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Simultaneous removal of phenol and dichromate from aqueous solution through a phenol-Cr(VI) coupled redox fuel cell reactor
Separation and Purification Technology 172 (2017) 152–157
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Simultaneous removal of phenol and dichromate from aqueous solution through a phenol-Cr(VI) coupled redox fuel cell reactor Hui-Min Zhang a,b, Wei Xu a, Zheng Fan a, Xin Liu a, Zu-Cheng Wu a,⇑, Ming-Hua Zhou c,⇑ a Department of Environmental Engineering, Laboratory of Electrochemistry and Energy Storage, State Key Laboratory Clean Energy Utilization, Zhejiang University, Hangzhou 310027, PR China b Institute of Environmental Engineering, East China Jiao Tong University, Nanchang, Jiangxi 330013, PR China c Key Laboratory of Pollution Process and Environmental Criteria, Ministry of Education, College of Environmental Science and Engineering, Nankai University, Tianjin 300071, PR China
a r t i c l e
i n f o
a b s t r a c t
Article history: Received 31 May 2016 Received in revised form 22 July 2016 Accepted 7 August 2016 Available online 9 August 2016
To eliminate contaminants like phenol and dichromate from aqueous solutions is still a hot research interest. In contrast to a traditional method within high intensity of energy input, both dichromate and phenol were removed within self-powered process through a phenol-Cr(VI) coupled redox fuel cell reactor, which was uniquely assembled on Ni/C anode the electro-oxidation of phenol takes place to give electrons and then go through the external circuit reducing Cr(VI) to Cr(III) on the cathode. Taking an example of 0.94 g L1 phenol in 0.2 M phosphate buffer solution (PBS) serving as the anolyte and 0.15 g L1 Cr(VI) in 0.5 M H2SO4 serving as the catholyte, a net power density of 0.184 W m2 at 0.61 V can be gained. A removal efficiency of 98.6% for 0.94 g L1 phenol within 132 h and 99.8% for 0.30 g L1 Cr(VI) within 60 h can be readily achieved in the system. This confirmed in the fed-batch mode that either the contaminants of phenol and Cr(VI) in solutions is effectively removed or the energy contained in chemicals as the form of electricity has been exactly retrieved. Ó 2016 Elsevier B.V. All rights reserved.
Keywords: Coupled redox fuel cell Dichromate removal Elimination of phenol Nickel/carbon electrode
1. Introduction Phenol, which is highly poisonous for the ecosystem, often is derived from oil refinery waste, coal conversion plants, chemical plants, and municipal waste treatment plants. Stringent environmental regulations have drawn the attention of chemists and environmental researchers to develop efficient technologies to control phenol. Different approaches, such as electrochemical process [1,2], photocatalysis [3,4], and wet-air oxidation (WAO) [5–7], have been implemented to remove phenol. Although these approaches are efficient and applicable to remove phenol, the issue of energy consumption has never been ignored. Of electrochemical oxidation methods, phenol was electrochemically oxidized on an anode with energy consumption around 1.88 kW h/g-phenol [8,9]. Similarly, removal of Cr(VI) by electrochemical reduction will consume equally electricity [10]. Importantly, in order to resolve the increasingly critical environmental pollution and energy shortage issues, recovering energy from the waste rather than energy consumption has been attracted a great ⇑ Corresponding authors. E-mail addresses: (M.-H. Zhou).
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(Z.-C.
http://dx.doi.org/10.1016/j.seppur.2016.08.007 1383-5866/Ó 2016 Elsevier B.V. All rights reserved.
Wu),
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attention world-widely [11–14]. Development of new energy harvesting ways from waste would be a new frontier. Noticed that an acidic Cr(VI) has a positive oxidation potential (1.33 V vs. SHE) and can reasonably be used as a potential oxidizer. In our previous work, Cr(VI) removal has been accompanied energy production in an alkaline fuel cell reactor, where acidic Cr(VI) was used as oxidizer to oxidize organic fuels and to produce electricity [15–17]. Phenol is really a hazardous substance but similarly a carbon-containing compound with a ring, and the standard potential of the C6H5OH/(CO2, H+) redox pair is +0.107 V vs. SHE, which was calculated from thermodynamic data [18]. Thus, such waste as phenol prompted us to consider the possibility of a phenol-Cr (VI) fuel-cell-like device to be removed and to generate net energy, namely coupled redox fuel cell (CRFC), where phenol can be the fuel, and Cr(VI) can be the oxidizer. Phenol electro-oxidation, which occurs at the anode, relies on the electrocatalysts. Pt-noble metals and Pt-non-noble metals alloys exhibit good electrocatalyst ability [19–23]. However, platinum is highly expensive and demonstrates poor electrochemical performance for phenol oxidation because a strongly adherent insulating film forms and inactivates the electrode surface [24,25]. Non-precious transition metals, such as nickel, which costs much less than platinum and exhibits good catalytic ability in
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electricity generation [26,27]. Nickel hydroxide can catalyze the oxidative removal of phenol from water [7]. Therefore, a nanonickel was similarly employed herein to be as a catalyst for phenol anodic-oxidation to testify its electrocatalytic activities. The objective of the work is to examine the property of CRFC both the aqueous anode in phenol and the aqueous cathode in Cr(VI) solutions by using conductive wires, which connect the anode and the cathode to form a circuit. We further testify the approaches to reduce the toxicity of contaminants and to characterize the performance in terms of phenol and Cr(VI) removal, electricity production, and the operational sustainability of this system. 2. Experimental section 2.1. Phenol-Cr(VI) CRFC reactor assembly Carbon fiber cloth (Jilin Tansu, China) was treated according to the previous literature and used as the cathode [15,27]. Homemade catalyst of 50% Ni/C was cast on the treated carbon fiber cloth with a 5 mg cm2 Ni loading mass, and the resulting electrode was used as the anode [28]. The effective geometric area of each electrode was approximately 10.0 cm2. Two electrodes were inserted in the anode chamber and the cathode chamber, the volumes of which are 11.0 mL. A cylinder-shaped U vycor tube (length: 10 cm, inner diameter: 10 mm, both ends sealed with ceramic cores) filled with a saturated KNO3 solution was used as the ion bridge. The ions bridge was placed between the two chambers. The anode chamber was filled with phenol that dissolves in the PBS solution (0.2 M KH2PO4 + 0.2 M Na2HPO4, pH 8), and the cathode chamber was filled with Cr(VI), which contained H2SO4 aqueous solution. The CRFC reactor is placed in one glass container filled with nitrogen. The voltage output is one of the main performance indicators of CRFC. Appropriate voltages can be obtained when a constant external resistor was fixed at 985 X under every close circuit. The components of anolyte and catholyte of every control experiment under the open circuits (without resistance and conductive wires between the two electrodes) are same as the close circuit. In all experiments, pH changes in anolyte and catholyte were undergoing tracing observation. PBS can keep the pH unchanged in anolyte. Acidity in catholyte was measured to be no obvious change. At the anode, the electrons released from phenol oxidation were transferred through the external circuit to the cathode, where the electrons were used to reduce Cr(VI) to Cr(III). The ion bridge connected the two poles and formed a closed loop. The operating emperature was maintained at approximately 25 °C.
