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Electricity production during the treatment of real electroplating wastewater containing Cr6+ using microbial fuel cell
Process Biochemistry 43 (2008) 1352–1358
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Process Biochemistry journal homepage: www.elsevier.com/locate/procbio
Electricity production during the treatment of real electroplating wastewater containing Cr6+ using microbial fuel cell Zhongjian Li, Xingwang Zhang, Lecheng Lei * Institute of Environmental Pollution Control Technologies, Xixi Campus, Zhejiang University, Hangzhou 310028, PR China
A R T I C L E I N F O
A B S T R A C T
Article history: Received 30 January 2008 Received in revised form 11 July 2008 Accepted 8 August 2008
Microbial fuel cell (MFC) was employed to dispose Cr6+ in real electroplating wastewater and generate electricity simultaneously. The experiments were carried out in a dual-chamber MFC. Under the condition of pH 2 and using graphite paper as the cathode electrode chromium removal and power density were highest. Moreover, increasing initial concentration of Cr6+ could enhance the power density. The results indicated that when treating a real electroplating wastewater containing Cr6+ with the initial concentration of 204 ppm in the MFC, the maximum power density of 1600 mW/m2 was generated at a columbic efficiency of 12%. In addition, 99.5% Cr6+ and 66.2% total Cr were removed through reduction of Cr2O72 to Cr2O3 precipitating on the surface of cathode electrode. MFC was proved to be a promising technology for removing Cr6+ from electroplating wastewater. ß 2008 Elsevier Ltd. All rights reserved.
Keywords: Microbial fuel cell (MFC) Electroplating wastewater Chromium Electricity generation XPS SEM
1. Introduction One of the critical pollution problems arising from the electroplating industry is the generation of wastewater containing heavy metals such as cadmium, copper and chromium [1]. Heavy metals especially chromium pose a serious risk to human, animals, and the environment. Hexavalent chromium usually occurs as highly soluble and highly toxic chromate anions, and is suspected carcinogens and mutagens. The accumulation of Cr6+ in living tissues throughout the food chain causes many serious health problems. Potable waters containing more than 0.05 mg/l Cr6+ are considered toxic [2,3]. Therefore, it is necessary to treat electroplating wastewater containing chromium prior to its discharge. Several treatment techniques such as chemical precipitation, coagulation–flocculation, ion exchange, membrane filtration [4] and biosorption [5] have been applied to dispose chromium in electroplating wastewater. Although conventional metal removal techniques are effective, they encountered with some major disadvantages including high energy requirements, excessive chemicals consumption, and generation of a large quantity of toxic waste sludge. It is proposed here that electroplating wastewater containing Cr6+ can be treated using microbial fuel cells (MFCs) which have been developed to generate electricity from wastewater. Microbial
* Corresponding author. Tel.: +86 571 88273090; fax: +86 571 88273693. E-mail address: [email protected] (L. Lei). 1359-5113/$ – see front matter ß 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.procbio.2008.08.005
fuel cells are of interest as a technology for the conversion of chemical energy to electricity [6–9]. The most commonly used MFCs in the laboratory are dual-chamber MFCs, which consist of an anode chamber and a cathode chamber separated by a proton exchange membrane (PEM). Microorganisms grow at the anode under anaerobic conditions. These microorganisms oxidize the organic matter and transfer electrons to the anode that then pass through an external circuit to the cathode producing current. Protons migrate through the proton exchange membrane to the cathode where they combine with oxygen and electrons to form water [7]. Recently, MFCs have been developed to generate electricity directly from complex organic wastewater such as food wastewater [10,11], domestic wastewater [12–14], swine wastewater [15], chemical wastewater [16,17] and other wastewaters [18–23]. Min et al. [15] treated swine wastewater by a dualchamber MFC. They found that 86% removal of COD and 83% removal of NH4–N was achieved and the maximum power density was 45 mW/m2. Liu et al. [24] employed a single-chamber MFC to treat domestic wastewater. The maximum power density generated during the treatment process was 26 mW/m2 and 80% COD was removed. Previous studies have demonstrated that the energy-efficient treatment of wastewater is one of the most promising applications of MFCs. However, in most of those systems, wastewater as the electron donors was treated in the anode chamber by microorganisms. The effect of cathode chamber on electricity generation was just to form the pass way for electrons and protons. Oxygen [8,9,25], hexacynoferrate [26] or permanganate [27] was commonly used as electron acceptors.
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Therefore, if using certain pollutant with the ability of accepting electrons and protons as oxidant, the cathode chamber of MFCs, as well as the anode chamber could harvest considerable energy from pollutants. This would greatly enhance the environmental benefit of MFCs [28]. Based on this hypothesis, electroplating wastewater containing Cr6+ was chosen as the cathodic electron acceptor in this study. Under acidic conditions, Cr6+ ion accepts six electrons and thus is reduced to Cr3+ ion as illustrated in the following equations: Cr2 O7 2 þ 8Hþ þ 6e ¼ Cr2 O3 þ 4H2 O; Cr2 O3 þ 6Hþ ¼ 2Cr3þ þ 3H2 O
1:33 V
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PEM which was sequentially boiled in H2O2 (30%), deionized water, 0.5 M H2SO4, and then deionized water (each time for 1 h) as described by Liu and Logan [29] was held between the two rectangular chambers with a rubber gasket to prevent leakage. The anode chamber sealed with a rubber stopple was mixed with a magnetic stir bar. Plain carbon felt (2 cm 5 cm, 2 mm thick, Liaoning, China) with rough surface and porosity was placed in the anode chamber to accumulate the electron-transferring bacteria. Plain graphite paper (2 cm 5 cm, 0.2 mm thick, Jiangsu, China) was inserted into the cathode chamber to act as a cathode (except as noted). The electrodes were connected with a copper wire coming out of the rubber stopple to provide the connection points for the external circuit. An air-sparger was fixed in the bottom of cathode chamber for air sparging during inoculation or nitrogen sparging during the wastewater treatment.
