Degradation of Organic Pollutants in a Photoelectrocatalytic System

Jun 28, 2010 - way to elevate the photocatalytic efficiency. Here we report a novel bioelectrochemical system to effectively reduce p-nitrophenol as a...
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Environ. Sci. Technol. 2010, 44, 5575–5580

Degradation of Organic Pollutants in a Photoelectrocatalytic System Enhanced by a Microbial Fuel Cell SHI-JIE YUAN, GUO-PING SHENG,* WEN-WEI LI, ZHI-QI LIN, RAYMOND J. ZENG, ZHONG-HUA TONG, AND HAN-QING YU Department of Chemistry, University of Science & Technology of China, Hefei, 230026, China

Received April 23, 2010. Revised manuscript received June 11, 2010. Accepted June 16, 2010.

Photocatalytic oxidation mediated by TiO2 is a promising oxidation process for degradation of organic pollutants, but suffers from the decreased photocatalytic efficiency attributed to the recombination of photogenerated electrons and holes. Thus, a cost-effective supply of external electrons is an effective way to elevate the photocatalytic efficiency. Here we report a novel bioelectrochemical system to effectively reduce p-nitrophenol as a model organic pollutant with utilization of the energy derived from a microbial fuel cell. In such a system, there is a synergetic effect between the electrochemical and photocatalytic oxidation processes. Kinetic analysis shows that the system exhibits a more rapid p-nitrophenol degradation at a rate two times the sum of rates by the individual photocatalytic and electrochemical methods. The system performance is influenced by both external resistor and electrolyte concentration. Either a lower external resistor or a lower electrolyte concentration results in a higher p-nitrophenol degradation rate. This system has a potential for the effective degradation of refractory organic pollutants and provides a new way for utilization of the energy generated from conversion of organic wastes by microbial fuel cells.

Introduction Photocatalytic oxidation mediated by semiconductors such as TiO2 is a promising conversion process for hazardous waste remediation (1-4) and water disinfection (5-7). Upon UV irradiation, photoexcitation promotes electrons from the valence band to the conduction band of semiconductors, leaving high-oxidizing photogenerated holes behind. The photogenerated holes react with adsorbed OH- or H2O to produce OH · , which is able to degrade various refractory pollutants (8, 9). However, the concentration of photogenerated holes is usually of a low level, attributed to a high degree of recombination between the photogenerated electrons and holes, which decreases the photocatalytic efficiency (10). To sort out this problem, several methods have been adopted, such as coupling different semiconductors (1, 11), metal doping (12), and electrochemical methods (9, 13). Among them, the electrochemical method of applying an external positive anodic bias, i.e., constructing a photoelectrocatalytic (PEC) system, can significantly increase the * Corresponding author fax: +86-551-3601592; e-mail: gpsheng@ ustc.edu.cn. 10.1021/es101317z

 2010 American Chemical Society

Published on Web 06/28/2010

photocatalytic efficiency (5, 13, 14). In this case, a costeffective supply of external electricity is essential to elevate the photocatalytic efficiency. Microbial fuel cell (MFC) is a bioelectrochemical system that utilizes electrochemically active microorganisms to produce electricity from organic substrates (15-17). In an MFC the open circuit voltage could reach only 0.80 V, and such a low-voltage electricity could be used for other electrochemical processes (18, 19). Thus, it is expected that the electricity generated from an MFC could be used to supply extra electricity to the PEC system and to achieve a higher photocatalytic efficiency. Here, we report a new bioelectrochemical system for utilization of MFC electricity and an effective degradation of organic pollutants, with p-nitrophenol as a target pollutant. p-Nitrophenol, which has been recognized as a refractory, hazardous, and priority toxic pollutant by the U.S. Environmental Protection Agency, was chosen as the model pollutant to evaluate the performance of this system. The bioelectrochemical system is composed of a coupled TiO2-mediated photoelectrocatalytic oxidation reactor and an air-cathode MFC. The MFC is used to provide an external anodic bias to the PEC reactor for the degradation of p-nitrophenol. This work may present an efficient and cost-effective approach to establish such an MFC-assisted photoelectrocatalytic (MPEC) oxidation process for the degradation of organic pollutants.