electrocatalysts were characterized by transmission electron microscopy (TEM, JEOL JEM-1230) at a voltage of 120 kv. The sample was dispersed in alcohol solvent by ultrasonic action and dropped onto a carbon-coated copper grid. Scanning Electron Microscopes (SEM) were conducted by Hitachi TM1000. 2.3. Analysis The phenol and its intermediates were analyzed on a Varian C18 (HPLC, Varian, USA) column operated at 35 °C with the elution solvents A (aqueous 1% acetic acid) and B (99% acetonitrile + 1% acetic acid) and a flow-rate of 1 ml min1. The gradient is 0–10 min, 100–100% A; 10–15 min, 100–30% A. The UV–vis spectra of the anolyte were measured using the UNICO 2820 UV–vis scanning spectrophotometer. The Cr(VI) sample analysis, polarization curves and cathodic efficiency was conducted according to the previous literature [15]. 3. Results and discussion 3.1. The performance of the phenol-Cr(VI) CRFC reactor The polarization and power density curves of the phenol-Cr(VI) CRFC was shown in Fig. 1. In 0.2 M PBS solution when 940 mg L1 phenol was employed as the fuel and 150 mg L1 Cr(VI) in 0.25 M H2SO4 was served as the catholyte, phenol-Cr(VI) CRFC was assembled to give an open circuit voltage (OCV) of 1.02 V. This can be theoretically calculated that at standard condition phenol oxidation at anode was E+ = 0.107 V (vs. SHE, assumption of CO2 as oxidation product) and Cr2O2 reduction is E- = 1.33 V 7 (vs. SHE), i.e. E0 = 1.223 V. Actually, a peak power density of 0.184 W m2 at 0.61 V was obtained. This difference was due to existing of a polarization loss. Polarization curves exhibited a common profile of a low activation polarization loss, which illustrates that the electricity generation primarily ascribed to the ionic conductivity of the electrolyte until the reaction reached the mass transport limit [30]. It can also be clearly observed that CRFC cell performance was significantly related to Ni/C catalyst loading mass, which indicates that the prepared Ni/C catalyst possesses a high electrocatalytic activity for phenol oxidation. When a loading of 5.0 mg Ni cm2 was used as the anode catalyst, the variation of the cell voltage with time decreased gradually but maintained a level of above 31 mV (Fig. 2A, blue curve). Comparably, in the control experiment (no nickel loading on the anode), neither electricity nor phenol
50 wt.% Ni/C electrocatalyst was prepared using a chemical reaction between NiSO4 and NaBH4 [28,29]. NiSO4 (5.0 ml, 0.4 M) and sodium citrate (10.0 ml, 0.1 M) were added to 250 ml de-ionized water in a three-neck flask. Under vigorous stirring, carbon powder (112.0 mg, Vulcan XC-72R) was added with continuous stirring for 2.5 h. Next, after half an hour of ultrasonic dispersion, a homogeneous solution was obtained. Fresh NaBH4 solution (50.0 ml, 5 wt.%) was added dropwise into this solution. The final products were collected by filtration and washed several times with de-ionized water and alcohol. The prepared electrocatalysts were dried at 45 °C for 12 h in a vacuum oven. X-ray diffraction (XRD) pattern measurements were performed using an X-ray diffractometer (Shimadzu XRD-6000), which was equipped with a Cu Ka radiation source (k = 1.54056 Å) and operated at 40 kV and 40 mA. The 2h angular regions between 10° and 90° were explored at a scan rate of 5° min1. The obtained Ni/C
1.0
Voltage (V)
1.2 0.15
0.8 0.10 0.6 0.05
0.4
Ni loading: 5 mg cm-2 Ni loading: 2.5 mg cm-2 Ni loading: 0.5 mg cm-2
0.2
Power density (W m-2 )
0.20
2.2. Preparation of the Ni/C catalyst
0.00
0.0 0.0
0.1
0.2
0.3
0.4
0.5
Current density (A m-2) Fig. 1. Polarization and power density curves of the phenol-Cr(VI) CRFC, anode: Ni/C (50 wt.%), 5.0 mg Ni cm2 (j), 2.5 mg Ni cm2 (N) and 0.5 mg Ni cm2 (.). Anolyte: 940 mg L1 phenol in 0.2 M PBS (pH 8); catholyte: 150 mg L1 Cr(VI) in 0.25 M H2SO4.
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0.4
470 mg L-1 Phenol 940 mg L-1 Phenol
A
♥ Ni(OH)2
1880 mg L-1 Phenol
♦C ♦
Intensity (a.u.)
0.3
Voltage (V)
♣ Ni
♣
0.2
♥ ♥
♣
0.1
0.0 0
10
20
30
10
Time (h)
40
50
60
70
80
Fig. 3. XRD of the Ni/C. -1
50 mg L Cr(VI) -1 150 mg L Cr(VI) -1 300 mg L Cr(VI)
B 0.3
Voltage (V)
30
2 Theta (deg.)
0.4
0.2
0.1
0.0 0
10
20
30
40
50
60
Time (h) 0.4
0.05 M H2SO4
C
0.10 M H2SO4 0.25 M H2SO4
0.3
Voltage (V)
20
0.50 M H2SO4 0.2
0.1
0.0 0
5
10
15
20
25
30
Time (h) Fig. 2. (A) Variation of the cell voltage with time under different initial phenol concentrations (470 mg L1 (black), 940 mg L1 (blue), and 1880 mg L1 (red)) in 0.2 M PBS (pH 8), and the Cr(VI) concentration was maintained at 150 mg L1 in 0.5 M H2SO4. (B) Variation of the cell voltage with time under different initial Cr(VI) concentrations (50 mg L1 (black), 150 mg L1 (blue) and 300 mg L1 (red)) in 0.5 M H2SO4, and the phenol concentration was maintained at 940 mg L1 in 0.2 M PBS (pH 8). (C) Variation of the cell voltage with time under different pH values (0.05 M H2SO4 (green), 0.10 M H2SO4 (black), 0.25 M H2SO4 (blue) and 0.5 M H2SO4 (red)) with a phenol concentration of 940 mg L1 in 0.2 M PBS (pH 8) and a Cr(VI) concentration of 150 mg L1 in 0.5 M H2SO4. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
reduction in the anolyte and Cr(VI) reduction in the catholyte occurred. Electrochemical reactions occurred in the CRFC with nickel loading on the anode. At anode nickel catalyzed phenol to
release electrons, which transferred the external circuit to the cathode. At cathode the electrons were consumed by Cr(VI) electro-reduction to Cr(III). Apparently, the electricity generation of the corresponding CRFC is significantly regulated by catalytic oxidation of phenol on the anode. For instance, when the Ni loading was decreased to 0.5 mg cm2, the CRFC yielded a low peak power density of 0.057 W m2. The electricity generation of phenol-Cr(VI) CRFC attributes to Ni anode. Fig. 3 showed the XRD pattern of the Ni/C catalyst, which proved the formation of nickel. The first relatively wide peak, which is located at the 2h value of 25°, is attributed to the carbon (0 0 2) facet of the Vulcan XC-72R carbon powder support. The strongest diffraction peak at the 2h value of approximately 45° can be indexed to the nickel (1 0 1) facets, which indicates the formation of nickel. The peaks at 2h 60° and 72° signify nickel particle having a hexagonal structure. A nickel hydroxide peak is also observed at 2h near 34°, revealing that some nickel was prepared in the form of Ni(OH)2. SEM and TEM analyses were done in order to get information on the particle size and uniformity. A typical TEM image of Ni/C and its corresponding histogram are shown in Fig. 4(A). It is observed that most of the nanoparticles are round in shape with primary particle size of 30 nm. Ni/C nanoparticles were homogeneously dispersed over the surface of carbon black with only few agglomerations. In the SEM image of Fig. 4(B), numerous Ni particles were uniformly distributed on carbon though some agglomerations were observed. The secondary particle size of the prepared Ni/C is about several 101 lm. 3.2. Phenol removal and electricity production To investigate phenol removal on Ni/C and the electricity production in CRFCs, different concentrations of phenol were filled in the anode while maintaining the Cr(VI) concentration in the catholyte at 150 mg L1. The output voltages above 30 mV of these phenol-Cr(VI) CRFCs were recorded. As summarized in Fig. 2(A) and Table 1, when the initial phenol concentration varied from 470 mg L1 to 1880 mg L1, the OCV of 0.93–1.03 V can be obtained. With an external resistor, the voltage output was maintained above 0.031 V for 21.65 h until the phenol reduced from 470 mg L1 to 361.4 mg L1. When the initial concentration was increased to 1880 mg L1, 0.31 V was measured; next, the voltage slightly decreased to 0.036 V after 30.5 h; the present phenol concentration was 1588.9 mg L1. Obviously, with the initial phenol concentration changing from 470 mg L1 to 1880 mg L1, the absolute phenol removal amount significantly increased from
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Fig. 4. (A) TEM of the Ni/C; (B) SEM of the Ni/C. Table 1 Effect of the initial phenol concentration in 0.2 M PBS with a constant amount of 150 mg L1 Cr(VI) in 0.5 M H2SO4. Initial phenol conc. (mg L1)
Cr(VI) conc. at time t (mg L1)
Phenol conc. at time t (mg L1)
Cr(VI) removal efficiency (%)
Phenol removal efficiency (%)
OCV (V)
Cathodic efficiency (%)
470a 940b 1880c
10.42 2.72 0.703
367.4 781.1 1588.9
93.1 98.2 99.5
23.1 16.9 15.5
0.93 1.06 1.03
94.08 90.47 75.15
a = 21.65 h, b = 25.3 h, c = 30.5 h.
2.0
A a b c d
Abs. (a.u.)