(1) 2.2. Inoculation
(2)
The equations indicated that in acidic environment Cr6+ has higher oxidation potential (1.33 V vs. SHE) than oxygen (1.23 V) and hexacynoferrate (0.36 V) [26]. So it can be deduced that potassium dichromate is a more favorable electron acceptor theoretically. To the best of our knowledge, potassium dichromate has not been used as the cathodic electron acceptor in MFCs. In this work, the objective is to produce electricity directly from electroplating wastewater using a MFC combined with accomplishing wastewater treatment. Therefore, several factors that might affect the removal of chromium and electricity generation including pH, initial Cr6+ ion concentration and different electrode materials were investigated using synthetic wastewater in a dualchamber MFC. Also, to demonstrate the feasibility of this novel MFC treatment process, we examined the treatment efficiency of a real electroplating wastewater containing Cr6+. The reduced production of Cr6+ was analyzed by SEM and XPS.
Inoculation was conducted in the dual-chamber MFC mentioned above. Anaerobic sludge collected from the anaerobic digester of SiBao wastewater treatment plant in Hangzhou, China, was used as an inoculum in the anode chamber. Anaerobic sludge has been shown to be a suitable biocatalyst for electricity production [24,26]. For inoculation, a mixed solution of 50 ml anaerobic sludge with 150 ml sodium acetate medium was injected into the anode chamber. The anode medium contained the following (per liter): KH2PO4, 13.60 g; CH3COONa, 1.00 g; NaCl, 11.70 g; NaOH, 2.33 g [15]; NH4Cl, 0.45 g; MgCl26H2O, 0.17 g; FeCl36H2O, 1.00 mg; MnCl24H2O, 23.0 mg; CaCl2, 15.0 mg [7] and 1 ml of a trace element solution as reported by Logan et al. [25]. The cathode medium contained the following (per liter): KH2PO4, 13.60 g; NaCl, 11.70 g; NaOH, 2.33 g. KH2PO4 and NaOH were added in both anode and cathode medium forming PBS (pH 7). And NaCl was added to increase the solution conductivity. The cathode chamber was sparged with air (80 ml/min) using an air pump, during the inoculation stage. The electrodes were connected via an external circuit containing a single resistor (R = 1000 V). Experiments were conducted in a 35 8C temperature-controlled box (SPX-250B-Z, Shanghai, China). The anode chamber was added with 80 mg sodium acetate every day until the stable output voltage increase to 240–260 mV, which indicated bacteria have colonized the electrodes. 2.3. Operation of MFC
2. Materials and methods 2.1. MFC construction The experimental set-up was shown in Fig. 1. MFC comprised two (anode and cathode) 220 ml (h = 11 cm, w ¼ 4 cm, l = 5 cm, effective volume = 200 ml) plexiglass rectangular chambers. The chambers were physically separated by proton exchange membrane (NafionTM 117, Dupont Co.) with a 45 cm2 surface area. The
After the inoculation was finished, the anode chamber was refilled with 200 ml anode medium which was sparged with N2 for 0.5 h (60 ml/min) to remove dissolved oxygen. Synthetic wastewater was refilled in the cathode chamber and continuously sparged with N2 (80 ml/min) to fully mix the cathode medium. The synthetic wastewater (per liter) contained KH2PO4, 13.6g and NaCl, 11.7 g with different amount of K2Cr2O7. The pH was adjusted with H2SO4 (50%). Plain graphite paper (2 cm 5 cm, 0.2 mm thick) was used as the cathode electrode except where stated otherwise. Electrodes were connected via a circuit containing a resistance of 1000 V (except as noted) to measure electricity generation and Cr6+ removal. To determine the effect of various parameters on power generation and Cr6+ removal, pH (1–6) and initial concentration of Cr6+ (50–500 ppm), different electrode materials including graphite paper, carbon paper (120, Toray Co., Japan) and carbon felt were investigated. Different external resistance (10–9000 V) was applied in order to obtain polarization curve for determination of the maximum power generation. 2.4. Calculations Voltage was measured using a multimeter with a data acquisition system (UNT UT-70B, China) and converted to power density, P (W/m2), according to P¼
y2 RA
where R is the resistance, y is the voltage, and A is the cross-sectional area (projected) of the anode. The Columbic efficiency was calculated as Ec ¼
CP 100% CT
where Cp is the total coulombs calculated by integrating the current over time. CT is the theoretical amount of coulombs that can be produced from sodium acetate, calculated as CT ¼
bCVF M
where F is Faraday’s constant (96485 C/mol e), M is relative molecular mass (82 g/ mol by sodium acetate), b is the number of mol of electrons produced per mol of substrate (b = 8 mol e/mol based on sodium acetate), C (mg/l) is the overall removal of sodium acetate, y (200 ml) is the anode liquid volume. 2.5. Analytical methods Fig. 1. Schematic prototype of the MFC used to generate electricity from electroplating wastewater.
Acetate was analyzed using ion chromatograph (IC1000 Techcomp, China) equipped with an anion separation column (SI-90 Shodex, Japan). Samples were
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filtered through a 0.22 mm pore diameter membrane. Cr6+ was determined by colorimetry using diphenylcarbazide. Total chromium was analyzed using atom absorption spectrometry (AA-6800 Shimadzu, Japan). Microorganisms on anodic electrodes and cathodic electrodes were examined using a scanning electron microscope (SEM) (XL-30 Philips, Holland). Determination of the chemical composition of cathodic reduced products was done by using an X-ray photoelectron spectrometer (XPS) (PHI 5000C ESCA System, USA).
3. Results and discussion 3.1. The start up of MFC using Cr6+ ion as the electron acceptor During the start-up stage, stable power generation was obtained after two cycles. Fig. 2 showed the output voltage (R = 1000 V) steadily increased from 100 mV at 232.4 h to about 240 mV at 233.5 h when the anode chamber was refilled with the fresh anode medium (point A). And then the synthetic wastewater containing 50 ppm potassium dichromate (pH 2) was refilled the cathode chamber and N2 was continuously sparged at a flow rate of 80 ml/min (point B). This led to a substantial increase of voltage to 605 mV. Such a sharp increase of voltage was most likely due to the high redox potential (1.33 V) of Cr2O72 in acidic conditions as illustrated in Eq. (1). The SEM images (Fig. 3) shows that anodic electrode surface is covered by bacilliform bacteria which are responsible for electron transfer and current generation in the MFC.
Fig. 3. SEM image of electron-transfer bacteria attached to the surface of the anode.