Materials and Methods Reactor Setup and Operation. The layout of the MPEC system is shown in Figure S1 (Supporting Information). The MFC was a single-chamber air-cathode configuration with carbon paper (GEFC Co., China) as the anode (3 × 7 cm) and carbon paper loaded with Pt (2 mg cm-2) as the cathode (2 × 2 cm). The bioanodes were cultivated in another air-cathode MFC with anaerobic sludge as inoculums and acetate as substrate for 2 months, thus a consortium of electrochemically active microorganisms was well enriched on the carbon paper. The anode chamber was filled with 430 mL of autoclaved anode medium as described previously (20). All organic and inorganic reagents used were of analytical grade unless otherwise stated. All solutions were prepared with water from a water purification system (Millipore Co., USA). A quartz glass cylinder, 6.5 cm high, 6 cm in diameter and filled with 150 mL of p-nitrophenol solution at 2.3 mM, was used as the PEC reactor. A self-prepared photoelectrode and a carbon paper were respectively used as the photoanode and cathode and placed in parallel in the reactor. A saturated calomel electrode (SCE) was inserted as the reference electrode (all the potentials are referred to SCE unless mentioned otherwise). The photoelectrode was prepared by painting TiO2 (P25) on a carbon paper. A mixture of TiO2 powder and poly(ethylene glycol) at a weight ratio of 10% was added to 100 mL of ethanol and ultrasonicated for 30 min. The mixture was then uniformly spread on a carbon paper (2.6 × 5.3 cm). After evaporation of ethanol at 120 °C, the sample was heated to 430 °C in a muffle furnace for 2 h to completely remove poly(ethylene glycol). The prepared electrodes had 0.075 g TiO2 loaded on their surface. A 30-W low-pressure mercury lamp was fixed outside the reactor against photoelectrode surface about 10 cm distant. Na2SO4 solution (0.01 M) was used as the support electrolyte, except when the electrolyte concentrations of 0.05, 0.1, and 0.3 M were used to evaluate the effect of solution ionic conductivity on the system performance (Table 1). The solutions in the PEC reactor were constantly stirred. All batch VOL. 44, NO. 14, 2010 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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TABLE 1. MPEC System Performance under Different Conditionsa R (Ω)

CPEC (mol L-1)

k (hr-1)

PPhotoanode (V)

VPEC (V)

current density (mA m-2)

VMFC (V)

0 0 10b 10 300 1000 10 10 10

0.01 0.01 0.01 0.01 0.01 0.01 0.05 0.1 0.3

0.198 ( 0.002 0.411 ( 0.013 0.015 ( 0.002 0.409 ( 0.016 0.387 ( 0.012 0.336 ( 0.009 0.390 ( 0.010 0.359 ( 0.013 0.299 ( 0.006

-0.169 ( 0.010 0.212 ( 0.010 0.270 ( 0.009 0.211 ( 0.011 0.199 ( 0.007 0.164 ( 0.012 0.185 ( 0.020 0.151 ( 0.023 -0.014 ( 0.035

0.489 ( 0.005 0.528 ( 0.005 0.489 ( 0.011 0.469 ( 0.004 0.424 ( 0.004 0.457 ( 0.020 0.435 ( 0.014 0.326 ( 0.013

20.0 ( 1.3 51.9 ( 0.5 49.0 ( 1.0 45.7 ( 1.3 91.8 ( 2.5 125.9 ( 1.4 213.1 ( 6.6

0.489 ( 0.005 0.528 ( 0.005 0.489 ( 0.011 0.494 ( 0.003 0.502 ( 0.002 0.458 ( 0.021 0.437 ( 0.018 0.329 ( 0.016

a R is the resistance between the MFC and the PEC reactor; CPEC is the supporting electrolyte concentration in the PEC reactor; k is the rate constant of degradation reaction; PPhotoanode is the photoanode potential; VPEC is the input voltage of PEC reactor; VMFC is the output voltage of MFC. *b. b In the absence of UV irradiation.