1.6
Control 48 h 60 h 132 h
1.2
0.8 cd
Phenol
b
0.4
a
0.0 200
250
300
350
400
Wavelength (nm)
B
Fig. 5. (A) UV/Visible Spectrum and (B) HPLC analysis of the anolyte before and after treatment of phenol solution by a CRFC. (a, black) control of 940 mg L1 in 0.2 M PBS (pH 8); (b, red) fuel: 940 mg L1 in 0.2 M PBS (pH 8), oxidant: 150 mg L1 Cr(VI) in 0.5 M H2SO4, running 48.1 h; (c, blue) fuel: 940 mg L1 in 0.2 M PBS (pH 8), oxidant: 300 mg L1 Cr(VI) in 0.5 M H2SO4, running 60 h; (d, green) fuel: secondhand fuel from (c), fresh oxidant: 300 mg L1 Cr(VI) in 0.5 M H2SO4, running 72 h. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
108.6 mg L1 to 291.2 mg L1, although the removal efficiency decreased from 23.1% to 15.5%, and the cathodic efficiency from 94.08% to 75.15%. Correspondingly, Cr(VI) removal efficiency in the catholyte reached above 93% (Table 1). To examine the phenol removal efficiency via electro-oxidation on Ni/C in the CRFC system further, the UV–vis spectrum of the anolyte in CRFCs that operated for different time was analyzed. From the UV–vis spectrum results (Fig. 5(A)), it can be observed that the phenol concentration decreased with the running time. Under the condition of (b) in Fig. 5(A), after 48.1 h, the phenol concentration in the anolyte was 630.9 mg L1 (Table 1). When we prolonged the time to 60 h, as in the case of (c) in Fig. 3(A), more phenol (528.1 mg L1) (Table 1) was degraded. Correspondingly, the Cr(VI) concentration in the catholyte was reduced to 0.62 mg L1. When we renewed Cr(VI) in the catholyte with 300 mg L1 of fresh Cr(VI) and run this CRFC again for another 72 h, as shown in Fig. 5(A) (d), only 12.7 mg L1 of phenol can be detected (the corresponding Cr(VI) was 1.52 mg L1). As a result, it took 132 h to finish the phenol oxidation. If we further prolonged the time, the phenol would be reduced to a much lower concentration. In the phenol electro-oxidation process, oxidation products were generated with releasing electrons continuously. HPLC analysis was conducted to determine these products in Fig. 5(B). The intermediates conclude hydroquinone, benzoquinone, maleic acid and fumaric acid. Due to its ecotoxicity, the formation of benzoquinone as an intermediate during phenol oxidation could be a problem. However, the maximum concentration was measured below 0.03 mg L1 in all experiments, which was smaller than the lowest LC50 for fishes reported in the literature [31]. Consideration of the joint toxicity of these products, due to insignificant observation of those quinone or hydroquinone intermediate implying they are rapidly oxidized, could be ignored [31]. Especially, they would be converted to low-molecular-weight carboxylic acids with time. In UV visible spectroscopy scan results of Fig. 5(A), signal response near 220 nm amplifying with the running time was observed, which probably were the response of carboxylic acids. Using phenol-Cr(VI) CRFC can realize the phenol oxidation; the main products are organic acids of low toxicity.
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Table 2 Effect of the initial Cr(VI) concentration in 0.5 M H2SO4 with a constant amount of 940 mg L1 phenol in 0.2 M PBS. Initial Cr(VI) conc. (mg L1)
Cr(VI) conc. at time t (mg L1)
Phenol conc. at time t (mg L1)
Cr(VI) removal efficiency (%)
Phenol removal efficiency (%)
OCV (V)
Cathodic efficiency (%)
10a 50b 150c 300d
0.10 0.36 0.35 0.50
860.5 712.2 630.9 411.9
99.0 99.3 99.8 99.8
8.46 24.2 32.9 56.2
0.90 0.95 1.06 1.03
95.62 89.81 89.11 77.5
a = 18 h, b = 30.5 h, c = 48.1 h, d = 60 h.
The phenol electro-oxidation occurs at the anode to release electrons. Those electrons will transfer through the external circuit to the cathode, which renders the electro-reduction of Cr(VI) to Cr(III) according to the following reaction: 3þ þ Cr2 O2 þ 7H2 O 7 þ 14H þ 6e ! 2Cr
ð1Þ
When the initial phenol concentration in the anolyte was maintained at 940 mg L1, Fig. 2(B) shows the voltage output under different initial Cr(VI) concentrations in the catholyte. When the initial Cr(VI) concentration varied from 50 mg L1 to 300 mg L1, the OCV of 0.95–1.03 V could be obtained. After an external resistor was loaded, the corresponding voltage output was 0.212 V and 0.363 V. After 30 h, the voltage output was below 30 mV for the initial Cr(VI) concentration of 50 mg L1, but for the concentration of 300 mg L1, the voltage was above 100 mV. Consequently, with the Cr(VI) reduction reaction, the voltage decreased from 0.363 V to 17 mV, which corresponded to a Cr(VI) removal efficiency of 99.8% (Table 2). Higher initial Cr(VI) concentrations resulted in higher Cr(VI) and phenol removal amount. Differently, cathodic electron utilization efficiency decreased from 95.62% to 99.7% with increasing the initial Cr(VI) levels and extension of time. It is noticed that lower initial Cr(VI) concentration of 10 mg L1, after 18 h running, the finial concentration reduced to 0.1 mg L1, which would meet the requirement of the European discharge standard and US EPA criteria [32,33]. When varying the Cr(VI) from 300 to 10 mg L1, the OCV insignificantly decreased from 1.03 V to 0.90 V at 0.5 M H2SO4. That means the OCV is not much dependent on the oxidant concentration. The process is extremely useful for contaminants reduction although the electricity generation is not remarkable. Sufficient phenol producing electrons can maintain high Cr(VI) removal, more than 99% removal efficiency at initial Cr(VI) concentration of 300 mg L1 can be easily achieved, as well as much higher Cr(VI) concentration can be predictably reduced. If the abundance of Cr(VI) was in presence in the catholyte, more phenol removal also can be reached. Twice refilling 300 mg L1 Cr(VI) in the catholyte can result in 98.6% removal of 940 mg L1 phenol. All these results imply that this method can be flexibly designed for wastewater treatment, and is practically useful for pretreatment of contaminants removal. From Eq. (1), pH is another important factor affecting the voltage output and the Cr(VI) removal. When the Cr(VI) concentration was maintained at 150 mg L1 Cr(VI), the H2SO4 concentration was changed from 0.05 M to 0.5 M, Fig. 2(C) shows that the OCV changed from 0.96 V to 1.06 V, and the initial potential slightly changed from 0.307 V to 0.376 V with a load. Next, all cell potentials tend to be identical after 10 h. Under typical conditions, the acid concentration changes the reduction potential according to the Nernst equation from 1.19V (0.05 M H2SO4) to 1.33 V (0.5 M H2SO4). A phenol oxidation under the fixed conditions has the same reduction potential. Consequently, the combination of the two half reactions can generate a difference in the OCV and the operating potential. 0.05 M H2SO4 provides sufficient protons for the Cr(VI)
Table 3 Effect of the initial pH in 150 mg L1 Cr(VI) concentration and 940 mg L1 phenol in 0.2 M PBS. H2SO4 Conc. (M)
Cr(VI) conc. at time t (mg L1)
Phenol conc. at time t (mg L1)
Cr(VI) removal efficiency (%) Rc
Phenol removal efficiency (%) Rc0
OCV (V)
Cathodic efficiency (%)
0.05a 0.10a 0.25a 0.5a
6.49 5.47 1.03 1.02
760.2 677.2 658.9 647.4
96.7 96.4 99.3 99.3
19.1 28.0 29.9 31.1
0.96 1.02 1.03 1.06
97.05 96.64 97.74 98.30
a = 30 h.
reduction under the giving conditions of 150 mg L1 Cr(VI). Thus, the Cr(VI) and phenol removal has close values under different pH values in catholyte, which can be observed in Table 3. Identically, the cathodic efficiency held at 97% for all pH. 3.4. Sustainability of phenol and Cr(VI) removal in CRFCs To evaluate the sustainability of electricity production with phenol and Cr(VI) removal in the CRFC, a fed-batch mode test was performed with a CRFC, i.e., by refilling the phenol solution (940 mg L1 in 0.2 M PBS) and the Cr(VI) solution (150 mg L1 in 0.5 M H2SO4) every 25 h. Five cycles were continuously performed. The voltage output is shown in Fig. 6. An OCV of 1.02 V can be obtained in the first cycle. The output voltage remains almost the same in all cycles. During the test, a stable phenol and Cr(VI) removal, which was greater than 23% and 96% every cycle with ca. 25 h, respectively, was achieved (Table 4). In another CRFC, 300 mg L1 Cr(VI) was filled into the catholyte twice, which removed 98.6% of 940 mg L1 phenol with a total time of 132 h. All of these tests indicate that continuous electricity generation and the removal of phenol and 0.4
1st 4th 2nd
0.3
3th
Voltage (V)
3.3. Cr(VI) removal
5th
0.2
0.1
0.0 0
20
40
60
80
100
120
140
Time (h) Fig. 6. Voltage output with time of a fed-batch mode test (anolyte: 940 mg L1 phenol in 0.2 M PBS (pH 8), catholyte 150 mg L1 in 0.5 M H2SO4).
H.-M. Zhang et al. / Separation and Purification Technology 172 (2017) 152–157 Table 4 Results of the continuous phenol and Cr(VI) removal in a fed-batch mode CRFC. Number of cycles
Phenol conc. (mg L1) Start
End
1a 2b 3b 4b 5b
940 940 940 940 940
Average
940
Phenol removal (%)
Cr(VI) Conc. (mg L1)
Cr(VI) removal (%)
Start
End
647.4 687.5 705.0 712.6 720.4
31.1 26.9 25.0 24.2 23.4
150 150 150 150 150
1.02 5.20 5.22 5.82 6.49
99.3 96.5 96.5 96.1 95.7
694.6
26.1
150
4.75
96.8
a = 30 h, b = 25.5 h.
