Novel trickling microbial fuel cells for electricity generation from wastewater.

There are two main challenges associated with the scale-up of air-cathode microbial fuel cells (MFCs): performance reduction and cathode leakage/flooding. In this study, a novel 13.4 L reactor that contains 4 tubular MFCs was designed and operated in a trickling mode for 65 days under different conditions. The trickling water flow through the horizontally aligned MFCs alleviated the hydraulic pressure applied to the air-cathodes. With a total cathode working area of over 1700 cm2, this reactor generated power densities up to 1 W/m2 with coulombic efficiencies over 50% using acetate. Using a brewery waste stream as carbon source, an average power density of 0.27 W/m2 was generated with ∼60% COD removal at hydraulic retention time of 1.6 h. The decent performance of this reactor compared with other air-cathode MFCs at the similar scale and the alleviated hydraulic pressure on air-cathodes demonstrate the great potential of this design and operation for future MFC optimization and scaling up.

Accelerated tests for evaluating the air-cathode aging in microbial fuel cells.

Air-cathode stability is a key factor affecting the feasibility of microbial fuel cells (MFCs) in applications. However, there is no quick and effective method to evaluate the robustness and durability of the MFC air cathodes. In this study, a three-phase decrease of power density was observed in multiple MFCs that have been operated for about a year. Quantification of the contributions of cathode biofilm and salt accumulation to the current decrease suggested that the biofouling was the major contributor to the cathode aging during the first 200 days, and salt accumulation gradually outpaced biofouling afterward. An accelerated test method was then developed using fast-growing Escherichia coli, simulated soluble microbial products (SMPs), and a concentrated medium solution. Using this method, the cathode aging can be evaluated quickly within hours/days compared to over a year of operation, benefiting the development of high-performing and durable cathode materials.

Millimeter scale electron conduction through exoelectrogenic mixed species biofilms.

The functioning of many natural and engineered environments is dependent on long distance electron transfer mediated through electrical currents. These currents have been observed in exoelectrogenic biofilms and it has been proposed that microbial biofilms can mediate electron transfer via electrical currents on the centimeter scale. However, direct evidence to confirm this hypothesis has not been demonstrated and the longest known electrical transfer distance for single species exoelectrogenic biofilms is limited to 100 µm. In the present study, biofilms were developed on electrodes with electrically non-conductive gaps from 50 µm to 1 mm and the in situ conductance of biofilms was evaluated over time. Results demonstrated that the exoelectrogenic mixed species biofilms in the present study possess the ability to transfer electrons through electrical currents over a distance of up to 1 mm, 10 times further than previously observed. Results indicate the possibility of interspecies interactions playing an important role in the spatial development of exoelectrogenic biofilms, suggesting that these biological networks might remain conductive even at longer distance. These findings have significant implications in regards to future optimization of microbial electrochemical systems.

Redox Conductivity of Current-Producing Mixed Species Biofilms.

While most biological materials are insulating in nature, efficient extracellular electron transfer is a critical property of biofilms associated with microbial electrochemical systems and several microorganisms are capable of establishing conductive aggregates and biofilms. Though construction of these conductive microbial networks is an intriguing and important phenomenon in both natural and engineered systems, few studies have been published related to conductive biofilms/aggregates and their conduction mechanisms, especially in mixed-species environments. In the present study, current-producing mixed species biofilms exhibited high conductivity across non-conductive gaps. Biofilm growth observed on the inactive electrode contributed to overall power outputs, suggesting that an electrical connection was established throughout the biofilm assembly. Electrochemical gating analysis of the biofilms over a range of potentials (-600-200 mV, vs. Ag/AgCl) resulted in a peak-manner response with maximum conductance of 3437 ± 271 µS at a gate potential of -360 mV. Following removal of the electron donor (acetate), a 96.6% decrease in peak conductivity was observed. Differential responses observed in the absence of an electron donor and over varying potentials suggest a redox driven conductivity mechanism in mixed-species biofilms. These results demonstrated significant differences in biofilm development and conductivity compared to previous studies using pure cultures.

Enhanced performance and mechanism study of microbial electrolysis cells using Fe nanoparticle-decorated anodes.