tests were conducted in duplicate at room temperatures of around 20 °C. The MFC and the PEC reactor were connected in series with a resistance between the photoelectrode and the air-cathode of the MFC to measure the circuit current. All measurements were performed at 2.3 mM p-nitrophenol and 0.01 M Na2SO4 with a 10 Ω external resistance, unless mentioned otherwise. Analysis. The circuit current was calculated from the voltage across the resistance which was continuously recorded with a potentiostat (660C, CH Instruments, Inc., USA) connected to a computer. The current density was calculated according to the projected surface area of the photoelectrode. The potentiostat was also used to perform an analysis of linear sweep voltammetry (LSV) and electrochemical impedance spectroscopy (EIS) in the solutions. LSV experiments were carried out at a potential scan rate of 10 mV/s. The impedance data were collected as a function of frequency scanned from the highest (105 Hz) to the lowest (0.01 Hz) and with initial potential at open circuit and +0.45 V. The EIS results were fitted by ZSimp Win software according to the corresponding equivalent circuits (Figure S2 in Supporting Information). The output voltage of MFC and the potential of PEC reactor electrodes were measured using a multimeter (UT39A, UNIT Inc., China). The morphology of the prepared photoelectrodes with TiO2 loaded on the surface was observed using scanning electron microscopy (SEM; FEI Co., The Netherlands), and its crystal structure was characterized by X-ray diffraction (XRD; X’ Pert PRO, Philips Co., Netherlands). The adsorption spectra of p-nitrophenol in the degradation process were measured using a UV-vis spectroscopy (UV-2401, Shimaze Co., Japan). The intermediates of p-nitrophenol degradation were qualitatively determined by an HPLC (1100, Agilent Inc., USA) with a Hypersil-ODS inverse phase column at 246 nm using a VWD detector, on the basis of the standard addition method. The mobile phase was a mixture of water with 0.1% acetic acid and methanol (60:40) delivered at a flow rate of 0.8 mL min-1. The total organic carbon concentration was measured using a TOC analyzer (VCPN, Shimadzu Co., Japan).

Results Characterization of the MPEC System. SEM and XRD were employed to characterize the surface morphology and structure of the photoelectrode used in the MPEC system. The photoelectrode, covered with TiO2, displayed a crackedmud structure, which increased the actual electrode surface area (Figure 1A and B). The XRD spectrum of the photoelectrode, in which the peaks correspond to anatase and rutile phases (Figure 1C), shows structure similar to that of the commercial TiO2 nanoparticles P25 (21). This suggests that the coating of TiO2 on carbon surface would not change its structure and that the photoelectrode could provide a large contact area and thus had a high potential to improve the photoelectrocatalytic efficiency. 5576

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FIGURE 1. SEM images of: (A) the raw photoelectrode; (B) the photoelectrode coated with TiO2; and (C) XRD spectrum of the TiO2 on the photoelectrode. A represents anatase and R represents rutile. LSV experiment was carried out with the PEC reactor to compare the induced photocurrent and electrochemical current of the system with or without UV irradiation (Figure 2A). The current was low in the absence of UV irradiation. In contrast, when irradiation was applied, a much higher current was achieved. Such an increase in current was attributed to the flux of photogenerated electrons. A larger current was generated when the applied potential bias was higher than +0.2 V, and the electricity difference increased from 0.122 mA at +0.2 V to the peak of 0.296 mA at +0.56 V. Figure 2B shows the EIS response of the PEC system under various conditions. Only one arc and one peak were observed, implying that the degradation process was a simple electrode reaction. The size of the arc radius on the EIS Nyquist plot reflects the rate of the electrode reaction (22). It had the biggest radius for the case without UV and positive bias, and