Cr(VI) in a CRFC are feasible, and the contaminant removal performance is sustainable. 4. Conclusions Different from the traditional approaches for phenol removal, our work is putting forward a new method of energy extraction from waste by assembling pollutant coupled redox fuel cell. Recovering electricity and simultaneously reducing toxicity of phenol and Cr(VI) is of great significant. The result of the phenol-Cr(VI) CRFC is an example of energy extraction from wastewater. Although the production of electricity is modest probably due to the sluggish electrode reaction and high internal resistance of the device, which could be predictably reduced to improve the performance on electricity generation, whatever simultaneous removal of phenol and Cr(VI) from aqueous solution within self-powered process is achievable. Acknowledgements This work is financially supported by the National Natural Science Foundation of China (Nos. 21473158, 51178225), Postdoctoral Science Foundation of China (No. 2015M581941), Youth Science Foundation of Jiangxi Province, China (20161BAB216137) and Science Foundation of Jiangxi Province Department of Education, China (GJJ150520). References [1] B.H. Wang, H.J. Wu, G.X. Zhang, S. Licht, STEP wastewater treatment: a solar thermal electrochemical process for pollutant oxidation, ChemSusChem 5 (2012) 2000–2010. [2] J.R. Sun, H.Y. Lu, H.B. Lin, Electrochemical oxidation of aqueous phenol at low concentration using Ti/BDD electrode, Sep. Purif. Technol. 88 (2012) 116–120. [3] C.A. Aguilar, T. Pandiyan, J.A. Arenas-Alatorre, Oxidation of phenols by TiO2-Fe3O4-M(M = Ag or Au) hybrid composites under visible light, Sep. Purif. Technol. 149 (2015) 265–278. [4] S. Kunduz, G.S.P. Soylu, Highly active BiVO4 nanoparticles: the enhanced photocatalytic properties under natural sunlight for removal of phenol from wastewater, Sep. Purif. Technol. 141 (2015) 221–228. [5] Y. Yan, S.S. Jiang, H.P. Zhang, Efficient catalytic wet peroxide oxidation of phenol over Fe-ZSM-5 catalyst in a fixed bed reactor, Sep. Purif. Technol. 133 (2014) 365–374. [6] R.R.N. Marques, F. Stuber, K.M. Smith, A. Fabregat, C. Bengoa, J. Font, A. Fortuny, S. Pullket, G.D. Fowler, N.J.D. Graham, Sewage sludge based catalysts for catalytic wet air oxidation of phenol: preparation, characterisation and catalytic performance, Appl. Catal. B – Environ. 101 (2011) 306–316. [7] M. Saeed, M. Ilyas, Oxidative removal of phenol from water catalyzed by nickel hydroxide, Appl. Catal. B – Environ. 129 (2013) 247–254.
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[8] Y.H. Cui, Y.J. Feng, X.Y. Li, Kinetics and efficiency analysis of electrochemical oxidation of phenol: influence of anode materials and operational conditions, Chem. Eng. Technol. 34 (2011) 265–272. [9] Y. Yavuz, A.S. Koparal, Electrochemical oxidation of phenol in a parallel plate reactor using ruthenium mixed metal oxide electrode, J. Hazard. Mater. 36 (2006) 296–302. [10] Z.Y. Zhang, D. Liba, L. Alvarado, A.C. Chen, Separation and recovery of Cr(III) and Cr(VI) using electrodeionization as an efficient approach, Sep. Purif. Technol. 137 (2014) 86–93. [11] M. Tilahun, O. Sahu, M. Kotha, H. Sahu, Cogeneration of renewable energy from biomass (utilization of municipal solid waste as electricity production: gasification method), Mater. Renew. Sustain. Energy 4 (2015). [12] B. Baran, M.S. Mamis, B.B. Alagoz, Utilization of energy from waste potential in Turkey as distributed secondary renewable energy source, Renew. Energy 90 (2016) 493–500. [13] Q.P. Chen, J.H. Li, X.J. Li, K. Huang, B.X. Zhou, W.M. Cai, W.F. Shangguan, Visible-light responsive photocatalytic fuel cell based on WO3/W photoanode and Cu2O/Cu photocathode for simultaneous wastewater treatment and electricity generation, Environ. Sci. Technol. 46 (2012) 11451–11458. [14] Y.L. Xu, Y. He, C. Yang, Y. Wang, L.J.P. Jia, Photocatalytic fuel cell (PFC) and dye self-photosensitization photocatalytic fuel cell (DSPFC) with BiOCl/Ti photoanode under UV and visible light irradiation, Environ. Sci. Technol. 47 (2013) 3490–3497. [15] H.M. Zhang, W. Xu, Z.C. Wu, M.H. Zhou, T. Jin, Removal of Cr(VI) with cogeneration of electricity by an alkaline fuel cell reactor, J. Phys. Chem. C 117 (2013) 14479–14484. [16] B.B. Yu, H.M. Zhang, W. Xu, G. Li, Z.C. Wu, Remediation of chromium-slag leakage with electricity cogeneration via a urea-Cr(VI) cell, Sci. Rep. 4 (2014) (2014) 6. [17] W. Xu, H.M. Zhang, G. Li, Z.C. Wu, A urine/Cr(VI) fuel cell—electrical power from processing heavy metal and human urine, J. Electroanal. Chem. 784 (2016) (2016) 38–44. [18] D.R. Lide, in: Handbook of Chemistry and Physics, CRC PRESS, 2003–2004, pp. 842–897. [19] M.M. Tusi, N.S.O. Polanco, S.G. da Silva, E.V. Spinace, A.O. Neto, The high activity of PtBi/C electrocatalysts for ethanol electro-oxidation in alkaline medium, Electrochem. Commun. 13 (2011) 143–146. [20] L. Jiang, A. Hsu, D. Chu, R. Chen, Ethanol electro-oxidation on Pt/C and PtSn/C catalysts in alkaline and acid solutions, Int. J. Hydrogen Energy 35 (2010) 365– 372. [21] S.Y. Shen, T.S. Zhao, J.B. Xu, Y.S. Li, Synthesis of PdNi catalysts for the oxidation of ethanol in alkaline direct ethanol fuel cells, J. Power Sour. 195 (2010) 1001– 1006. [22] P.S. Roy, J. Bagchi, S.K. Bhattacharya, The size-dependent anode-catalytic activity of nickel-supported palladium nanoparticles for ethanol alkaline fuel cells, Catal. Sci. Technol. 2 (2012) 2302–2310. [23] A.M.C. Luna, A. Bonesi, W.E. Triaca, A. Di Blasi, A. Stassi, V. Baglio, V. Antonucci, A.S. Arico, Investigation of a Pt-Fe/C catalyst for oxygen reduction reaction in direct ethanol fuel cells, J. Nanopart. Res. 12 (2010) 357–365. [24] M. Ferreira, H. Varela, R.M. Torresi, G. Tremiliosi, Electrode passivation caused by polymerization of different phenolic compounds, Electrochim. Acta 52 (2006) 434–442. [25] S. Patra, N. Munichandraiah, Electro-oxidation of phenol on polyethylenedioxythiophene conductive-polymer-deposited stainless steel substrate, J. Electrochem. Soc. 155 (2008) F23–F30. [26] D.L. Li, M. Koike, L. Wang, Y. Nakagawa, Y. Xu, K. Tomishige, Regenerability of hydrotalcite-derived nickel-iron alloy nanoparticles for syngas production from biomass tar, ChemSusChem 7 (2014) 510–522. [27] H.M. Zhang, W. Xu, D.L. Feng, Z.M. Liu, Z.C. Wu, Self-powered denitration of landfill leachate through ammonia/nitrate coupled redox fuel cell reactor, Bioresour. Technol. 203 (2016) 56–61. [28] W. Xu, H.M. Zhang, G. Li, Z.C. Wu, Nickel-cobalt bimetallic anode catalysts for direct urea fuel cell, Sci. Rep. 4 (2014) 6. [29] H.M. Zhang, W. Xu, G. Li, Z.M. Liu, Z.C. Wu, B.G. Li, Assembly of coupled redox fuel cells using copper as electron acceptors to generate power and its in-situ retrieval, Sci. Rep. 6 (2016) 21059. [30] M. Winter, R.J. Brodd, What are batteries, fuel cells, and supercapacitors?, Chem Rev. 104 (2004) 4245–4269. [31] Z.C. Wu, M.H. Zhou, Partial degradation of phenol by advanced electrochemical oxidation process, Environ. Sci. Technol. 35 (2001) 2698–2703. [32] European Commission Integrated Pollution Prevention and Control: Surface Treatment of Metals and Plastics, 2006, p. VI. [33] US EPA National Recommended Water Quality Criteria. (accessed 03.2014).