3.2. Effects of pH on power density and removal efficiency of Cr6+ ion To determine the effect of pH, the power density and the removal of chromium were examined using a synthetic wastewater containing 50 ppm potassium dichromate. The pH of synthetic wastewater was adjusted to 1–6 with H2SO4 (50%). Fig. 4 illustrates that Cr6+ removal increases from 28% to 98.3%, as the pH decreases from 6 to 2. And green deposit could be evidently observed on the surface of cathode. However, the removal was not further increased at lower pH (pH 1). This was attributed to the H+ in cathode chamber diffused to anode chamber at pH 1, leading to a decrease in pH of anodic medium from 6.8 to 1.8 over the course of experiment. The low pH of anodic medium deactivated the microorganism and reduced the performance of MFC. The removal of total chromium followed a similar trend. The highest removal of 49.1% total chromium was achieved at pH 2, while removal decreasing to 18% at pH 6 and to 0 at pH 1. We supposed that Cr3+
Fig. 4. Maximum power density generated and chromium removal as a function of pH (50 ppm potassium dichromate).
was removed in the form of trivalent chromium oxides depositing on the surface of cathode. Moreover, trivalent chromium oxides dissolved in the cathode medium pH 1. Fig. 4 also shows the effect of pH on the maximum power density. The maximum power density was increased from 67.6 to 705 mW/m2 as pH decreased from 6 to 1. This revealed that low pH played a positive effect on power generation when potassium dichromate was used as electron acceptor in the MFC. The conclusion was confirmed by the power density generated over 9 h experiments with an external resistance of 1000 V (Fig. 5). The sharp decrease of power density at pH 1 observed in Fig. 5 was possibly due to the diffusion of H+ to anode chamber mentioned above. 3.3. Effects of initial Cr6+ concentration on output power density and cathode potential
Fig. 2. Electricity generation by MFCs during startup using potassium dichromate. (A) Refill the anode chamber with fresh anode medium. (B) Refill the cathode chamber with synthetic wastewater containing 50 ppm potassium dichromate, pH 2.
To determine the effect of initial Cr6+ concentration on the performance of MFC, Cr6+ concentration was set in the range of 50– 500 ppm. Based on the above-mentioned results, the pH of cathode medium was adjusted to 2 and then the effect of Cr6+ concentration on cathode OCP was examined by placing a saturated calomel electrode in the cathode chamber. Fig. 6 shows that increasing the initial Cr6+ concentration from 50 to 500 ppm increases the maximum power density by a factor of 3.54 from 602 to 2116 mW/ m2 and the OCP increases from 619 mV to 814 mV. The
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Fig. 5. Comparison of power generated in different pH (50 ppm potassium dichromate, R = 1000 V).
Fig. 6. Maximum power density and open circle cathode potential as a function of initial concentration of potassium dichromate (pH 2).
improvement in maximum power density might be due to the increase of cathode OCP which was influenced by Cr6+ concentration and pH. The phenomena were generally consistent with the trends expected from analysis using the Nernst equation based on Eq. (1):
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Fig. 7. Comparison of power generated using different cathode materials (50 ppm potassium dichromate, pH 2, R = 1000 V).
water containing 50 ppm potassium dichromate at optimal pH of 2 was used to assess the effect of different electrode materials on the performance of MFC. As shown in Fig. 7, the power density of graphite paper cathode slowly increased to the maximum (348 mW/m2) at the first 2 h, and gradually decreased to 5.7 mW/m2 in the next 7 h as Cr6+ was reduced. For carbon paper cathode, the power density of achieved the maximum value (291.6 mW/m2) in 10 min, and decrease slowly to 13.3 mW/m2 within the next 8.5 h. As for the carbon felt cathode, the maximum power density (678.9 mW/m2) was generated in 10 min, and decrease sharply to a low level (<50 mW/m2). The sharp decrease could be ascribed to some changes on the surface of carbon felt, which were induced by electricity and oxidation effects. The internal resistances kept about 300 V, not affected by the electrode materials. The maximum power density of graphite paper, carbon paper and carbon felt were 616, 408.3 and 761 mW/m2, respectively, for an external resistance of 300 V. From Fig. 8, it can be seen that the removal of Cr6+ in 9 h is 98.3% for graphite paper cathode, 86.3% for carbon paper and 40.6% for carbon felt. The graphite paper had the best performance on removing Cr6+. Moreover, 49% of total chromium could be reduced using graphite paper, while only 26.2% and 27.6% total chromium could be removed using carbon paper and carbon felt, respectively. After 9 h, a green precipitate could be observed on the surface of
E ¼ 1:33 þ 0:01 log½Cr2 O7 2 0:08 pH Another reason for the increase in maximum power density might be the decrease in internal resistance as a result of increase in ionic strength [30]. At 50 ppm Cr6+, the internal resistance was 300 V and then decreased to 100 V as the Cr6+ concentration increased to 500 ppm. You et al. adopted 100 ppm potassium permanganate as the electron acceptor in a dual-chamber MFC. And a maximum power density of 115.6 mW/m2 was generated [27]. Gu et al. revealed that a maximum power density of 12.4 mW/m2 was generated using 50 ppm chlorophenol as the electron acceptor [28]. In contrast to potassium permanganate and chlorophenol, potassium dichromate was a more favorable electron acceptor, showing a higher power density of MFC. 3.4. Effects of different electrode materials on power density and removal of Cr6+ Graphite paper, carbon paper and carbon felt were chosen as the cathode electrodes in this investigation. A synthetic waste-
Fig. 8. Chromium removal obtained using different cathode materials (50 ppm potassium dichromate, pH 2).