FIGURE 2. (A) Linear sweep voltammetric curves of the PEC reactor at 10 mV/s in the presence (a) and absence of UV irradiation (b); and (B) EIS of the PEC reactor in the absence of UV condition with (EC) and without (Dark) +0.45 V anodic bias; and in the presence of UV with (PEC) and without (PC) +0.45 V anodic bias. was considerably reduced under UV irradiation. Thus, the electron-transfer resistance (Rt) of the single electrochemical reaction and photocatalytic degradation reaction were determined to be 1.35 × 104 and 7.56 × 103 Ω, respectively. With a positive anodic bias of +0.45 V applied, the electrontransfer resistance was reduced to a very low value (Rt ) 526 Ω). In this case, a much faster degradation rate of pnitrophenol was obtained under the photoelectrocatalytic conditions. Degradation of p-Nitrophenol. Figure 3A illustrates the UV-vis absorption spectra of p-nitrophenol in aqueous solution prior to and after degradation in the MPEC system. A main absorption band for p-nitrophenol was at 317 nm, the intensity of which reflects its concentration in solution. The peak intensity decreased by about 90% within 6.25 h, confirming the rapid degradation of p-nitrophenol in this system. A comparison of the p-nitrophenol removals in the electrochemical, photocatalytic, and MPEC processes is shown in Figure 3B. The linear relationship of ln(Ct/C0) vs t shows that the p-nitrophenol degradation in the different systems all followed the pseudo-first-order kinetics (9): ln

Ct ) -kt C0

where Ct/C0 is the normalized p-nitrophenol concentration, t is the reaction time (hr), and k is the reaction rate constant (hr-1). The MPEC system had the greatest degradation rate constant of 0.411 h-1, significantly higher than that for the individual photocatalytic (0.198 h-1) and electrochemical (0.015 h-1) methods (Table 1). This indicates that the MPEC

FIGURE 3. (A) UV-vis spectra of a typical degradation process; and (B) the relative concentration profiles of p-nitrophenol and TOC during a typical degradation process in the MPEC system (EC: with MFC to supply the extra power but without UV irradiation; PC: with UV irradiation but without MFC; MPEC: with both of UV irradiation and extra power supply from the MFC). system could effectively restrain the recombination of the photogenerated electrons and holes and improve the catalytic efficiency, which is in agreement with the EIS results. 1,2,4-Benzenetriol, benzoquinone, p-nitrosophenol, and p-nitrocatechol were identified as the main intermediates in the PEC degradation of p-nitrophenol (Figure S3). As shown in Figure 3B, the total organic carbon concentration of solution decreased, and its reduction rate was slightly slower than that of p-nitrophenol, indicating the retarded mineralization of p-nitrophenol. This observation is also supported by the intermediate profiles according to the peak area evolution in the HPLC analysis, which increased in the initial 2.5 h, but then decreased to a low level. Current, Voltage, and Half-Cell Potential in the MPEC System. The changes of current, voltage, and half-cell potential of the MPEC system under UV irradiation in a typical batch test are shown in Figure 4. The current increased after the start of illumination and decreased continuously with time, while the MFC output voltage showed an opposite trend. The current charges of the system at initial 0.2 h are shown in the inlet of Figure 5A. The current initially exhibited a rapid reduction, followed by a gradual decline, and finally became almost stable (27.6 mA m-2). This is attributed to the fact that the MFC was in open circuit for several minutes to connect with the PEC reactor. After the system was connected, the MFC output voltage decreased from the open circuit state to a stable one, and the input voltage of the MPEC reactor decreased correspondingly. As soon as the UV irradiation was applied, the current substantially increased and then reached a relatively stable level (63.9 mA m-2), attributed to the photogenerated current (Figure 5A). An increase in the system current led to the decreased MFC VOL. 44, NO. 14, 2010 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 4. Profiles in a typical degradation process: (A) current of the MPEC system; (B) output voltage of the MFC and input voltage of the PEC reactor; and (C) potentials of the photoanode and the cathode in the PEC reactor. output voltage and the subsequent decreased input voltage of the PEC reactor (Figure 5B). The electrode potential of the PEC reactor also changed following the change of the input voltage (Figure 5C). It should be noted that in Figure 4A there are some points deviated from the main line, which was caused by the sampling for the UV-vis measurements. The average voltage and potential were obtained on time integration from Figure 4 and are shown in Table 1. At 2.3 mM of p-nitrophenol and 0.01 M Na2SO4 in solutions with a 10 Ω resistance, the average potential of the photoanode was +0.211 V and the input voltage of the PEC reactor was 0.489 V. This bias potential facilitated the effective separation of the photogenerated electron-hole pair and thus enhanced the p-nitrophenol degradation. A current density of 51.9 mA m-2 was obtained, much higher than that without UV irradiation (20.0 mA m-2). System Performance with Different Resistors and Support Electrolytes. When the extra electricity from the MFC was supplied without UV irradiation, only electrochemical reaction of p-nitrophenol took place on the electrode surface. At a lower ionic strength (0.01 M) and higher ohmic resistance of the support electrolyte, the current density and the p-nitrophenol degradation rate constant were at a lower level, while the MFC had a higher output voltage of 0.528 V (Table 1). Compared with the electrochemical conditions, the 5578

