. Bocos, O. Iglesias, M. Pazos, and M. Ángeles Sanromán, Nickel foam a suitable alternative to increase the
generation
of
Fenton's
reagents,
Process
Saf.
Environ.
101
(2016),
pp.
34-44.doi:10.1016/j.psep.2015.04.011 [25] S. Song, M. Wu, Y. Liu, Q. Zhu, P. Tsiakaras, and Y. Wang, Efficient and Stable Carbon-coated Nickel Foam
Cathodes
for
the
Electro-Fenton
Process,
Electrochim.
Acta.
176
(2015),
pp.
811-818.doi:10.1016/j.electacta.2015.07.029 [26] W. Liu, Z. Ai and L. Zhang, Design of a neutral three-dimensional electro-Fenton system with foam nickel as particle electrodes for wastewater treatment, J. Hazard. Mater. 243 (2012), pp. 257-264. [27] A. Wang, J. Qu, J. Ru, H. Liu, and J. Ge, Mineralization of an azo dye Acid Red 14 by electro-Fenton's reagent
using
an
activated
carbon
fiber
cathode,
Dyes
Pigments.
65
(2005),
pp.
227-233.doi:10.1016/j.dyepig.2004.07.019 [28] L. Liang, Y. An, M. Zhou, F. Yu, M. Liu, and G. Ren, Novel rolling-made gas-diffusion electrode loading trace transition metal for efficient heterogeneous electro-Fenton-like, J. Environ. Chem. Eng. 4 (2016), pp. 4400-4408.doi:10.1016/j.jece.2016.10.006 [29] S. Şahinkaya, COD and color removal from synthetic textile wastewater by ultrasound assisted electro-Fenton oxidation process, J. Ind. Eng. Chem. 19 (2013), pp. 601-605.doi:10.1016/j.jiec.2012.09.023 [30] C. Su, A. Chang, L. M. Bellotindos, and M. Lu, Degradation of acetaminophen by Fenton and electro-Fenton
processes
in
aerator
reactor,
Sep.
Purif.
Technol.
99
(2012),
pp.
8-13.doi:10.1016/j.seppur.2012.07.004 [31]P. V. Nidheesh and R. Gandhimathi, Trends in electro-Fenton process for water and wastewater treatment:
An overview, Desalination. 299 (2012), pp. 1-15.doi:10.1016/j.desal.2012.05.011 [32] E. Rosales, O. Iglesias, M. Pazos, and M. A. Sanroman, Decolourisation of dyes under electro-Fenton process
using
Fe
alginate
gel
beads,
J.
Hazard.
Mater.
213-214
(2012),
pp.
369-77.doi:10.1016/j.jhazmat.2012.02.005 [33] M. Zhou, Q. Tan, Q. Wang, Y. Jiao, N. Oturan, and M. A. Oturan, Degradation of organics in reverse osmosis
concentrate
by
electro-Fenton
process,
J.
Hazard.
Mater.
215-216
(2012),
pp.
287-93.doi:10.1016/j.jhazmat.2012.02.070 [34] F. X. Deng, J. X. Yang, Y. S. Zhu, F. Ma, and S. Qiu, A Rapid Azo Dye Decolorization of Methyl Orange by the Foam Zero-Valent Nickel, Environ. Prog. Sustain. 37 (2018), pp. 686-694.doi:10.1002/ep [35] E. Bocos, D. Pérez-Álvarez, M. Pazos, M. C. Rodríguez-Argüelles, and M. Á. Sanromán, Coated nickel foam electrode for the implementation of continuous electro-Fenton treatment, J. Chem. Technol. Biotechnol. 91 (2016), pp. 685-692.doi:10.1002/jctb.4626 [36] F. Yang, K. Cheng, X. Xue, J. Yin, G. Wang, and D. Cao, Three-dimensional porous Ni film electrodeposited on Ni foam: High performance and low-cost catalytic electrode for H2O2 electrooxidation in KOH solution, Electrochim. Acta. 107 (2013), pp. 194-199.doi:10.1016/j.electacta.2013.05.152 [37] T. Placke, G. Schmuelling, R. Kloepsch, P. Meister, O. Fromm, P. Hilbig, H. Meyer, and M. Winter, In situ X-ray Diffraction Studies of Cation and Anion Inter-calation into Graphitic Carbons for Electrochemical Energy Storage Applications, Z. Anorg. Allg. Chem. 640 (2014), pp. 1996-2006.doi:10.1002/zaac.201400181 [38] H. Y. Yue, H. Zhang, S. Huang, X. Y. Lin, X. Gao, J. Chang, L. H. Yao, and E. J. Guo, Synthesis of ZnO nanowire arrays/3D graphene foam and application for determination of levodopa in the presence of uric acid, Biosens. Bioelectro. 89 (2017), pp. 592-597.doi:10.1016/j.bios.2016.01.078 [39] H. Y. Yue, H. Zhang, J. Chang, X. Gao, S. Huang, L. H. Yao, X. Y. Lin, and E. J. Guo, Highly sensitive and selective uric acid biosensor based on a three-dimensional graphene foam/indium tin oxide glass electrode,
