Article pubs.acs.org/EF
Nitrification, Denitrification, and Power Generation Enhanced by Photocatalysis in Microbial Fuel Cells in the Absence of Organic Compounds Xuan Xu,*,†,‡ Bi Zhou,†,‡ Fangying Ji,†,‡ Qiulin Zou,†,‡ Yunsong Yuan,†,‡ Zhan Jin,†,‡ Deqiang Zhao,†,‡ and Jun Long†,‡ †
Key Laboratory of Three Gorges Reservoir Region’s Eco-Environment, Ministry of Education, and ‡National Centre for International Research of Low-Carbon and Green Buildings, Chongqing University, Chongqing 400045, People’s Republic of China ABSTRACT: In this study, a novel reactor using a biocathode microbial fuel cell and photocatalysis was designed for simultaneous nitrogen removal and power generation in the absence of organic compounds. To enhance the redox potential of microbial fuel cells, two pieces of conductive glass coated with TiO2 and SiO2 were attached to the anode and cathode electrodes, respectively. Nitrification and denitrification occurred in the anode and cathode chambers, respectively. As the sole electron donor, ammonium supported the current production without organic compounds in the anode chamber. In the cathode chamber, the denitrification process occurred with nitrate as the electron acceptor. Moreover, with the effect of ultraviolet irradiation of the conductive glasses, which is in contact with the anode and cathode, nitrification and power generation were both enhanced. The nitrification rate was increased from 0.58 to 1.34 mg L−1 h−1, and the denitrification rate was increased from 0.5 to 1.2 mg L−1 h−1. In addition, the current generation increased from 0.012 to 0.032 mA (an average current). This study demonstrated that photocatalysis can be applied to enhance the nitrification, denitrification, and power generation in microbial fuel cells in the absence of organic carbon.
1. INTRODUCTION Much attention is focused on green renewable energy production because of the global energy crisis and environmental pollution, and wastewater containing a high content of organic matter represents an ideal alternative source for energy production.1 Microbial fuel cells (MFCs) are an emerging technology, in which the energy contained in organic matter is converted directly into useful electrical power2−5 or other useful chemicals, such as H2,6 H2O2,7 methane,8 etc. In the process of power generation, electrons and protons are produced from organic electron donors through anaerobic microbial catalysis in the anode. The electrons and protons are transferred through an external circuit and proton exchange membrane, respectively, to the cathode, where they reduce the final electron acceptor, typically oxygen. In addition to organic matter, wastewater also contains a large amount of nitrogen, mainly in the forms of ammonium (NH4+) and nitrate (NO3−). Many studies suggest that nitrate as the final electron acceptor in the cathode chamber of MFCs can achieve simultaneous nitrogen removal and power generation,9−12 and those electrons are from the organic compounds degraded in the anode. Clauwaert et al.13 developed a MFC in which microorganisms in the cathode perform complete denitrification using electrons supplied by other microorganisms that oxidize acetate in the anode, and the power output of this MFC is approximately 8 W/m3 net cathodic compartment (NCC). Previous studies have proposed that MFCs offer an efficient system to convert nitrate into nitrogen, when the amount of organic matter in the anode is sufficient.14−16 However, treatment of wastewater containing high concentrations of ammonium N under carbon-limited conditions poses a © XXXX American Chemical Society
challenge. If the nitrate in the cathode chamber can gain electrons from ammonium in the anode, nitrification, denitrification, and power generation can be achieved in the absence of organic substances in MFCs. Currently, considerable controversy surrounds the use of ammonium as the electron donor in the anode of MFCs. In a study by Min et al.,17 high levels of ammonium were reportedly removed in a MFC used to treat swine wastewater. However, Kim et al.18 concluded that ammonium was not a substrate for electricity generation, and its removal was largely due to either ammonium volatilization in an air cathode MFC or ammonium ion diffusion from the anode to the cathode in a two-chambered MFC. However, He et al.19 put forward different views. In their study, they found that, with the decrease in ammonium, nitrate and nitrite increased. However, to date, only a few reports have investigated the production of electricity from ammonium oxidation in MFCs.17−20 Denitrification and power generation are all dependent upon the electrical power produced by ammonium oxidation in the anode chamber. However, ammonium oxidation often occurs under aerobic conditions,21 and the anode chamber of MFCs is usually anaerobic.22 To improve the oxidation of ammonium in the anode chamber, an innovative MFC was developed, in which another technology, photocatalysis, was introduced to increase the redox potential. Under illumination by a suitable photosource, a p−n junction can create and separate the electron−hole pairs.23,24 The holes flow into the negative field, while the electrons move to the positive field.25 In this study, a Received: August 5, 2014 Revised: January 9, 2015