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FIGURE 5. Profiles of the initial 0.2 h in a typical degradation process: (A) current of the MPEC system; (B) output voltage of the MFC and input voltage of the PEC reactor; and (C) potentials of the photoanode and the cathode in the PEC reactor. presence of UV irradiation not only increased the degradation rate in the PEC reactor, but also caused an increase in the MFC output current density by 159.2%. With an increase in resistance connected in the MPEC system, the input voltage of PEC reactor and the photoanode potential decreased. The degradation rate decreased as the result of an increase in the recombination rate of the photogenerated electrons and the holes. When the resistance between the MFC and the PEC reactor increased from 0 to 1000 Ω with 0.01 M Na2SO4 as the support electrolyte, the p-nitrophenol degradation rate decreased by 18.2% and the maximum degradation rate of 0.411 h-1 was obtained without the external resistance (Table 1). To measure the current, a 10 Ω resistance was then used in the subsequent experiments. Nevertheless, the concentration of the support electrolyte in the PEC reactor also affected the MPEC system performance. The p-nitrophenol degradation rate constant reduced by 26.9% with an increase in the support electrolyte concentration from 0.01 to 0.3 M at a fixed connected resistance of 10 Ω. The decrease in the p-nitrophenol in the MPEC system was attributed to the decrease in the MFC output voltage. The solution conductivity increased with the increasing support electrolyte concentration in the PEC reactor. This would decrease the external resistance of the MFC, and thus resulted in a decrease in the input voltage of the PEC. A decrease in the positive anodic bias applied on

FIGURE 6. Working principles of the MPEC system (p-Nph represents p-nitrophenol). the photocatalytic cell would increase the recombination between the photogenerated electrons and the holes, and decrease the photocatalytic efficiency. As a consequence, it thus reduced the p-nitrophenol degradation rate.

Discussion In the MPEC system several half-reactions are involved: the organic oxidation in the MFC anode (reaction 1); the oxygen reduction in the MFC and the PEC cathodes (reaction 2); and pollutant degradation in the PEC reactor (reactions 3-6) (20, 23). + CH3COO- + 4H2O f 2HCO3 + 9H + 8e

(1)

O2 + 4H+ + 4e- f 2H2O

(2)

+ TiO2 + hv f hvb + ecb

(3)

+ hvb + H2O f OH· + H+

(4)

+ + OH- f OH· hvb

(5)

+ C6H4OHNO2 + 28OH· f 6CO2 + 16H2O + NO3 + H (6)