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Separation and Purification Technology journal homepage: www.elsevier.com/locate/seppur
Simultaneous removal of phenol and dichromate from aqueous solution through a phenol-Cr(VI) coupled redox fuel cell reactor Hui-Min Zhang a,b, Wei Xu a, Zheng Fan a, Xin Liu a, Zu-Cheng Wu a,⇑, Ming-Hua Zhou c,⇑ a Department of Environmental Engineering, Laboratory of Electrochemistry and Energy Storage, State Key Laboratory Clean Energy Utilization, Zhejiang University, Hangzhou 310027, PR China b Institute of Environmental Engineering, East China Jiao Tong University, Nanchang, Jiangxi 330013, PR China c Key Laboratory of Pollution Process and Environmental Criteria, Ministry of Education, College of Environmental Science and Engineering, Nankai University, Tianjin 300071, PR China
a r t i c l e
i n f o
a b s t r a c t
Article history: Received 31 May 2016 Received in revised form 22 July 2016 Accepted 7 August 2016 Available online 9 August 2016
To eliminate contaminants like phenol and dichromate from aqueous solutions is still a hot research interest. In contrast to a traditional method within high intensity of energy input, both dichromate and phenol were removed within self-powered process through a phenol-Cr(VI) coupled redox fuel cell reactor, which was uniquely assembled on Ni/C anode the electro-oxidation of phenol takes place to give electrons and then go through the external circuit reducing Cr(VI) to Cr(III) on the cathode. Taking an example of 0.94 g L1 phenol in 0.2 M phosphate buffer solution (PBS) serving as the anolyte and 0.15 g L1 Cr(VI) in 0.5 M H2SO4 serving as the catholyte, a net power density of 0.184 W m2 at 0.61 V can be gained. A removal efficiency of 98.6% for 0.94 g L1 phenol within 132 h and 99.8% for 0.30 g L1 Cr(VI) within 60 h can be readily achieved in the system. This confirmed in the fed-batch mode that either the contaminants of phenol and Cr(VI) in solutions is effectively removed or the energy contained in chemicals as the form of electricity has been exactly retrieved. Ó 2016 Elsevier B.V. All rights reserved.
Keywords: Coupled redox fuel cell Dichromate removal Elimination of phenol Nickel/carbon electrode
1. Introduction Phenol, which is highly poisonous for the ecosystem, often is derived from oil refinery waste, coal conversion plants, chemical plants, and municipal waste treatment plants. Stringent environmental regulations have drawn the attention of chemists and environmental researchers to develop efficient technologies to control phenol. Different approaches, such as electrochemical process [1,2], photocatalysis [3,4], and wet-air oxidation (WAO) [5–7], have been implemented to remove phenol. Although these approaches are efficient and applicable to remove phenol, the issue of energy consumption has never been ignored. Of electrochemical oxidation methods, phenol was electrochemically oxidized on an anode with energy consumption around 1.88 kW h/g-phenol [8,9]. Similarly, removal of Cr(VI) by electrochemical reduction will consume equally electricity [10]. Importantly, in order to resolve the increasingly critical environmental pollution and energy shortage issues, recovering energy from the waste rather than energy consumption has been attracted a great ⇑ Corresponding authors. E-mail addresses: (M.-H. Zhou).
[email protected]
(Z.-C.
http://dx.doi.org/10.1016/j.seppur.2016.08.007 1383-5866/Ó 2016 Elsevier B.V. All rights reserved.
Wu),
[email protected]
attention world-widely [11–14]. Development of new energy harvesting ways from waste would be a new frontier. Noticed that an acidic Cr(VI) has a positive oxidation potential (1.33 V vs. SHE) and can reasonably be used as a potential oxidizer. In our previous work, Cr(VI) removal has been accompanied energy production in an alkaline fuel cell reactor, where acidic Cr(VI) was used as oxidizer to oxidize organic fuels and to produce electricity [15–17]. Phenol is really a hazardous substance but similarly a carbon-containing compound with a ring, and the standard potential of the C6H5OH/(CO2, H+) redox pair is +0.107 V vs. SHE, which was calculated from thermodynamic data [18]. Thus, such waste as phenol prompted us to consider the possibility of a phenol-Cr (VI) fuel-cell-like device to be removed and to generate net energy, namely coupled redox fuel cell (CRFC), where phenol can be the fuel, and Cr(VI) can be the oxidizer. Phenol electro-oxidation, which occurs at the anode, relies on the electrocatalysts. Pt-noble metals and Pt-non-noble metals alloys exhibit good electrocatalyst ability [19–23]. However, platinum is highly expensive and demonstrates poor electrochemical performance for phenol oxidation because a strongly adherent insulating film forms and inactivates the electrode surface [24,25]. Non-precious transition metals, such as nickel, which costs much less than platinum and exhibits good catalytic ability in
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electricity generation [26,27]. Nickel hydroxide can catalyze the oxidative removal of phenol from water [7]. Therefore, a nanonickel was similarly employed herein to be as a catalyst for phenol anodic-oxidation to testify its electrocatalytic activities. The objective of the work is to examine the property of CRFC both the aqueous anode in phenol and the aqueous cathode in Cr(VI) solutions by using conductive wires, which connect the anode and the cathode to form a circuit. We further testify the approaches to reduce the toxicity of contaminants and to characterize the performance in terms of phenol and Cr(VI) removal, electricity production, and the operational sustainability of this system. 2. Experimental section 2.1. Phenol-Cr(VI) CRFC reactor assembly Carbon fiber cloth (Jilin Tansu, China) was treated according to the previous literature and used as the cathode [15,27]. Homemade catalyst of 50% Ni/C was cast on the treated carbon fiber cloth with a 5 mg cm2 Ni loading mass, and the resulting electrode was used as the anode [28]. The effective geometric area of each electrode was approximately 10.0 cm2. Two electrodes were inserted in the anode chamber and the cathode chamber, the volumes of which are 11.0 mL. A cylinder-shaped U vycor tube (length: 10 cm, inner diameter: 10 mm, both ends sealed with ceramic cores) filled with a saturated KNO3 solution was used as the ion bridge. The ions bridge was placed between the two chambers. The anode chamber was filled with phenol that dissolves in the PBS solution (0.2 M KH2PO4 + 0.2 M Na2HPO4, pH 8), and the cathode chamber was filled with Cr(VI), which contained H2SO4 aqueous solution. The CRFC reactor is placed in one glass container filled with nitrogen. The voltage output is one of the main performance indicators of CRFC. Appropriate voltages can be obtained when a constant external resistor was fixed at 985 X under every close circuit. The components of anolyte and catholyte of every control experiment under the open circuits (without resistance and conductive wires between the two electrodes) are same as the close circuit. In all experiments, pH changes in anolyte and catholyte were undergoing tracing observation. PBS can keep the pH unchanged in anolyte. Acidity in catholyte was measured to be no obvious change. At the anode, the electrons released from phenol oxidation were transferred through the external circuit to the cathode, where the electrons were used to reduce Cr(VI) to Cr(III). The ion bridge connected the two poles and formed a closed loop. The operating emperature was maintained at approximately 25 °C.