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graphite paper and carbon paper, while few precipitate could be found on carbon felt. The removal of Cr6+ and total chromium was related with the output power density. Higher power density produced faster removing of chromium. During 9 h treatment, the redox potential of NAD+/NADH in the anode varied between 320 and 355 mV, which was similar to that reported by Rabaey and Verstraete [31]. Hence, compared with carbon paper and carbon felt, the MFC using graphite paper cathode with higher cathode potential showed the highest power density and the most efficient removal of chromium. 3.5. Treatment of real electroplating wastewater using MFC system To demonstrate the feasibility of this new type of MFC treatment process, we examined the treatment of a real electroplating wastewater. All the real wastewater used in this study was collected from an actual electroplating plant. The concentration of potassium dichromate was about 204 ppm and the pH was 2.5. After 25 h treatment, the removal of Cr6+ and total chromium reached 99.5% and 66.2%, respectively. Table 1 compared the removal of Cr6+ in the literatures with that obtained in this study. In contrast to other treatment process, the main advantages of MFC for electroplating wastewater treatment were (1) simultaneous electricity generation, (2) higher Cr6+ concentration tolerance compared to other biological treatment and (3) no toxic waste sludge production. The power density generated over the process was showed in Fig. 9. The maximum power density was 1600 mW/ m2 at a current density of 0.4 mA/cm2 with an external resistance of 100 V (Fig. 10). The obtained maximum power density was higher than that reported by other researchers in the treatment of wastewater using MFC, e.g. the maximum of 142.41 mW/m2 was obtained during the chemical wastewater treatment using MFC reported by Mohan et al. [17], 25 mW/m2 generated during urban wastewater treatment reported by Rodrigo et al. [14] And the maximum of 218 mW/m2 generated during domestic wastewater treatment reported by Min and Angelidaki [39]. The columbic efficiency of the system was 12% which was comparable to <15% reported by Liu et al. [9], indicating there was substantial COD removal that was not associated with power generation. With respect to reaction mechanism, the removal of Cr6+ was accomplished by the whole MFC system, including the proton and electron generation phase. Under anaerobic condition, microorganisms in anode chamber oxidized acetate to generate protons and electrons (Eq. (3)): CH3 COO þ 4H2 O ! 2HCO3 þ 9Hþ þ 8e
(3)
And then electrons were transferred to anode and finally to cathode through the circuit, while proton was transferred to Table 1 Cr6+ removal reported in the literatures and that obtained in this study Methods
Initial concentration (mg/l)
Removal (%)
Reference
Nanofiltration Biosorption Biosorption Fluidized zero valent iron process Combined electrocoagulation– electroflotation Electrocoagulation Biosorption by surfactant modified coconut coir pith Microbial fuel cells
17.09 30 100 534
96.6 86.61–91.03 95 99.9
[32] [33] [34] [35]
10
97
[36]
24 20
92 96
[37] [38]
99.5
This study
204
Fig. 9. Power generated in the MFC using real electroplating wastewater as electron acceptor (pH 2, R = 1000 V).
Fig. 10. Power generation as a function of the current density using potassium dichromate (pH 2).
cathode chamber through the PEM. Cr6+ was reduced by the protons and electrons as illustrated in Eq. (1). The reduction of Cr6+ is strongly dependent on pH. Therefore, decreasing the pH makes the reaction more favorable. For practical application, the wastewater containing Cr6+ can be mixed with acid cleaning wastewater which is also discharged from electroplating industry to decrease the pH value. Thus, no additional chemicals are needed in the treatment process. Also, a continuous operation was conducted in sequencing batch model for three cycles without significant reduction in capacity for power generation and Cr6+ removal. These suggested that MFC should be an economical and sustainable method for removing Cr6+ from electroplating wastewater. The electricity produced by MFC can be used for monitoring the process. The high output power density and efficient removal of chromium makes this MFC treatment process useful not only for treating pollutants but also for electricity production. In addition, trivalent chromium oxide recovered in MFC treatment systems following purification can have additional market value. Thus, the system may also be applied in a metal recovery process. The system used here was a very basic type of MFC that has been used to examine factors that can increase power generation in these types of applications. However, the development of an economical treatment system
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on the graphite paper, XPS analysis was applied. As shown in Fig. 12, binding energies peaks corresponded to 575.6 eV, pointing to a fact that Cr2O3 was the main cathode products [40]. 4. Conclusion Treatment of electroplating wastewater containing Cr6+ and production electricity was carried out successfully in the cathode chamber of dual-chamber microbial fuel cell. pH value affected the power density and chromium removal obviously and the optimal pH value was 2. In addition, higher concentration of Cr6+ could enhance the power density. Compared with carbon paper and carbon felt cathode, the graphite paper cathode showed the best performance on chromium removal and electricity generation. During the 25 h treatment of real electroplating wastewater containing 204 ppm Cr6+, the maximum power density generated was up to 1600 mW/ m2 accompanied by 99.5% of Cr6+ and 66.2% of total chromium removal, respectively. The removal of chromium from wastewater was mainly due to the deposition of Cr2O3 on the surface of cathode. The considerable high power density, higher Cr6+ concentration tolerance compared to other biological treatment and no hazardous waste sludge production demonstrated that microbial fuel cell was a promising technology to treat electroplating wastewater and simultaneously generate electricity. Acknowledgements
Fig. 11. SEM image of (A) plain cathode and (B) surface of cathode in the MFC using potassium dichromate as electron acceptor (500 ppm potassium dichromate pH 2, reacting time of 35 h).
The authors would like to acknowledge financial support for this work provided by the National Science Foundation of China (Nos. 20576120, 90610005 and U0633003), project of Zhejiang Province (No. 2007C13061) and ‘‘86300 project of China (No. 2007AA06Z339). References
will require a system architecture that is scaleable. Improvements in system architecture are under investigation and will be reported in due course. 3.6. Characteristics of the cathode surface SEM images clearly show deposits formed on the surface of the cathode (Fig. 11). To identify the chemical form of those deposits
Fig. 12. XPS analysis for the surface of the cathode (500 ppm potassium dichromate pH 2, reacting time of 35 h).