The half-reactions above are interaffected and a balance among different reactions is expected in a stabilized system. The protons needed for reaction 2 could come from reaction 1; and the consumed electrons could come from the photoanode by reaction 3 under UV irradiation, and from the PEC reactor by reaction 6 (Figure 6). The current thus tends to decrease with the decreased p-nitrophenol concentration in the degradation process. Therefore, these half-reactions can influence each other and some reactions cannot occur in the absence of either MFC or UV irradiation. For example, in the photocatalytic reactor without MFC, a low degradation rate constant of 0.198 h-1 is obtained due to a higher degree of recombination between the photogenerated electrons and holes. In this case,

the absence of reaction 2 cannot consume the photogenerated electrons efficiently. Similarly, without the UV irradiation, the degradation rate is also very low. Thus, the PEC reactor acts like an electrochemical reactor. No efficient OH · is produced for the pollutant degradation. However, in the presence of MFC and UV irradiation, the current increases immediately (Figure 5A). Such an increase could be explained as a phenomenon known as “current-doubling” (13, 24). OH · produced from the photogenerated holes oxidizes p-nitrophenol to CO2 and water, followed by injection of electrons into the electrode (reactions 3-6). The electrons released in this system are transferred from the photoanode of the PEC to the MFC cathode and oxidized by oxygen (reaction 2). For the MPEC system, the MFC could obviously restrain the recombination of the electrons and holes, which are generated by the photoanode under UV irradiation (Figure 5). Then, the photogenerated holes are captured by water or hydroxyl groups, followed by the formation of OH · (reactions 4 and 5) and the injection of electrons into the air-cathode of the MFC for oxygen reduction (reaction 2). OH · would react with the organic molecules to form reactive intermediates for complete oxidation (reaction 6). It should be noted that the processes occurring in the PEC cathode are also important. The electrons produced by the bacteria from organics oxidation should be transmitted to the PEC cathode. The electrons accumulated on the PEC cathode are then consumed by the oxygen reduction reaction to achieve an electron-neutrality between the MFC and the PEC reactor. However, the formation of p-nitrosophenol during the p-nitrophenol degradation (Figure S3) suggests that a small amount of p-nitrophenol was reduced to p-nitrosophenol in the PEC cathode. The intermediate products were then degraded in the photoanode (13). In this study, the established MPEC system has shown a potential for effective degradation of refractory pollutants. The results demonstrate, for the first time, that the power generated from an MFC could be utilized to supply the extra bias potential to reduce the recombination between the photogenerated electrons and holes, and thus enhance the photocatalytic efficiency. The energy captured by microorVOL. 44, NO. 14, 2010 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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ganisms from organic wastes is utilized to enhance the degradation of biorefractory compounds in the PEC reactor. Compared with the conventional PEC system, the MPEC system has two advantages: (i) the pollutant degradation rate is high; and (ii) the system is effectively operated without extra electricity supply, but with the electric energy produced by the MFC. Energy captured by microorganisms from easily biodegradable pollutions in MFCs is used to enhance the removal of the toxic and biorefractory compounds by photocatalysis. The kinetic analysis also shows that the MPEC system established in this study can remove p-nitrophenol more efficiently and rapidly. Thus, the system is expected to have a higher treatment capacity than conventional reactors. In addition, a new design of an MPEC system configuration and modification of the photocatalyst are expected to enhance the system performance. It affords a more practical means of wastewater treatment, especially for wastewaters rich in toxic and nondegradable compounds, which are not readily treated by conventional methods. In the present work, acetate was used as the substrate for electricity generation in the MFC. Actually, the intermediates of p-nitrophenol degradation in the MPEC system are a potential substrate for the MFC. If they could be effectively degraded in the MFC anode, the treatment capacity of this system would be further enhanced. This warrants a further investigation.

Acknowledgments We thank the NSFC (50625825), the Chinese Academy of Sciences (KZCX2-YW-QN504), the Outstanding Young Scientists Foundation of Anhui Province (10040606Y27), and the Fundamental Research Funds for the Central Universities for the partial support of this study.

Supporting Information Available Additional figures. This material is available free of charge via the Internet at http://pubs.acs.org.

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