Anal. Biochem. 488 (2015), pp. 22-27.doi:10.1016/j.ab.2015.07.007 [40] S. Alwarappan and A. Kumar, Graphene-based materials: Science and Technology: CRC Press, 2013. [41] Y. Chen, P. He, P. Huang, L. Wang, X. Yi, and F. Dong, SnO2-modified MnO2 Electrode Materials for Electrochemical Capacitor, in ECS Transactions. vol. 28, X. Zhang, D. Chu, P. S. Fedkiw, and C. Wang, Eds., 2010, pp. 107-115. [42] X. Li, G. Han, Y. Liu, B. Dong, X. Shang, W. Hu, Y. Chai, Y. Liu, and C. Liu, In situ Grown Pyramid Structures of Nickel Diselenides Dependent on Oxidized Nickel Foam as Efficient Electrocatalyst for Oxygen Evolution Reaction, Electrochim. Acta. 205 (2016), pp. 77-84.doi:10.1016/j.electacta.2016.04.108 [43] S. You, M. Ma, W. Wang, D. Qi, X. Chen, J. Qu, and N. Ren, 3D Macroporous Nitrogen-Enriched Graphitic Carbon Scaffold for Efficient Bioelectricity Generation in Microbial Fuel Cells, Adv. Energy Mater. (2016), p. 1601364.doi:10.1002/aenm.201601364 [44] Y. Liu, Z. Yu, Y. Hou, Z. Peng, L. Wang, Z. Gong, J. Zhu, and D. Su, Highly efficient Pd-Fe/Ni foam as heterogeneous Fenton catalysts for the three-dimensional electrode system, Catal. Commun. 86 (2016), pp. 63-66.doi:10.1016/j.catcom.2016.08.012 [45] M. Kim, I. Oh and J. Kim, Carbonization temperature dependence of pore structure of silicon carbide spheres and their electrochemical capacitive properties as supercapacitors, Ceram. Int. 42 (2016), pp. 3947-3958.doi:10.1016/j.ceramint.2015.11.063 [46] W. Xing, S. Z. Qiao, R. G. Ding, F. Li, G. Q. Lu, Z. F. Yan, and H. M. Cheng, Superior electric double layer
capacitors
using
ordered
mesoporous
carbons,
Carbon.
44
(2006),
pp.
216-224.doi:10.1016/j.carbon.2005.07.029 [47] M. Biswal, A. Banerjee, M. Deo, and S. Ogale, From dead leaves to high energy density supercapacitors, Energ. Environ. Sci. 6 (2013), p. 1249.doi:10.1039/c3ee22325f [48] D. Wang, F. Watanabe and W. Zhao, Reduced Graphene Oxide-NiO/Ni Nanomembranes as Oxygen
Evolution
Reaction
Electrocatalysts,
ECS
J.
Solid
State
Sc.
6
(2017),
pp.
M3049-M3054.doi:10.1149/2.0101706jss [49] P. H. Matter, E. Wang, M. Arias, E. J. Biddinger, and U. S. Ozkan, Oxygen reduction reaction activity and surface properties of nanostructured nitrogen-containing carbon, J. Mol. Catal. A: Chem. 264 (2007), pp. 73-81.doi:10.1016/j.molcata.2006.09.008 [50] J. Van Drunen, B. K. Pilapil, Y. Makonnen, D. Beauchemin, B. D. Gates, and G. Jerkiewicz, Electrochemically Active Nickel Foams as Support Materials for Nanoscopic Platinum Electrocatalysts, ACS Appl. Mater. Inter. 6 (2014), pp. 12046-12061.doi:10.1021/am501097t [51] E. Brillas, Electro-Fenton, UVA Photoelectro-Fenton and Solar Photoelectro-Fenton Treatments of Organics in Waters Using a Boron-Doped Diamond Anode: A Review, J. Mex. Chem. Soc. 58 (2014), pp. 239-255. [52] Y. Fan, Z. Ai and L. Zhang, Design of an electro-Fenton system with a novel sandwich film cathode for wastewater treatment, J. Hazard. Mater. 176 (2010), pp. 678-84.doi:10.1016/j.jhazmat.2009.11.085 [53] P. V. Nidheesh, R. Gandhimathi and N. S. Sanjini, NaHCO3 enhanced Rhodamine B removal from aqueous solution by graphite – graphite electro Fenton system, Sep. Purif. Technol. 132 (2014), pp. 568-576.doi:10.1016/j.seppur.2014.06.009 [54] J. Shi, Z. Ai and L. Zhang, Fe@Fe2O3 core-shell nanowires enhanced Fenton oxidation by accelerating the Fe(III)/Fe(II) cycles, Water Res. 59 (2014), pp. 145-153.doi:10.1016/j.watres.2014.04.015 [55] M. Sun, Y. Liu, W. Xiang, and L. Zhai, Electricity-induced catalytic oxidation of RhB by O2 at a graphite anode, Electrochim. Acta. 158 (2015), pp. 314-320.doi:10.1016/j.electacta.2015.01.156 [56] H. Wu, Z. Ai and L. Zhang, Anoxic and oxic removal of humic acids with Fe@Fe2O3 core – shell nanowires: A comparative study, Water Res. 52 (2014), pp. 92-100.doi:10.1016/j.watres.2013.12.041 [57] B. Lai, Y. Zhou, J. Wang, Z. Yang, and Z. Chen, Application of excitation and emission matrix
fluorescence (EEM) and UV–vis absorption to monitor the characteristics of Alizarin Red S (ARS) during electro-Fenton
degradation
process,
2805-2813.doi:10.1016/j.chemosphere.2013.09.056
Chemosphere.