A
DOI: 10.1021/ef501755d Energy Fuels XXXX, XXX, XXX−XXX
Energy & Fuels
Article
of the MFC. A 50 Ω resistor was connected between the anode and cathode electrodes using copper wire. The cell voltage and current were recorded every minute by a thermocouple acquisition module (TDAM7018, Yokei, Shenzhen, China). 2.2. Inoculation and Operation. The cathode and anode were inoculated with denitrifying and nitrifying sludge-treated domestic wastewater, respectively, in a sequencing batch reactor. Both chambers were inoculated with an initial mixed liquor suspended solid concentration of 3 g L−1. The operation of the MFC reactors was conducted according to a batch-fed mode, and the hydraulic retention time was 12 h. Ammonium chloride and sodium nitrate were added to the anode and cathode chambers as the electron donor and acceptor, respectively. Sodium bicarbonate was used as the inorganic carbon for the growth of autotrophic bacteria in both chambers. The nutrient solution and trace nutrient solution were modeled according to those described by He et al.19 During the startup, sodium acetate and ammonium chloride were supplied to the anode chamber at concentrations of 0.215 and 0.19 g L−1, respectively, while sodium acetate and sodium nitrate were supplied at concentrations of 0.215 and 0.3 g L−1, respectively, to the cathode chamber. After 2 months of the startup procedure, the concentration of sodium acetate was gradually decreased to 0 g L−1, always keeping the same concentration in both chambers, to investigate the effects of the presence of organic compounds on MFC performance. Ammonium and nitrate were still used as the electron donor and acceptor in the anode and cathode chambers, respectively. To reduce the effect of residues on microorganisms, the residues from the previous day were washed out before inflow. Both compartments were flushed with N2 gas prior to operation to achieve totally anaerobic conditions. All experiments were performed at room temperature. 2.3. Chemical Measurement. The concentrations of chemical oxygen demand (COD), ammonia nitrogen (NH4+-N), and nitrate nitrogen (NO3−-N) were determined using a HACH-COD/DR2010 UV−vis spectrophotometer (HACH Co., Loveland, CO). To evaluate the electricity generation performance of the MFC, the cell voltage and current were recorded every minute by a thermocouple acquisition module (TDAM7018, Yokei). A third electrode was connected to the Ag/AgCl reference electrode (232 INESA Scientific Instrument Co., Ltd., Shanghai, China) to determine the electrode potential. The potential difference (ΔV) of the MFC was calculated as ΔV = EC − EA, where EC is the cathodic potential and EA is the anodic potential.
structure similar to a p−n junction was used to create and separate the electron−hole pairs. When this structure is connected to the electrode of the MFC, the holes flow into the anode, while the electrons move to the cathode. This can increase or decrease the oxidation potential of the anode (cathode) electrode, and as a result, nitrification (denitrification) can be accelerated in the anode (cathode) chamber in the absence of oxygen. In this study, we aimed to investigate this concept and to establish whether nitrification, denitrification, and power generation can be achieved simultaneously in a MFC employing photocatalysis when treating a waste stream with a low organic content and high ammonium content. For this purpose, two conductive glasses coated individually with TiO2 (a n-type semiconductor)24 and SiO2 (a p-type semiconductor)26 were used to increase the oxidation reduction potential of the electrode under ultraviolet (UV) radiation. The effects of several operational parameters, such as the sodium acetate concentration, current generation, and nitrogen removal, were investigated. The effect of UV irradiation on the MFC performance was also studied.
2. MATERIALS AND METHODS 2.1. MFC Construction. Two-chamber MFC employing photocatalysis were constructed, as shown in Figure 1. The MFC consisted
3. RESULTS 3.1. Effect of COD on MFC Performance. To study the MFC performance in the presence and absence of COD, ammonium N and nitrate N were used as the inorganic donor and acceptor in the anode and cathode chambers, respectively. Figures 2 and 3 show the results of the experiments in which the COD concentration was varied in the batch feed. Figure 2 shows the current intensity with various COD concentrations, and current production was shown to be dependent upon the concentration of acetate. When the COD concentration was decreased from 120 ± 4 to 40 ± 5 mg L−1, current production did not obviously change, and the maximum nominal current was 1 ± 0.025 mA. However, with lower concentrations of COD (