Anode properties are critical for the performance of microbial electrolysis cells (MECs). In the present study, Fe nanoparticle-modified graphite disks were used as anodes to investigate the effects of nanoparticles on the performance of Shewanella oneidensis MR-1 in MECs. Results demonstrated that the average current densities produced with Fe nanoparticle-decorated anodes up to 5.89-fold higher than plain graphite anodes. Whole genome microarray analysis of the gene expression showed that genes encoding biofilm formation were significantly up-regulated as a response to nanoparticle-decorated anodes. Increased expression of genes related to nanowires, flavins, and c-type cytochromes indicates that enhanced mechanisms of electron transfer to the anode may also have contributed to the observed increases in current density. The majority of the remaining differentially expressed genes associated with electron transport and anaerobic metabolism demonstrate a systemic response to increased power loads.

Nanoparticle decorated anodes for enhanced current generation in microbial electrochemical cells.

The development of highly efficient anode materials is critical for enhancing the current output of microbial electrochemical cells. In this study, Au and Pd nanoparticle decorated graphite anodes were developed and evaluated in a newly designed multi-anode microbial electrolysis cell (MEC). The anodes decorated with Au nanoparticles produced current densities up to 20-fold higher than plain graphite anodes by Shewanella oneidensis MR-1, while those of Pd-decorated anodes with similar morphologies produced 50-150% higher than the control. Significant positive linear regression was obtained between the current density and the particle size (average Feret's diameter and average area), while the circularity of the particles showed negative correlation with current densities. On the contrary, no significant correlation was evident between the current density and the particle density based on area fraction and particle counts. These results demonstrated that nano-decoration can greatly enhance the performance of microbial anodes, while the chemical composition, size and shape of the nanoparticles determined the extent of the enhancement.

Quantification of the internal resistance distribution of microbial fuel cells.

Identifying the limiting factors in a microbial fuel cell (MFC) system requires qualifying the contribution of each component of an MFC to internal resistance. In this study, a new method was developed to calculate the internal resistance distribution of an MFC. Experiments were conducted to identify the limiting factors in single-chamber MFCs by varying the anode surface areas, cathode surface areas, and phosphate buffer concentrations. For the MFCs with equally sized electrodes (7 cm2) and 200 mM phosphate buffer, the anode contributed just 5.4% of the internal resistance, while the cathode and the electrolyte each contributed 47.3%, indicating that the anode was not the limiting factor in power generation. The limitation of the cathode was further revealed by the 780% higher area-specific resistance (284.4 omega cm2) than the 32.3 omega cm2 of the anode. The electrolyte limitation was also evidenced by the greatly increased contribution of electrolyte in internal resistance from 47.3 to 78.2% when the concentration of phosphate buffer was decreased from 200 to 50 mM. An anodic power density of 6860 mW/m2 was achieved at a current density of 2.62 mA/cm2 using the MFCs with an anode/cathode area ratio of 1/14 and 200 mM phosphate buffer. The method was also successfully applied to analyze the internal resistance distribution of the two chamber MFCs from a previously reported study. The comparison of the internal resistances of the two air cathode systems indicates that the much lower resistances, including anode, cathode, and membrane resistances, contributed to the much better performance of the single-chamber MFCs than the two-chamber system.

Hydrogen production using single-chamber membrane-free microbial electrolysis cells.

Microbial electrohydrogenesis provides a new approach for hydrogen generation from renewable biomass. Membranes were used in all the reported microbial electrolysis cells (MECs) to separate the anode and cathode chambers. To reduce the potential losses associated with membrane and increase the energy recovery of this process, single-chamber membrane-free MECs were designed and used to investigate hydrogen production by one mixed culture and one pure culture: Shewanella oneidensis MR-1. At an applied voltage of 0.6 V, this system with a mixed culture achieved a hydrogen production rate of 0.53 m(3)/day/m(3) (0.11 m(3)/day/m(2)) with a current density of 9.3A/m(2) at pH 7 and 0.69 m(3)/day/m(3) (0.15m(3)/day/m(2)) with a current density of 14 A/m(2) at pH 5.8. Stable hydrogen production from lactic acid by S. oneidensis was also observed. Methane was detected during the hydrogen production process with the mixed culture and negatively affected hydrogen production rate. However, by employing suitable approaches, such as exposure of cathodes to air, the hydrogenotrophic methanogens can be suppressed. The current density and volumetric hydrogen production rate of this system have potential to increase significantly by further reducing the electrode spacing and increasing the ratio of electrode surface area/cell volume.

Sustainable power generation in microbial fuel cells using bicarbonate buffer and proton transfer mechanisms.