93
(2013),
pp.
Figure Captions Figure 1. Experimental set-up Figure 2. Concentration of H2O2 (a) and CE (b) using nickel foam and graphite as cathodes in the electro-Fenton system. Conditions: pH = 3, [Na2SO4] = 0.05 mol/L, V = 500 mL, and current density = 1 mA/cm2 Figure 3. SEM image of nickel foam at 50× magnification (a), 500× magnification (b), and of graphite at 100× magnification (c), 1000× magnification (d) Figure 4. XRD pattern of nickel foam and graphite Figure 5. CV curves of nickel foam (a) and graphite electrode (b) with different scan rates. Conditions: pH = 3, [Na2SO4] = 0.05 mol/L and V = 50 mL. (a) and (c), Scanning rates from bottom to top: 5, 10, 20, 30, 40 and 50 mV/s. (b), Relationship of the reduction peak current vs. scan rate. (d), Relationship of the reduction peak current vs. square-root of the scan rate Figure 6. Nyquist plots of nickel foam and graphite electrodes. Conditions: pH = 3, [Na2SO4] = 0.05 mol/L and V = 50 mL. Inset magnifies the data for clear view Figure 7. Impedance phase angle as a function of frequency of nickel foam and graphite electrodes. Operating conditions: pH = 3, [Na2SO4] = 0.05 mol/L and V = 50 mL Figure 8. Capacitance obtained from EIS of nickel foam and graphite electrodes as a function of frequency. Conditions: pH = 3, [Na2SO4] = 0.05 mol/L and V = 50 mL Figure 9. Tafel plots of nickel foam and graphite cathode. Operating conditions: pH = 3, [Na 2SO4] = 0.05 mol/L and V = 50 mL Figure 10. Degradation of RhB with different cathode in electro-Fenton system. Conditions: pH = 3, [Na2SO4] = 0.05 mol/L, [RhB] = 20 mg/L, [Fe2+] = 0.1 mmol/L V = 500 mL, and current density = 1 mA/cm2 Figure 11. EEM fluorescence spectra of the samples with different reaction time. First row – graphite
electrode and second row – nickel foam electrode. Conditions: pH = 3, [Na2SO4] = 0.05 mol/L, [RhB] = 20 mg/L, [Fe2+] = 0.1 mmol/L V = 500 mL, and current density = 1 mA/cm2 Figure 12. ESR spectra of DMPO-·OH adduct generated in electro-Fenton system. Conditions: pH = 3, [Na2SO4] = 0.05 mol/L, [Fe2+] = 0.1 mmol/L V = 500 mL, and current density = 1 mA/cm2
Table 1. Simulated parameters of the elements in equivalent circuits of nickel foam and graphite Element
Nickel foam
Graphite
Value
Error %
Value
Error %
Rs (Ω)
10.15
0.68
10.15
0.02
CPE-T (F)
0
1.00
0.01
0
CPE-P (F)
0.75
0.36
0.68
0
Rct (Ω)
146.6
4.06
237.8
16.31
Table 2. Tafel slopes values for nickel foam and graphite as cathode in electrochemical system
Electrode
j0
b 2
mA/cm
mV/dec
R2
Nickel foam 0.3367
-129.68
0.9952
Graphite
-221.32
0.9980
0.0015
Table 3. Comparison of different processes for RhB removal Electrode
Solution
Anode: Pt (2 cm2) Cathode: ACF+Fe2+-chitosan (2 cm×3 cm)
j 2
Electrolysis
Removal
Reference
(mg/L)
(mA/cm )
time (min)
efficiency (%)
5
1.7
120
93
[52]
10
1.2
180
99
[53]
5
ΔE=1.2 V
120
91.5
[54]
25
E=2.0 V
120
100
[55]
20
1.0
60
92.5
This study
0.05 mol/L Na2SO4 at pH=6.2
Anode: Graphite (5
5 mg/L NaHCO3
cm×5 cm) Cathode:
and 10 mg/L
Graphite (5 cm×5 cm)
Concentration
2+
Fe at pH=3
2
Anode: Pt (2 cm ) Cathode:
0.05 mol/L
Fe@Fe2O3/CNT (3
Na2SO4 at pH=6
cm2) Anode: Carbon paper (3 cm×3 cm) Cathode:
0.05 mol/L
Graphite rod (Φ5 mm×5
Na2SO4 at pH=3
cm) Anode: Carbon rod (Φ2
0.05 mol/L
cm×10.3 cm) Cathode:
Na2SO4 and 0.1
Nickel foam (30 cm×10
mmol/L Fe2+ at
cm)
pH=3
