| Microbial fuel cell (MFC), an advanced technology converting biomass into electricity, has many advantages, such as high efficiency, mild condition, extensive material and no secondary pollution. Undoubtedly, it provides a practical way to relieve both the problems of energy crisis and environmental pollution. Among various types of MFCs, sediment microbial fuel cell (SMFC) has gained more and more attention due to its unique structure. It is becoming a hot and focal point how their output power density to meet the demands of small electrical equipments is improved. Obviously, the limit to anode mass transfer of SMFCs seriously influences their electricity-generating performances. Therefore, magnetic stirring was exerted in SMFCs'anodic area to improve it. In this study, the effects of stirring rates (100, 200, 300 and 400 r/min) in anodic area of SMFCs on their overall performances, in terms of variations in cell performances, behaviors of producing electricity, sludge properties and characteristics of anode electrodes were investigated. Furthermore, the electron transfer mechanism was investigated by cyclic voltammetry (CV). The main research contents and results are as follows:(1) Influence of stirring rate on the cell performance of SMFCs: The power density of the SMFC with a stirring rate of 100 r/min amounted to the highest peak value (98.8 mW/m2) after the steady operation and it also showed the highest COD removal rate (84.2±0.2%). The power density of 400 r/min stirring rate was the lowest (35.9 mW/m2). It was indicated that for SMFC system, the higher was the COD removal rate, the higher was the power density. Moreover, the ammonia nitrogen removal improved as the stirring rate increased and the highest ammonia nitrogen removal achieved 35.9±0.7%.(2) Influence of stirring rate on the behavior of producing electricity of SMFCs: An appropriate stirring rate in anodic area could mixed the nutrient well, decreased mass transfer losses and ohmic losses, and influence the cathode potential and anode potential. The SMFC with the stirring rate of 100 r/min showed the highest open-circle potential (OCP) of cathode (346 mV), the lowest OCP of anode (-250 mV), the highest OCP (603 mV) and the lowest internal resistance (175 ?). The discharge behavior of the system at 100 r/min stirring rate was the most stabled.(3) Influence of stirring rate on the sludge properties of SMFCs: Different stirring conditions significantly affected the sludge parameters and had an indirect influence on the power generation performances of SMFCs. Sludge relative hydrophobicity (RH) and the volume average floc size of sludge decreased with increasing the stirring rates, but the content of EPS in sludge and the ratio of soluble microbial products (SMP) to bound EPS (BEPS) increased, which is not beneficial to the settleability of SMFCs to some certain extent.(4) Preliminary investigation of electron transfer mechanism: Stirring rates influenced the ratios of protein to polysaccharide of sludge on the anode surfaces and the MLVSS adsorbed. Taken the case of 100 r/min as an example, the ratio of protein to polysaccharide of sludge on the anode surface amounted to the maximum (0.914), and the MLVSS (111.0 mg/cm2) adsorbed on anode was highest. According to CV examination, the anode of SMFC with the stirring rate of 100 r/min showed the highest electrochemical activity. The electron transfer mechanism of our SMFCs was biofilm-controlled mechanism and the electrochemical activities were due to some kinds of cell-membrane associated mediating compounds with the redox potential of -0.250 V (vs. SCE). |
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