electrocatalysts were characterized by transmission electron microscopy (TEM, JEOL JEM-1230) at a voltage of 120 kv. The sample was dispersed in alcohol solvent by ultrasonic action and dropped onto a carbon-coated copper grid. Scanning Electron Microscopes (SEM) were conducted by Hitachi TM1000. 2.3. Analysis The phenol and its intermediates were analyzed on a Varian C18 (HPLC, Varian, USA) column operated at 35 °C with the elution solvents A (aqueous 1% acetic acid) and B (99% acetonitrile + 1% acetic acid) and a flow-rate of 1 ml min1. The gradient is 0–10 min, 100–100% A; 10–15 min, 100–30% A. The UV–vis spectra of the anolyte were measured using the UNICO 2820 UV–vis scanning spectrophotometer. The Cr(VI) sample analysis, polarization curves and cathodic efficiency was conducted according to the previous literature [15]. 3. Results and discussion 3.1. The performance of the phenol-Cr(VI) CRFC reactor The polarization and power density curves of the phenol-Cr(VI) CRFC was shown in Fig. 1. In 0.2 M PBS solution when 940 mg L1 phenol was employed as the fuel and 150 mg L1 Cr(VI) in 0.25 M H2SO4 was served as the catholyte, phenol-Cr(VI) CRFC was assembled to give an open circuit voltage (OCV) of 1.02 V. This can be theoretically calculated that at standard condition phenol oxidation at anode was E+ = 0.107 V (vs. SHE, assumption of CO2 as oxidation product) and Cr2O2 reduction is E- = 1.33 V 7 (vs. SHE), i.e. E0 = 1.223 V. Actually, a peak power density of 0.184 W m2 at 0.61 V was obtained. This difference was due to existing of a polarization loss. Polarization curves exhibited a common profile of a low activation polarization loss, which illustrates that the electricity generation primarily ascribed to the ionic conductivity of the electrolyte until the reaction reached the mass transport limit [30]. It can also be clearly observed that CRFC cell performance was significantly related to Ni/C catalyst loading mass, which indicates that the prepared Ni/C catalyst possesses a high electrocatalytic activity for phenol oxidation. When a loading of 5.0 mg Ni cm2 was used as the anode catalyst, the variation of the cell voltage with time decreased gradually but maintained a level of above 31 mV (Fig. 2A, blue curve). Comparably, in the control experiment (no nickel loading on the anode), neither electricity nor phenol
50 wt.% Ni/C electrocatalyst was prepared using a chemical reaction between NiSO4 and NaBH4 [28,29]. NiSO4 (5.0 ml, 0.4 M) and sodium citrate (10.0 ml, 0.1 M) were added to 250 ml de-ionized water in a three-neck flask. Under vigorous stirring, carbon powder (112.0 mg, Vulcan XC-72R) was added with continuous stirring for 2.5 h. Next, after half an hour of ultrasonic dispersion, a homogeneous solution was obtained. Fresh NaBH4 solution (50.0 ml, 5 wt.%) was added dropwise into this solution. The final products were collected by filtration and washed several times with de-ionized water and alcohol. The prepared electrocatalysts were dried at 45 °C for 12 h in a vacuum oven. X-ray diffraction (XRD) pattern measurements were performed using an X-ray diffractometer (Shimadzu XRD-6000), which was equipped with a Cu Ka radiation source (k = 1.54056 Å) and operated at 40 kV and 40 mA. The 2h angular regions between 10° and 90° were explored at a scan rate of 5° min1. The obtained Ni/C
1.0
Voltage (V)
1.2 0.15
0.8 0.10 0.6 0.05
0.4
Ni loading: 5 mg cm-2 Ni loading: 2.5 mg cm-2 Ni loading: 0.5 mg cm-2
0.2
Power density (W m-2 )
0.20
2.2. Preparation of the Ni/C catalyst
0.00
0.0 0.0
0.1
0.2
0.3
0.4
0.5
Current density (A m-2) Fig. 1. Polarization and power density curves of the phenol-Cr(VI) CRFC, anode: Ni/C (50 wt.%), 5.0 mg Ni cm2 (j), 2.5 mg Ni cm2 (N) and 0.5 mg Ni cm2 (.). Anolyte: 940 mg L1 phenol in 0.2 M PBS (pH 8); catholyte: 150 mg L1 Cr(VI) in 0.25 M H2SO4.
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0.4
470 mg L-1 Phenol 940 mg L-1 Phenol
A
♥ Ni(OH)2
1880 mg L-1 Phenol
♦C ♦
Intensity (a.u.)
0.3
Voltage (V)
♣ Ni
♣
0.2
♥ ♥
♣
0.1
0.0 0
10
20
30
10
Time (h)
40
50
60
70
80
Fig. 3. XRD of the Ni/C. -1
50 mg L Cr(VI) -1 150 mg L Cr(VI) -1 300 mg L Cr(VI)
B 0.3
Voltage (V)
30
2 Theta (deg.)
0.4
0.2
0.1
0.0 0
10
20
30
40
50
60
Time (h) 0.4
0.05 M H2SO4
C
0.10 M H2SO4 0.25 M H2SO4
0.3
Voltage (V)
20
0.50 M H2SO4 0.2
0.1
0.0 0
5
10
15
20
25
30
Time (h) Fig. 2. (A) Variation of the cell voltage with time under different initial phenol concentrations (470 mg L1 (black), 940 mg L1 (blue), and 1880 mg L1 (red)) in 0.2 M PBS (pH 8), and the Cr(VI) concentration was maintained at 150 mg L1 in 0.5 M H2SO4. (B) Variation of the cell voltage with time under different initial Cr(VI) concentrations (50 mg L1 (black), 150 mg L1 (blue) and 300 mg L1 (red)) in 0.5 M H2SO4, and the phenol concentration was maintained at 940 mg L1 in 0.2 M PBS (pH 8). (C) Variation of the cell voltage with time under different pH values (0.05 M H2SO4 (green), 0.10 M H2SO4 (black), 0.25 M H2SO4 (blue) and 0.5 M H2SO4 (red)) with a phenol concentration of 940 mg L1 in 0.2 M PBS (pH 8) and a Cr(VI) concentration of 150 mg L1 in 0.5 M H2SO4. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
reduction in the anolyte and Cr(VI) reduction in the catholyte occurred. Electrochemical reactions occurred in the CRFC with nickel loading on the anode. At anode nickel catalyzed phenol to
release electrons, which transferred the external circuit to the cathode. At cathode the electrons were consumed by Cr(VI) electro-reduction to Cr(III). Apparently, the electricity generation of the corresponding CRFC is significantly regulated by catalytic oxidation of phenol on the anode. For instance, when the Ni loading was decreased to 0.5 mg cm2, the CRFC yielded a low peak power density of 0.057 W m2. The electricity generation of phenol-Cr(VI) CRFC attributes to Ni anode. Fig. 3 showed the XRD pattern of the Ni/C catalyst, which proved the formation of nickel. The first relatively wide peak, which is located at the 2h value of 25°, is attributed to the carbon (0 0 2) facet of the Vulcan XC-72R carbon powder support. The strongest diffraction peak at the 2h value of approximately 45° can be indexed to the nickel (1 0 1) facets, which indicates the formation of nickel. The peaks at 2h 60° and 72° signify nickel particle having a hexagonal structure. A nickel hydroxide peak is also observed at 2h near 34°, revealing that some nickel was prepared in the form of Ni(OH)2. SEM and TEM analyses were done in order to get information on the particle size and uniformity. A typical TEM image of Ni/C and its corresponding histogram are shown in Fig. 4(A). It is observed that most of the nanoparticles are round in shape with primary particle size of 30 nm. Ni/C nanoparticles were homogeneously dispersed over the surface of carbon black with only few agglomerations. In the SEM image of Fig. 4(B), numerous Ni particles were uniformly distributed on carbon though some agglomerations were observed. The secondary particle size of the prepared Ni/C is about several 101 lm. 3.2. Phenol removal and electricity production To investigate phenol removal on Ni/C and the electricity production in CRFCs, different concentrations of phenol were filled in the anode while maintaining the Cr(VI) concentration in the catholyte at 150 mg L1. The output voltages above 30 mV of these phenol-Cr(VI) CRFCs were recorded. As summarized in Fig. 2(A) and Table 1, when the initial phenol concentration varied from 470 mg L1 to 1880 mg L1, the OCV of 0.93–1.03 V can be obtained. With an external resistor, the voltage output was maintained above 0.031 V for 21.65 h until the phenol reduced from 470 mg L1 to 361.4 mg L1. When the initial concentration was increased to 1880 mg L1, 0.31 V was measured; next, the voltage slightly decreased to 0.036 V after 30.5 h; the present phenol concentration was 1588.9 mg L1. Obviously, with the initial phenol concentration changing from 470 mg L1 to 1880 mg L1, the absolute phenol removal amount significantly increased from
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Fig. 4. (A) TEM of the Ni/C; (B) SEM of the Ni/C. Table 1 Effect of the initial phenol concentration in 0.2 M PBS with a constant amount of 150 mg L1 Cr(VI) in 0.5 M H2SO4. Initial phenol conc. (mg L1)
Cr(VI) conc. at time t (mg L1)
Phenol conc. at time t (mg L1)
Cr(VI) removal efficiency (%)
Phenol removal efficiency (%)
OCV (V)
Cathodic efficiency (%)
470a 940b 1880c
10.42 2.72 0.703
367.4 781.1 1588.9
93.1 98.2 99.5
23.1 16.9 15.5
0.93 1.06 1.03
94.08 90.47 75.15
a = 21.65 h, b = 25.3 h, c = 30.5 h.
2.0
A a b c d
Abs. (a.u.)