Phosphate buffer solution has been commonly used in MFC studiesto maintain a suitable pH for electricity-generating bacteria and/or to increase the solution conductivity. However, addition of a high concentration of phosphate buffer in MFCs could be expensive, especially for wastewater treatment. In this study, the performances of MFCs with cloth electrode assemblies (CEA) were evaluated using bicarbonate buffer solutions. A maximum power density of 1550 W/m3 (2770 mW/ m2) was obtained at a current density of 0.99 mA/cm2 using a pH 9 bicarbonate buffer solution. Such a power density was 38.6% higher than that using a pH 7 phosphate buffer at the same concentration of 0.2 M. Based on the quantitative comparison of free proton transfer rates, diffusion rates of pH buffer species, and the current generated, a facilitated proton transfer mechanism was proposed for MFCs in the presence of the pH buffers. The excellent performance of MFCs using bicarbonate as pH buffer and proton carrier indicates that bicarbonate buffer could be served as a low-cost and effective pH buffer for practical applications, especially for wastewater treatment.

High-performance planar pH fluorosensor for two-dimensional pH measurements in marine sediment and water.

A new plate fluorosensor foil was developed for two-dimensional pH measurement in marine sediments and overlying waters. The fluorescent dye 8-hydroxy-1,3,6-pyrenetrisulfonic acid trisodium salt (HPTS) was covalently linked onto a transparent poly(vinyl alcohol) membrane backed by polyester sheet. Both excitation and emission bands of the immobilized HPTS showed large red shifts in comparison to HPTS in free solution. The ratio of fluorescence emission intensities at 540 nm following successive excitation at 506 and 428 nm was pH-sensitive and correlated with pH changes from 5.5 to 8.6. These properties are robust and insensitive to factors such as intensity of light source, temperature, and oxygen concentration. The plate sensor responded rapidly (< 2 min), reversibly, and with high precision to pH changes in solution and in sediment. The stability of the sensor was also tested by continually changing the pH between 6 and 8. Performance of the sensor did not vary after > 200 pH cycles. Furthermore, the sensor foil retained its original properties after continuous exposure to natural marine sedimentfor 2 months. The plate optode has been successfully used to measure two-dimensional pH distributions in intertidal flat and subtidal sediment. Images are readily obtained with inexpensive light-emitting diode (LED) excitation and commercial-grade digital cameras with typical pixel resolution of approximately 50 x 50 microm over areas > 150 cm2.

Biodegradability of Atrazine, Cyanazine and Dicamba under methanogenic condition in three soils of China.

Persistence and degradation of the herbicides Atrazine, Cyanazine and Dicamba were measured in laboratory microcosms incubated under methanogenic condition using three soils of China. Results showed that Atrazine was more resistant to degradation than Cyanazine and Dicamba for the 300 days of incubation. Between 30% and 40% of the initially introduced chemicals were found to be not recoverable through solvent extraction of the incubated soils. Our results also indicated that the half-life of these herbicides in the three soils generally followed: Atrazine>Cyanazine>Dicamba. Biodegradation of Cyanazine and Dicamba was further substantiated by establishing enrichment cultures in which the degradation of the respective herbicides could be accelerated by the microorganisms. Our results suggest that biodegradation of xenobiotics can be established through enrichment culture transfer technique and non-extractability of chemicals should be taken into account in evaluation of chemicals' fate and risk.

Relationship between structures of substituted indolic compounds and their degradation by marine anaerobic microorganisms.

Degradation of selected indolic compounds including indole, 1-methylindole, 2-methylindole, and 3-methylindole was assessed under methanogenic and sulfate-reducing conditions using the serum-bottle anaerobic technique and marine sediment from Victoria Harbour, Hong Kong as an inoculum. Our results showed that indole degradation was achieved in 28 days by a methanogenic consortium and 35 days by a sulfate-reducing consortium. During degradation under both conditions, two intermediates were isolated, purified and identified as oxindole and isatin (indole-2,3-dione) suggesting that both methanogenic and sulfate-reducing bacteria use an identical degradation pathway. Degradation processes followed two steps of oxidation accomplished by hydroxylation and then dehydrogenation at 2- and then 3-position sequentially prior to the cleavage of the pyrrole ring between 2- and 3-positions. However, none of 1-methylindole or 2-methylindole was degraded under any conditions. 3-Methylindole (3-methyl-1H-indole, skatole) was transformed under methanogenic conditions and mineralized only under sulfate-reducing conditions. It is clear that methyl substitution on 1- or 2-position inhibits the initial attack by hydroxylation enzymes making them more persistent in the environment and posing longer toxic impact.