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[27] You SJ, Zhao QL, Zhang JN, Jiang JQ, Zhao SQ. A microbial fuel cell using permanganate as the cathodic electron acceptor. J Power Sources 2006; 162:1409–15. [28] Gu HY, Zhang XW, Li ZJ, Lei LC. Studies on treatment of chlorophenol-containing wastewater by microbial fuel cell. Chin Sci Bull 2007;52:3448–51. [29] Liu H, Logan BE. Electricity generation using an air-cathode single chamber microbial fuel cell in the presence and absence of a proton exchange membrane. Environ Sci Technol 2004;38:4040–6. [30] Liu H, Cheng SA, Logan BE. Power generation in fed-batch microbial fuel cells as a function of ionic strength, temperature, and reactor configuration. Environ Sci Technol 2005;39:5488–93. [31] Rabaey K, Verstraete W. Microbial fuel cells: novel biotechnology for energy generation. Trends Biotechnol 2005;23:291–8. [32] Wang Z, Liu GC, Fan ZF, Yang XT, Wang JX, Wang SC. Experimental study on treatment of electroplating wastewater by nanofiltration. J Membr Sci 2007;305:185–95. [33] Kumar R, Bishnoi NR, Garima BK. Biosorption of chromium(VI) from aqueous solution and electroplating wastewater using fungal biomass. Chem Eng J 2008;135:202–8. [34] Congeevaram S, Dhanarani S, Park J, Dexilin M, Thamaraiselvi K. Biosorption of chromium and nickel by heavy metal resistant fungal and bacterial isolates. J Hazard Mater 2007;146:270–7. [35] Chen SS, Cheng CY, Li CW, Chai PH, Chang YM. Reduction of chromate from electroplating wastewater from pH 1 to 2 using fluidized zero valent iron process. J Hazard Mater 2007;142:362–7. [36] Gao P, Chen XM, Shen F, Chen GH. Removal of chromium(VI) from wastewater by combined electrocoagulation–electroflotation without a filter. Sep Purif Technol 2005;43:117–23. [37] Adhoum N, Monser L, Bellakhal N, Belgaied JE. Treatment of electroplating wastewater containing Cu2+, Zn2+ and Cr (VI) by electrocoagulation. J Hazard Mater 2004;112:207–13. [38] Namasivayam C, Sureshkumar MV. Removal of chromium(VI) from water and wastewater using surfactant modified coconut coir pith as a biosorbent. Bioresource Technol 2007;99:2218–25. [39] Min B, Angelidaki I. Innovative microbial fuel cell for electricity production from anaerobic reactors. J Power Sources 2008;180:641–7. [40] Wagner CD, Riggs WM, Davis LE. Handbook of X-ray photoelectron spectroscopy. PerkinElmer Corporation; 1979.
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Process Biochemistry journal homepage: www.elsevier.com/locate/procbio
Electricity production during the treatment of real electroplating wastewater containing Cr6+ using microbial fuel cell Zhongjian Li, Xingwang Zhang, Lecheng Lei * Institute of Environmental Pollution Control Technologies, Xixi Campus, Zhejiang University, Hangzhou 310028, PR China
A R T I C L E I N F O
A B S T R A C T
Article history: Received 30 January 2008 Received in revised form 11 July 2008 Accepted 8 August 2008
Microbial fuel cell (MFC) was employed to dispose Cr6+ in real electroplating wastewater and generate electricity simultaneously. The experiments were carried out in a dual-chamber MFC. Under the condition of pH 2 and using graphite paper as the cathode electrode chromium removal and power density were highest. Moreover, increasing initial concentration of Cr6+ could enhance the power density. The results indicated that when treating a real electroplating wastewater containing Cr6+ with the initial concentration of 204 ppm in the MFC, the maximum power density of 1600 mW/m2 was generated at a columbic efficiency of 12%. In addition, 99.5% Cr6+ and 66.2% total Cr were removed through reduction of Cr2O72 to Cr2O3 precipitating on the surface of cathode electrode. MFC was proved to be a promising technology for removing Cr6+ from electroplating wastewater. ß 2008 Elsevier Ltd. All rights reserved.
Keywords: Microbial fuel cell (MFC) Electroplating wastewater Chromium Electricity generation XPS SEM
1. Introduction One of the critical pollution problems arising from the electroplating industry is the generation of wastewater containing heavy metals such as cadmium, copper and chromium [1]. Heavy metals especially chromium pose a serious risk to human, animals, and the environment. Hexavalent chromium usually occurs as highly soluble and highly toxic chromate anions, and is suspected carcinogens and mutagens. The accumulation of Cr6+ in living tissues throughout the food chain causes many serious health problems. Potable waters containing more than 0.05 mg/l Cr6+ are considered toxic [2,3]. Therefore, it is necessary to treat electroplating wastewater containing chromium prior to its discharge. Several treatment techniques such as chemical precipitation, coagulation–flocculation, ion exchange, membrane filtration [4] and biosorption [5] have been applied to dispose chromium in electroplating wastewater. Although conventional metal removal techniques are effective, they encountered with some major disadvantages including high energy requirements, excessive chemicals consumption, and generation of a large quantity of toxic waste sludge. It is proposed here that electroplating wastewater containing Cr6+ can be treated using microbial fuel cells (MFCs) which have been developed to generate electricity from wastewater. Microbial
* Corresponding author. Tel.: +86 571 88273090; fax: +86 571 88273693. E-mail address: [email protected] (L. Lei). 1359-5113/$ – see front matter ß 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.procbio.2008.08.005
fuel cells are of interest as a technology for the conversion of chemical energy to electricity [6–9]. The most commonly used MFCs in the laboratory are dual-chamber MFCs, which consist of an anode chamber and a cathode chamber separated by a proton exchange membrane (PEM). Microorganisms grow at the anode under anaerobic conditions. These microorganisms oxidize the organic matter and transfer electrons to the anode that then pass through an external circuit to the cathode producing current. Protons migrate through the proton exchange membrane to the cathode where they combine with oxygen and electrons to form water [7]. Recently, MFCs have been developed to generate electricity directly from complex organic wastewater such as food wastewater [10,11], domestic wastewater [12–14], swine wastewater [15], chemical wastewater [16,17] and other wastewaters [18–23]. Min et al. [15] treated swine wastewater by a dualchamber MFC. They found that 86% removal of COD and 83% removal of NH4–N was achieved and the maximum power density was 45 mW/m2. Liu et al. [24] employed a single-chamber MFC to treat domestic wastewater. The maximum power density generated during the treatment process was 26 mW/m2 and 80% COD was removed. Previous studies have demonstrated that the energy-efficient treatment of wastewater is one of the most promising applications of MFCs. However, in most of those systems, wastewater as the electron donors was treated in the anode chamber by microorganisms. The effect of cathode chamber on electricity generation was just to form the pass way for electrons and protons. Oxygen [8,9,25], hexacynoferrate [26] or permanganate [27] was commonly used as electron acceptors.