1.6
Control 48 h 60 h 132 h
1.2
0.8 cd
Phenol
b
0.4
a
0.0 200
250
300
350
400
Wavelength (nm)
B
Fig. 5. (A) UV/Visible Spectrum and (B) HPLC analysis of the anolyte before and after treatment of phenol solution by a CRFC. (a, black) control of 940 mg L1 in 0.2 M PBS (pH 8); (b, red) fuel: 940 mg L1 in 0.2 M PBS (pH 8), oxidant: 150 mg L1 Cr(VI) in 0.5 M H2SO4, running 48.1 h; (c, blue) fuel: 940 mg L1 in 0.2 M PBS (pH 8), oxidant: 300 mg L1 Cr(VI) in 0.5 M H2SO4, running 60 h; (d, green) fuel: secondhand fuel from (c), fresh oxidant: 300 mg L1 Cr(VI) in 0.5 M H2SO4, running 72 h. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
108.6 mg L1 to 291.2 mg L1, although the removal efficiency decreased from 23.1% to 15.5%, and the cathodic efficiency from 94.08% to 75.15%. Correspondingly, Cr(VI) removal efficiency in the catholyte reached above 93% (Table 1). To examine the phenol removal efficiency via electro-oxidation on Ni/C in the CRFC system further, the UV–vis spectrum of the anolyte in CRFCs that operated for different time was analyzed. From the UV–vis spectrum results (Fig. 5(A)), it can be observed that the phenol concentration decreased with the running time. Under the condition of (b) in Fig. 5(A), after 48.1 h, the phenol concentration in the anolyte was 630.9 mg L1 (Table 1). When we prolonged the time to 60 h, as in the case of (c) in Fig. 3(A), more phenol (528.1 mg L1) (Table 1) was degraded. Correspondingly, the Cr(VI) concentration in the catholyte was reduced to 0.62 mg L1. When we renewed Cr(VI) in the catholyte with 300 mg L1 of fresh Cr(VI) and run this CRFC again for another 72 h, as shown in Fig. 5(A) (d), only 12.7 mg L1 of phenol can be detected (the corresponding Cr(VI) was 1.52 mg L1). As a result, it took 132 h to finish the phenol oxidation. If we further prolonged the time, the phenol would be reduced to a much lower concentration. In the phenol electro-oxidation process, oxidation products were generated with releasing electrons continuously. HPLC analysis was conducted to determine these products in Fig. 5(B). The intermediates conclude hydroquinone, benzoquinone, maleic acid and fumaric acid. Due to its ecotoxicity, the formation of benzoquinone as an intermediate during phenol oxidation could be a problem. However, the maximum concentration was measured below 0.03 mg L1 in all experiments, which was smaller than the lowest LC50 for fishes reported in the literature [31]. Consideration of the joint toxicity of these products, due to insignificant observation of those quinone or hydroquinone intermediate implying they are rapidly oxidized, could be ignored [31]. Especially, they would be converted to low-molecular-weight carboxylic acids with time. In UV visible spectroscopy scan results of Fig. 5(A), signal response near 220 nm amplifying with the running time was observed, which probably were the response of carboxylic acids. Using phenol-Cr(VI) CRFC can realize the phenol oxidation; the main products are organic acids of low toxicity.
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Table 2 Effect of the initial Cr(VI) concentration in 0.5 M H2SO4 with a constant amount of 940 mg L1 phenol in 0.2 M PBS. Initial Cr(VI) conc. (mg L1)
Cr(VI) conc. at time t (mg L1)
Phenol conc. at time t (mg L1)
Cr(VI) removal efficiency (%)
Phenol removal efficiency (%)
OCV (V)
Cathodic efficiency (%)
10a 50b 150c 300d
0.10 0.36 0.35 0.50
860.5 712.2 630.9 411.9
99.0 99.3 99.8 99.8
8.46 24.2 32.9 56.2
0.90 0.95 1.06 1.03
95.62 89.81 89.11 77.5
a = 18 h, b = 30.5 h, c = 48.1 h, d = 60 h.
The phenol electro-oxidation occurs at the anode to release electrons. Those electrons will transfer through the external circuit to the cathode, which renders the electro-reduction of Cr(VI) to Cr(III) according to the following reaction: 3þ þ Cr2 O2 þ 7H2 O 7 þ 14H þ 6e ! 2Cr
ð1Þ
When the initial phenol concentration in the anolyte was maintained at 940 mg L1, Fig. 2(B) shows the voltage output under different initial Cr(VI) concentrations in the catholyte. When the initial Cr(VI) concentration varied from 50 mg L1 to 300 mg L1, the OCV of 0.95–1.03 V could be obtained. After an external resistor was loaded, the corresponding voltage output was 0.212 V and 0.363 V. After 30 h, the voltage output was below 30 mV for the initial Cr(VI) concentration of 50 mg L1, but for the concentration of 300 mg L1, the voltage was above 100 mV. Consequently, with the Cr(VI) reduction reaction, the voltage decreased from 0.363 V to 17 mV, which corresponded to a Cr(VI) removal efficiency of 99.8% (Table 2). Higher initial Cr(VI) concentrations resulted in higher Cr(VI) and phenol removal amount. Differently, cathodic electron utilization efficiency decreased from 95.62% to 99.7% with increasing the initial Cr(VI) levels and extension of time. It is noticed that lower initial Cr(VI) concentration of 10 mg L1, after 18 h running, the finial concentration reduced to 0.1 mg L1, which would meet the requirement of the European discharge standard and US EPA criteria [32,33]. When varying the Cr(VI) from 300 to 10 mg L1, the OCV insignificantly decreased from 1.03 V to 0.90 V at 0.5 M H2SO4. That means the OCV is not much dependent on the oxidant concentration. The process is extremely useful for contaminants reduction although the electricity generation is not remarkable. Sufficient phenol producing electrons can maintain high Cr(VI) removal, more than 99% removal efficiency at initial Cr(VI) concentration of 300 mg L1 can be easily achieved, as well as much higher Cr(VI) concentration can be predictably reduced. If the abundance of Cr(VI) was in presence in the catholyte, more phenol removal also can be reached. Twice refilling 300 mg L1 Cr(VI) in the catholyte can result in 98.6% removal of 940 mg L1 phenol. All these results imply that this method can be flexibly designed for wastewater treatment, and is practically useful for pretreatment of contaminants removal. From Eq. (1), pH is another important factor affecting the voltage output and the Cr(VI) removal. When the Cr(VI) concentration was maintained at 150 mg L1 Cr(VI), the H2SO4 concentration was changed from 0.05 M to 0.5 M, Fig. 2(C) shows that the OCV changed from 0.96 V to 1.06 V, and the initial potential slightly changed from 0.307 V to 0.376 V with a load. Next, all cell potentials tend to be identical after 10 h. Under typical conditions, the acid concentration changes the reduction potential according to the Nernst equation from 1.19V (0.05 M H2SO4) to 1.33 V (0.5 M H2SO4). A phenol oxidation under the fixed conditions has the same reduction potential. Consequently, the combination of the two half reactions can generate a difference in the OCV and the operating potential. 0.05 M H2SO4 provides sufficient protons for the Cr(VI)
Table 3 Effect of the initial pH in 150 mg L1 Cr(VI) concentration and 940 mg L1 phenol in 0.2 M PBS. H2SO4 Conc. (M)
Cr(VI) conc. at time t (mg L1)
Phenol conc. at time t (mg L1)
Cr(VI) removal efficiency (%) Rc
Phenol removal efficiency (%) Rc0
OCV (V)
Cathodic efficiency (%)
0.05a 0.10a 0.25a 0.5a
6.49 5.47 1.03 1.02
760.2 677.2 658.9 647.4
96.7 96.4 99.3 99.3
19.1 28.0 29.9 31.1
0.96 1.02 1.03 1.06
97.05 96.64 97.74 98.30
a = 30 h.
reduction under the giving conditions of 150 mg L1 Cr(VI). Thus, the Cr(VI) and phenol removal has close values under different pH values in catholyte, which can be observed in Table 3. Identically, the cathodic efficiency held at 97% for all pH. 3.4. Sustainability of phenol and Cr(VI) removal in CRFCs To evaluate the sustainability of electricity production with phenol and Cr(VI) removal in the CRFC, a fed-batch mode test was performed with a CRFC, i.e., by refilling the phenol solution (940 mg L1 in 0.2 M PBS) and the Cr(VI) solution (150 mg L1 in 0.5 M H2SO4) every 25 h. Five cycles were continuously performed. The voltage output is shown in Fig. 6. An OCV of 1.02 V can be obtained in the first cycle. The output voltage remains almost the same in all cycles. During the test, a stable phenol and Cr(VI) removal, which was greater than 23% and 96% every cycle with ca. 25 h, respectively, was achieved (Table 4). In another CRFC, 300 mg L1 Cr(VI) was filled into the catholyte twice, which removed 98.6% of 940 mg L1 phenol with a total time of 132 h. All of these tests indicate that continuous electricity generation and the removal of phenol and 0.4
1st 4th 2nd
0.3
3th
Voltage (V)
3.3. Cr(VI) removal
5th
0.2
0.1
0.0 0
20
40
60
80
100
120
140
Time (h) Fig. 6. Voltage output with time of a fed-batch mode test (anolyte: 940 mg L1 phenol in 0.2 M PBS (pH 8), catholyte 150 mg L1 in 0.5 M H2SO4).
H.-M. Zhang et al. / Separation and Purification Technology 172 (2017) 152–157 Table 4 Results of the continuous phenol and Cr(VI) removal in a fed-batch mode CRFC. Number of cycles
Phenol conc. (mg L1) Start
End
1a 2b 3b 4b 5b
940 940 940 940 940
Average
940
Phenol removal (%)
Cr(VI) Conc. (mg L1)
Cr(VI) removal (%)
Start
End
647.4 687.5 705.0 712.6 720.4
31.1 26.9 25.0 24.2 23.4
150 150 150 150 150
1.02 5.20 5.22 5.82 6.49
99.3 96.5 96.5 96.1 95.7
694.6
26.1
150
4.75
96.8
a = 30 h, b = 25.5 h.