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Therefore, if using certain pollutant with the ability of accepting electrons and protons as oxidant, the cathode chamber of MFCs, as well as the anode chamber could harvest considerable energy from pollutants. This would greatly enhance the environmental benefit of MFCs [28]. Based on this hypothesis, electroplating wastewater containing Cr6+ was chosen as the cathodic electron acceptor in this study. Under acidic conditions, Cr6+ ion accepts six electrons and thus is reduced to Cr3+ ion as illustrated in the following equations: Cr2 O7 2 þ 8Hþ þ 6e ¼ Cr2 O3 þ 4H2 O; Cr2 O3 þ 6Hþ ¼ 2Cr3þ þ 3H2 O
1:33 V
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PEM which was sequentially boiled in H2O2 (30%), deionized water, 0.5 M H2SO4, and then deionized water (each time for 1 h) as described by Liu and Logan [29] was held between the two rectangular chambers with a rubber gasket to prevent leakage. The anode chamber sealed with a rubber stopple was mixed with a magnetic stir bar. Plain carbon felt (2 cm 5 cm, 2 mm thick, Liaoning, China) with rough surface and porosity was placed in the anode chamber to accumulate the electron-transferring bacteria. Plain graphite paper (2 cm 5 cm, 0.2 mm thick, Jiangsu, China) was inserted into the cathode chamber to act as a cathode (except as noted). The electrodes were connected with a copper wire coming out of the rubber stopple to provide the connection points for the external circuit. An air-sparger was fixed in the bottom of cathode chamber for air sparging during inoculation or nitrogen sparging during the wastewater treatment.
(1) 2.2. Inoculation
(2)
The equations indicated that in acidic environment Cr6+ has higher oxidation potential (1.33 V vs. SHE) than oxygen (1.23 V) and hexacynoferrate (0.36 V) [26]. So it can be deduced that potassium dichromate is a more favorable electron acceptor theoretically. To the best of our knowledge, potassium dichromate has not been used as the cathodic electron acceptor in MFCs. In this work, the objective is to produce electricity directly from electroplating wastewater using a MFC combined with accomplishing wastewater treatment. Therefore, several factors that might affect the removal of chromium and electricity generation including pH, initial Cr6+ ion concentration and different electrode materials were investigated using synthetic wastewater in a dualchamber MFC. Also, to demonstrate the feasibility of this novel MFC treatment process, we examined the treatment efficiency of a real electroplating wastewater containing Cr6+. The reduced production of Cr6+ was analyzed by SEM and XPS.
Inoculation was conducted in the dual-chamber MFC mentioned above. Anaerobic sludge collected from the anaerobic digester of SiBao wastewater treatment plant in Hangzhou, China, was used as an inoculum in the anode chamber. Anaerobic sludge has been shown to be a suitable biocatalyst for electricity production [24,26]. For inoculation, a mixed solution of 50 ml anaerobic sludge with 150 ml sodium acetate medium was injected into the anode chamber. The anode medium contained the following (per liter): KH2PO4, 13.60 g; CH3COONa, 1.00 g; NaCl, 11.70 g; NaOH, 2.33 g [15]; NH4Cl, 0.45 g; MgCl26H2O, 0.17 g; FeCl36H2O, 1.00 mg; MnCl24H2O, 23.0 mg; CaCl2, 15.0 mg [7] and 1 ml of a trace element solution as reported by Logan et al. [25]. The cathode medium contained the following (per liter): KH2PO4, 13.60 g; NaCl, 11.70 g; NaOH, 2.33 g. KH2PO4 and NaOH were added in both anode and cathode medium forming PBS (pH 7). And NaCl was added to increase the solution conductivity. The cathode chamber was sparged with air (80 ml/min) using an air pump, during the inoculation stage. The electrodes were connected via an external circuit containing a single resistor (R = 1000 V). Experiments were conducted in a 35 8C temperature-controlled box (SPX-250B-Z, Shanghai, China). The anode chamber was added with 80 mg sodium acetate every day until the stable output voltage increase to 240–260 mV, which indicated bacteria have colonized the electrodes. 2.3. Operation of MFC
2. Materials and methods 2.1. MFC construction The experimental set-up was shown in Fig. 1. MFC comprised two (anode and cathode) 220 ml (h = 11 cm, w ¼ 4 cm, l = 5 cm, effective volume = 200 ml) plexiglass rectangular chambers. The chambers were physically separated by proton exchange membrane (NafionTM 117, Dupont Co.) with a 45 cm2 surface area. The
After the inoculation was finished, the anode chamber was refilled with 200 ml anode medium which was sparged with N2 for 0.5 h (60 ml/min) to remove dissolved oxygen. Synthetic wastewater was refilled in the cathode chamber and continuously sparged with N2 (80 ml/min) to fully mix the cathode medium. The synthetic wastewater (per liter) contained KH2PO4, 13.6g and NaCl, 11.7 g with different amount of K2Cr2O7. The pH was adjusted with H2SO4 (50%). Plain graphite paper (2 cm 5 cm, 0.2 mm thick) was used as the cathode electrode except where stated otherwise. Electrodes were connected via a circuit containing a resistance of 1000 V (except as noted) to measure electricity generation and Cr6+ removal. To determine the effect of various parameters on power generation and Cr6+ removal, pH (1–6) and initial concentration of Cr6+ (50–500 ppm), different electrode materials including graphite paper, carbon paper (120, Toray Co., Japan) and carbon felt were investigated. Different external resistance (10–9000 V) was applied in order to obtain polarization curve for determination of the maximum power generation. 2.4. Calculations Voltage was measured using a multimeter with a data acquisition system (UNT UT-70B, China) and converted to power density, P (W/m2), according to P¼
y2 RA
where R is the resistance, y is the voltage, and A is the cross-sectional area (projected) of the anode. The Columbic efficiency was calculated as Ec ¼
CP 100% CT
where Cp is the total coulombs calculated by integrating the current over time. CT is the theoretical amount of coulombs that can be produced from sodium acetate, calculated as CT ¼
bCVF M
where F is Faraday’s constant (96485 C/mol e), M is relative molecular mass (82 g/ mol by sodium acetate), b is the number of mol of electrons produced per mol of substrate (b = 8 mol e/mol based on sodium acetate), C (mg/l) is the overall removal of sodium acetate, y (200 ml) is the anode liquid volume. 2.5. Analytical methods Fig. 1. Schematic prototype of the MFC used to generate electricity from electroplating wastewater.
Acetate was analyzed using ion chromatograph (IC1000 Techcomp, China) equipped with an anion separation column (SI-90 Shodex, Japan). Samples were
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filtered through a 0.22 mm pore diameter membrane. Cr6+ was determined by colorimetry using diphenylcarbazide. Total chromium was analyzed using atom absorption spectrometry (AA-6800 Shimadzu, Japan). Microorganisms on anodic electrodes and cathodic electrodes were examined using a scanning electron microscope (SEM) (XL-30 Philips, Holland). Determination of the chemical composition of cathodic reduced products was done by using an X-ray photoelectron spectrometer (XPS) (PHI 5000C ESCA System, USA).
3. Results and discussion 3.1. The start up of MFC using Cr6+ ion as the electron acceptor During the start-up stage, stable power generation was obtained after two cycles. Fig. 2 showed the output voltage (R = 1000 V) steadily increased from 100 mV at 232.4 h to about 240 mV at 233.5 h when the anode chamber was refilled with the fresh anode medium (point A). And then the synthetic wastewater containing 50 ppm potassium dichromate (pH 2) was refilled the cathode chamber and N2 was continuously sparged at a flow rate of 80 ml/min (point B). This led to a substantial increase of voltage to 605 mV. Such a sharp increase of voltage was most likely due to the high redox potential (1.33 V) of Cr2O72 in acidic conditions as illustrated in Eq. (1). The SEM images (Fig. 3) shows that anodic electrode surface is covered by bacilliform bacteria which are responsible for electron transfer and current generation in the MFC.
Fig. 3. SEM image of electron-transfer bacteria attached to the surface of the anode.
3.2. Effects of pH on power density and removal efficiency of Cr6+ ion To determine the effect of pH, the power density and the removal of chromium were examined using a synthetic wastewater containing 50 ppm potassium dichromate. The pH of synthetic wastewater was adjusted to 1–6 with H2SO4 (50%). Fig. 4 illustrates that Cr6+ removal increases from 28% to 98.3%, as the pH decreases from 6 to 2. And green deposit could be evidently observed on the surface of cathode. However, the removal was not further increased at lower pH (pH 1). This was attributed to the H+ in cathode chamber diffused to anode chamber at pH 1, leading to a decrease in pH of anodic medium from 6.8 to 1.8 over the course of experiment. The low pH of anodic medium deactivated the microorganism and reduced the performance of MFC. The removal of total chromium followed a similar trend. The highest removal of 49.1% total chromium was achieved at pH 2, while removal decreasing to 18% at pH 6 and to 0 at pH 1. We supposed that Cr3+
Fig. 4. Maximum power density generated and chromium removal as a function of pH (50 ppm potassium dichromate).
was removed in the form of trivalent chromium oxides depositing on the surface of cathode. Moreover, trivalent chromium oxides dissolved in the cathode medium pH 1. Fig. 4 also shows the effect of pH on the maximum power density. The maximum power density was increased from 67.6 to 705 mW/m2 as pH decreased from 6 to 1. This revealed that low pH played a positive effect on power generation when potassium dichromate was used as electron acceptor in the MFC. The conclusion was confirmed by the power density generated over 9 h experiments with an external resistance of 1000 V (Fig. 5). The sharp decrease of power density at pH 1 observed in Fig. 5 was possibly due to the diffusion of H+ to anode chamber mentioned above. 3.3. Effects of initial Cr6+ concentration on output power density and cathode potential
Fig. 2. Electricity generation by MFCs during startup using potassium dichromate. (A) Refill the anode chamber with fresh anode medium. (B) Refill the cathode chamber with synthetic wastewater containing 50 ppm potassium dichromate, pH 2.
To determine the effect of initial Cr6+ concentration on the performance of MFC, Cr6+ concentration was set in the range of 50– 500 ppm. Based on the above-mentioned results, the pH of cathode medium was adjusted to 2 and then the effect of Cr6+ concentration on cathode OCP was examined by placing a saturated calomel electrode in the cathode chamber. Fig. 6 shows that increasing the initial Cr6+ concentration from 50 to 500 ppm increases the maximum power density by a factor of 3.54 from 602 to 2116 mW/ m2 and the OCP increases from 619 mV to 814 mV. The
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Fig. 5. Comparison of power generated in different pH (50 ppm potassium dichromate, R = 1000 V).
Fig. 6. Maximum power density and open circle cathode potential as a function of initial concentration of potassium dichromate (pH 2).
improvement in maximum power density might be due to the increase of cathode OCP which was influenced by Cr6+ concentration and pH. The phenomena were generally consistent with the trends expected from analysis using the Nernst equation based on Eq. (1):
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Fig. 7. Comparison of power generated using different cathode materials (50 ppm potassium dichromate, pH 2, R = 1000 V).
water containing 50 ppm potassium dichromate at optimal pH of 2 was used to assess the effect of different electrode materials on the performance of MFC. As shown in Fig. 7, the power density of graphite paper cathode slowly increased to the maximum (348 mW/m2) at the first 2 h, and gradually decreased to 5.7 mW/m2 in the next 7 h as Cr6+ was reduced. For carbon paper cathode, the power density of achieved the maximum value (291.6 mW/m2) in 10 min, and decrease slowly to 13.3 mW/m2 within the next 8.5 h. As for the carbon felt cathode, the maximum power density (678.9 mW/m2) was generated in 10 min, and decrease sharply to a low level (<50 mW/m2). The sharp decrease could be ascribed to some changes on the surface of carbon felt, which were induced by electricity and oxidation effects. The internal resistances kept about 300 V, not affected by the electrode materials. The maximum power density of graphite paper, carbon paper and carbon felt were 616, 408.3 and 761 mW/m2, respectively, for an external resistance of 300 V. From Fig. 8, it can be seen that the removal of Cr6+ in 9 h is 98.3% for graphite paper cathode, 86.3% for carbon paper and 40.6% for carbon felt. The graphite paper had the best performance on removing Cr6+. Moreover, 49% of total chromium could be reduced using graphite paper, while only 26.2% and 27.6% total chromium could be removed using carbon paper and carbon felt, respectively. After 9 h, a green precipitate could be observed on the surface of
E ¼ 1:33 þ 0:01 log½Cr2 O7 2 0:08 pH Another reason for the increase in maximum power density might be the decrease in internal resistance as a result of increase in ionic strength [30]. At 50 ppm Cr6+, the internal resistance was 300 V and then decreased to 100 V as the Cr6+ concentration increased to 500 ppm. You et al. adopted 100 ppm potassium permanganate as the electron acceptor in a dual-chamber MFC. And a maximum power density of 115.6 mW/m2 was generated [27]. Gu et al. revealed that a maximum power density of 12.4 mW/m2 was generated using 50 ppm chlorophenol as the electron acceptor [28]. In contrast to potassium permanganate and chlorophenol, potassium dichromate was a more favorable electron acceptor, showing a higher power density of MFC. 3.4. Effects of different electrode materials on power density and removal of Cr6+ Graphite paper, carbon paper and carbon felt were chosen as the cathode electrodes in this investigation. A synthetic waste-
Fig. 8. Chromium removal obtained using different cathode materials (50 ppm potassium dichromate, pH 2).