Cr(VI) in a CRFC are feasible, and the contaminant removal performance is sustainable. 4. Conclusions Different from the traditional approaches for phenol removal, our work is putting forward a new method of energy extraction from waste by assembling pollutant coupled redox fuel cell. Recovering electricity and simultaneously reducing toxicity of phenol and Cr(VI) is of great significant. The result of the phenol-Cr(VI) CRFC is an example of energy extraction from wastewater. Although the production of electricity is modest probably due to the sluggish electrode reaction and high internal resistance of the device, which could be predictably reduced to improve the performance on electricity generation, whatever simultaneous removal of phenol and Cr(VI) from aqueous solution within self-powered process is achievable. Acknowledgements This work is financially supported by the National Natural Science Foundation of China (Nos. 21473158, 51178225), Postdoctoral Science Foundation of China (No. 2015M581941), Youth Science Foundation of Jiangxi Province, China (20161BAB216137) and Science Foundation of Jiangxi Province Department of Education, China (GJJ150520). References [1] B.H. Wang, H.J. Wu, G.X. Zhang, S. Licht, STEP wastewater treatment: a solar thermal electrochemical process for pollutant oxidation, ChemSusChem 5 (2012) 2000–2010. [2] J.R. Sun, H.Y. Lu, H.B. Lin, Electrochemical oxidation of aqueous phenol at low concentration using Ti/BDD electrode, Sep. Purif. Technol. 88 (2012) 116–120. [3] C.A. Aguilar, T. Pandiyan, J.A. Arenas-Alatorre, Oxidation of phenols by TiO2-Fe3O4-M(M = Ag or Au) hybrid composites under visible light, Sep. Purif. Technol. 149 (2015) 265–278. [4] S. Kunduz, G.S.P. Soylu, Highly active BiVO4 nanoparticles: the enhanced photocatalytic properties under natural sunlight for removal of phenol from wastewater, Sep. Purif. Technol. 141 (2015) 221–228. [5] Y. Yan, S.S. Jiang, H.P. Zhang, Efficient catalytic wet peroxide oxidation of phenol over Fe-ZSM-5 catalyst in a fixed bed reactor, Sep. Purif. Technol. 133 (2014) 365–374. [6] R.R.N. Marques, F. Stuber, K.M. Smith, A. Fabregat, C. Bengoa, J. Font, A. Fortuny, S. Pullket, G.D. Fowler, N.J.D. Graham, Sewage sludge based catalysts for catalytic wet air oxidation of phenol: preparation, characterisation and catalytic performance, Appl. Catal. B – Environ. 101 (2011) 306–316. [7] M. Saeed, M. Ilyas, Oxidative removal of phenol from water catalyzed by nickel hydroxide, Appl. Catal. B – Environ. 129 (2013) 247–254.
157
[8] Y.H. Cui, Y.J. Feng, X.Y. Li, Kinetics and efficiency analysis of electrochemical oxidation of phenol: influence of anode materials and operational conditions, Chem. Eng. Technol. 34 (2011) 265–272. [9] Y. Yavuz, A.S. Koparal, Electrochemical oxidation of phenol in a parallel plate reactor using ruthenium mixed metal oxide electrode, J. Hazard. Mater. 36 (2006) 296–302. [10] Z.Y. Zhang, D. Liba, L. Alvarado, A.C. Chen, Separation and recovery of Cr(III) and Cr(VI) using electrodeionization as an efficient approach, Sep. Purif. Technol. 137 (2014) 86–93. [11] M. Tilahun, O. Sahu, M. Kotha, H. Sahu, Cogeneration of renewable energy from biomass (utilization of municipal solid waste as electricity production: gasification method), Mater. Renew. Sustain. Energy 4 (2015). [12] B. Baran, M.S. Mamis, B.B. Alagoz, Utilization of energy from waste potential in Turkey as distributed secondary renewable energy source, Renew. Energy 90 (2016) 493–500. [13] Q.P. Chen, J.H. Li, X.J. Li, K. Huang, B.X. Zhou, W.M. Cai, W.F. Shangguan, Visible-light responsive photocatalytic fuel cell based on WO3/W photoanode and Cu2O/Cu photocathode for simultaneous wastewater treatment and electricity generation, Environ. Sci. Technol. 46 (2012) 11451–11458. [14] Y.L. Xu, Y. He, C. Yang, Y. Wang, L.J.P. Jia, Photocatalytic fuel cell (PFC) and dye self-photosensitization photocatalytic fuel cell (DSPFC) with BiOCl/Ti photoanode under UV and visible light irradiation, Environ. Sci. Technol. 47 (2013) 3490–3497. [15] H.M. Zhang, W. Xu, Z.C. Wu, M.H. Zhou, T. Jin, Removal of Cr(VI) with cogeneration of electricity by an alkaline fuel cell reactor, J. Phys. Chem. C 117 (2013) 14479–14484. [16] B.B. Yu, H.M. Zhang, W. Xu, G. Li, Z.C. Wu, Remediation of chromium-slag leakage with electricity cogeneration via a urea-Cr(VI) cell, Sci. Rep. 4 (2014) (2014) 6. [17] W. Xu, H.M. Zhang, G. Li, Z.C. Wu, A urine/Cr(VI) fuel cell—electrical power from processing heavy metal and human urine, J. Electroanal. Chem. 784 (2016) (2016) 38–44. [18] D.R. Lide, in: Handbook of Chemistry and Physics, CRC PRESS, 2003–2004, pp. 842–897. [19] M.M. Tusi, N.S.O. Polanco, S.G. da Silva, E.V. Spinace, A.O. Neto, The high activity of PtBi/C electrocatalysts for ethanol electro-oxidation in alkaline medium, Electrochem. Commun. 13 (2011) 143–146. [20] L. Jiang, A. Hsu, D. Chu, R. Chen, Ethanol electro-oxidation on Pt/C and PtSn/C catalysts in alkaline and acid solutions, Int. J. Hydrogen Energy 35 (2010) 365– 372. [21] S.Y. Shen, T.S. Zhao, J.B. Xu, Y.S. Li, Synthesis of PdNi catalysts for the oxidation of ethanol in alkaline direct ethanol fuel cells, J. Power Sour. 195 (2010) 1001– 1006. [22] P.S. Roy, J. Bagchi, S.K. Bhattacharya, The size-dependent anode-catalytic activity of nickel-supported palladium nanoparticles for ethanol alkaline fuel cells, Catal. Sci. Technol. 2 (2012) 2302–2310. [23] A.M.C. Luna, A. Bonesi, W.E. Triaca, A. Di Blasi, A. Stassi, V. Baglio, V. Antonucci, A.S. Arico, Investigation of a Pt-Fe/C catalyst for oxygen reduction reaction in direct ethanol fuel cells, J. Nanopart. Res. 12 (2010) 357–365. [24] M. Ferreira, H. Varela, R.M. Torresi, G. Tremiliosi, Electrode passivation caused by polymerization of different phenolic compounds, Electrochim. Acta 52 (2006) 434–442. [25] S. Patra, N. Munichandraiah, Electro-oxidation of phenol on polyethylenedioxythiophene conductive-polymer-deposited stainless steel substrate, J. Electrochem. Soc. 155 (2008) F23–F30. [26] D.L. Li, M. Koike, L. Wang, Y. Nakagawa, Y. Xu, K. Tomishige, Regenerability of hydrotalcite-derived nickel-iron alloy nanoparticles for syngas production from biomass tar, ChemSusChem 7 (2014) 510–522. [27] H.M. Zhang, W. Xu, D.L. Feng, Z.M. Liu, Z.C. Wu, Self-powered denitration of landfill leachate through ammonia/nitrate coupled redox fuel cell reactor, Bioresour. Technol. 203 (2016) 56–61. [28] W. Xu, H.M. Zhang, G. Li, Z.C. Wu, Nickel-cobalt bimetallic anode catalysts for direct urea fuel cell, Sci. Rep. 4 (2014) 6. [29] H.M. Zhang, W. Xu, G. Li, Z.M. Liu, Z.C. Wu, B.G. Li, Assembly of coupled redox fuel cells using copper as electron acceptors to generate power and its in-situ retrieval, Sci. Rep. 6 (2016) 21059. [30] M. Winter, R.J. Brodd, What are batteries, fuel cells, and supercapacitors?, Chem Rev. 104 (2004) 4245–4269. [31] Z.C. Wu, M.H. Zhou, Partial degradation of phenol by advanced electrochemical oxidation process, Environ. Sci. Technol. 35 (2001) 2698–2703. [32] European Commission Integrated Pollution Prevention and Control: Surface Treatment of Metals and Plastics, 2006, p. VI. [33] US EPA National Recommended Water Quality Criteria.