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graphite paper and carbon paper, while few precipitate could be found on carbon felt. The removal of Cr6+ and total chromium was related with the output power density. Higher power density produced faster removing of chromium. During 9 h treatment, the redox potential of NAD+/NADH in the anode varied between 320 and 355 mV, which was similar to that reported by Rabaey and Verstraete [31]. Hence, compared with carbon paper and carbon felt, the MFC using graphite paper cathode with higher cathode potential showed the highest power density and the most efficient removal of chromium. 3.5. Treatment of real electroplating wastewater using MFC system To demonstrate the feasibility of this new type of MFC treatment process, we examined the treatment of a real electroplating wastewater. All the real wastewater used in this study was collected from an actual electroplating plant. The concentration of potassium dichromate was about 204 ppm and the pH was 2.5. After 25 h treatment, the removal of Cr6+ and total chromium reached 99.5% and 66.2%, respectively. Table 1 compared the removal of Cr6+ in the literatures with that obtained in this study. In contrast to other treatment process, the main advantages of MFC for electroplating wastewater treatment were (1) simultaneous electricity generation, (2) higher Cr6+ concentration tolerance compared to other biological treatment and (3) no toxic waste sludge production. The power density generated over the process was showed in Fig. 9. The maximum power density was 1600 mW/ m2 at a current density of 0.4 mA/cm2 with an external resistance of 100 V (Fig. 10). The obtained maximum power density was higher than that reported by other researchers in the treatment of wastewater using MFC, e.g. the maximum of 142.41 mW/m2 was obtained during the chemical wastewater treatment using MFC reported by Mohan et al. [17], 25 mW/m2 generated during urban wastewater treatment reported by Rodrigo et al. [14] And the maximum of 218 mW/m2 generated during domestic wastewater treatment reported by Min and Angelidaki [39]. The columbic efficiency of the system was 12% which was comparable to <15% reported by Liu et al. [9], indicating there was substantial COD removal that was not associated with power generation. With respect to reaction mechanism, the removal of Cr6+ was accomplished by the whole MFC system, including the proton and electron generation phase. Under anaerobic condition, microorganisms in anode chamber oxidized acetate to generate protons and electrons (Eq. (3)): CH3 COO þ 4H2 O ! 2HCO3 þ 9Hþ þ 8e
(3)
And then electrons were transferred to anode and finally to cathode through the circuit, while proton was transferred to Table 1 Cr6+ removal reported in the literatures and that obtained in this study Methods
Initial concentration (mg/l)
Removal (%)
Reference
Nanofiltration Biosorption Biosorption Fluidized zero valent iron process Combined electrocoagulation– electroflotation Electrocoagulation Biosorption by surfactant modified coconut coir pith Microbial fuel cells
17.09 30 100 534
96.6 86.61–91.03 95 99.9
[32] [33] [34] [35]
10
97
[36]
24 20
92 96
[37] [38]
99.5
This study
204
Fig. 9. Power generated in the MFC using real electroplating wastewater as electron acceptor (pH 2, R = 1000 V).
Fig. 10. Power generation as a function of the current density using potassium dichromate (pH 2).
cathode chamber through the PEM. Cr6+ was reduced by the protons and electrons as illustrated in Eq. (1). The reduction of Cr6+ is strongly dependent on pH. Therefore, decreasing the pH makes the reaction more favorable. For practical application, the wastewater containing Cr6+ can be mixed with acid cleaning wastewater which is also discharged from electroplating industry to decrease the pH value. Thus, no additional chemicals are needed in the treatment process. Also, a continuous operation was conducted in sequencing batch model for three cycles without significant reduction in capacity for power generation and Cr6+ removal. These suggested that MFC should be an economical and sustainable method for removing Cr6+ from electroplating wastewater. The electricity produced by MFC can be used for monitoring the process. The high output power density and efficient removal of chromium makes this MFC treatment process useful not only for treating pollutants but also for electricity production. In addition, trivalent chromium oxide recovered in MFC treatment systems following purification can have additional market value. Thus, the system may also be applied in a metal recovery process. The system used here was a very basic type of MFC that has been used to examine factors that can increase power generation in these types of applications. However, the development of an economical treatment system
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on the graphite paper, XPS analysis was applied. As shown in Fig. 12, binding energies peaks corresponded to 575.6 eV, pointing to a fact that Cr2O3 was the main cathode products [40]. 4. Conclusion Treatment of electroplating wastewater containing Cr6+ and production electricity was carried out successfully in the cathode chamber of dual-chamber microbial fuel cell. pH value affected the power density and chromium removal obviously and the optimal pH value was 2. In addition, higher concentration of Cr6+ could enhance the power density. Compared with carbon paper and carbon felt cathode, the graphite paper cathode showed the best performance on chromium removal and electricity generation. During the 25 h treatment of real electroplating wastewater containing 204 ppm Cr6+, the maximum power density generated was up to 1600 mW/ m2 accompanied by 99.5% of Cr6+ and 66.2% of total chromium removal, respectively. The removal of chromium from wastewater was mainly due to the deposition of Cr2O3 on the surface of cathode. The considerable high power density, higher Cr6+ concentration tolerance compared to other biological treatment and no hazardous waste sludge production demonstrated that microbial fuel cell was a promising technology to treat electroplating wastewater and simultaneously generate electricity. Acknowledgements
Fig. 11. SEM image of (A) plain cathode and (B) surface of cathode in the MFC using potassium dichromate as electron acceptor (500 ppm potassium dichromate pH 2, reacting time of 35 h).
The authors would like to acknowledge financial support for this work provided by the National Science Foundation of China (Nos. 20576120, 90610005 and U0633003), project of Zhejiang Province (No. 2007C13061) and ‘‘86300 project of China (No. 2007AA06Z339). References
will require a system architecture that is scaleable. Improvements in system architecture are under investigation and will be reported in due course. 3.6. Characteristics of the cathode surface SEM images clearly show deposits formed on the surface of the cathode (Fig. 11). To identify the chemical form of those deposits
Fig. 12. XPS analysis for the surface of the cathode (500 ppm potassium dichromate pH 2, reacting time of 35 h).
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