Dao-Bo Li

Preparation of a macroporous flexible three dimensional graphene sponge using an ice-template as the anode material for microbial fuel cells

A simple and effective method for the fabrication of a flexible macroporous 3D graphene sponge using an ice template is developed in this work. It was found that the porous structures of the 3D graphene architecture depended on the rate of ice crystal formation. At a low cooling rate, the inner walls of the graphene hydrogel were re-assembled into a hierarchical macroporous structure by the as-formed ice crystals, resulting in the formation of a macroporous graphene sponge after freeze-drying. The as-prepared graphene sponge was flexible and could recover from a 50% deformation. When the graphene sponge was used as the anode of a microbial fuel cell (MFC), the maximum power density reached 427.0 W m−3, which was higher than that of the MFC fabricated using carbon felt as the anode material. The macroporous structure of the graphene sponge ensured the microbes more easily diffused and propagated inside the materials, resulting in higher MFC performance.

Anodic Fenton process assisted by a microbial fuel cell for enhanced degradation of organic pollutants

The electro-Fenton process is efficient for degradation of organic pollutants, but it suffers from the high operating costs due to the need of power investment. Here, a new anodic Fenton system is developed for energy-saving and efficient treatment of organic pollutants by incorporating microbial fuel cell (MFC) into an anodic Fenton process. This system is composed of an anodic Fenton reactor and a two-chamber air-cathode MFC. The power generated from a two-chamber MFC is used to drive the anodic Fenton process for Acid Orange 7 (AO7) degradation through accelerating in situ generation of Fe(2+) from sacrificial iron. The kinetic results show that the MFC-assisted anodic Fenton process system had a significantly higher pseudo-first-order rate constant than those for the chemical Fenton methods. The electrochemical analysis reveals that AO7 did not hinder the corrosion of iron. The anodic Fenton process was influenced by the MFC performance. It was also found that increasing dissolved oxygen in the cathode improved the MFC power density, which in turn enhanced the AO7 degradation rate. These clearly demonstrate that the anodic Fenton process could be integrated with MFC to develop a self-sustained system for cost-effective and energy-saving electrochemical wastewater treatment.

Experimental and Theoretical Demonstrations for the Mechanism behind Enhanced Microbial Electron Transfer by CNT Network

Bioelectrochemical systems (BESs) share the principle of the microbially catalyzed anodic substrate oxidation. Creating an electrode interface to promote extracellular electron transfer from microbes to electrode and understanding such mechanisms are crucial for engineering BESs. In this study, significantly promoted electron transfer and a 10-times increase in current generation in a BES were achieved by the utilization of carbon nanotube (CNT) network, compared with carbon paper. The mechanisms for the enhanced current generation with the CNT network were elucidated with both experimental approach and molecular dynamic simulations. The fabricated CNT network was found to be able to substantially enhance the interaction between the c-type cytochromes and solid electron acceptor, indicating that the direct electron transfer from outer-membrane decaheme c-type cytochromes to electrode might occur. The results obtained in this study will benefit for the optimized design of new materials to target the outer membrane proteins for enhanced electron exchanges.

Photoautotrophic cathodic oxygen reduction catalyzed by a green alga, Chlamydomonas reinhardtii

Biofuel cells (BFCs) use enzymes and microbial cells to produce energy from bioavailable substrates and treat various wastewaters, and cathodic oxygen reduction is a key factor governing the efficiency of BFCs. In this study, we demonstrated that a green alga, Chlamydomonas reinhardtii, could directly mediate oxygen reduction. Cyclic voltammogram analysis revealed that the C. reinhardtii biofilm formed on a solid electrode was responsible for oxygen reduction without dosing of electron mediator. Furthermore, 4‐electron oxygen reduction pathway was found in this self‐sustained, light‐responded BFC. The results of this study could expand our understanding and viewpoints of biocathode catalysis, which is essential for novel catalyst design and development for BFCs. Biotechnol. Bioeng. 2013; 110: 173–179.

Manipulation of microbial extracellular electron transfer by changing molecular structure of phenazine-type redox mediators.

Phenazines, as a type of electron shuttle, are involved in various biological processes to facilitate microbial energy metabolism and electron transfer. They constitute a large group of nitrogen-containing heterocyclic compounds, which can be produced by a diverse range of bacteria or by artificial synthesis. They vary significantly in their properties, depending mainly on the nature and position of substitutent group. Thus, it is of great interest to find out the most favorable substituent type and molecular structure of phenazines for electron transfer routes. Here, the impacts of the substituent group on the reduction potentials of phenazine-type redox mediators in aqueous solution were investigated by quantum chemical calculations, and the calculation results were further validated with experimental data. The results show that the reaction free energy was substantially affected by the location of substituent groups on the phenazine molecule and the protonated water clusters. For the main proton addition process, the phenazines substituted with electron-donating groups and those with electron-withdrawing groups interacted with different protonated water clusters, attributed to the proximity effect of water molecules on proton transfer. Thus, high energy conversion efficiency could be achieved by controlling electron flow route with appropriate substituted phenazines to reduce the biological energy acquisition. This study provides useful information for designing efficient redox mediators to promote electron transfer between microbes and terminal acceptors, which are essential to bioenergy recovery from wastes and environmental bioremediation.

Enhancing Extracellular Electron Transfer of Shewanella oneidensis MR-1 through Coupling Improved Flavin Synthesis and Metal-Reducing Conduit for Pollutant Degradation

Dissimilatory metal reducing bacteria (DMRB) are capable of extracellular electron transfer (EET) to insoluble metal oxides, which are used as external electron acceptors by DMRB for their anaerobic respiration. The EET process has important contribution to environmental remediation mineral cycling, and bioelectrochemical systems. However, the low EET efficiency remains to be one of the major bottlenecks for its practical applications for pollutant degradation. In this work, Shewanella oneidensis MR-1, a model DMRB, was used to examine the feasibility of enhancing the EET and its biodegradation capacity through genetic engineering. A flavin biosynthesis gene cluster ribD-ribC-ribBA-ribE and metal-reducing conduit biosynthesis gene cluster mtrC-mtrA-mtrB were coexpressed in S. oneidensis MR-1. Compared to the control strain, the engineered strain was found to exhibit an improved EET capacity in microbial fuel cells and potentiostat-controlled electrochemical cells, with an increase in maximum current density by approximate 110% and 87%, respectively. The electrochemical impedance spectroscopy (EIS) analysis showed that the current increase correlated with the lower interfacial charge-transfer resistance of the engineered strain. Meanwhile, a three times more rapid removal rate of methyl orange by the engineered strain confirmed the improvement of its EET and biodegradation ability. Our results demonstrate that coupling of improved synthesis of mediators and metal-reducing conduits could be an efficient strategy to enhance EET in S. oneidensis MR-1, which is essential to the applications of DMRB for environmental remediation, wastewater treatment, and bioenergy recovery from wastes.

Promotion of Iron Oxide Reduction and Extracellular Electron Transfer in Shewanella oneidensis by DMSO

The dissimilatory metal reducing bacterium Shewanella oneidensis MR-1, known for its capacity of reducing iron and manganese oxides, has great environmental impacts. The iron oxides reducing process is affected by the coexistence of alternative electron acceptors in the environment, while investigation into it is limited so far. In this work, the impact of dimethyl sulphoxide (DMSO), a ubiquitous chemical in marine environment, on the reduction of hydrous ferric oxide (HFO) by S. oneidensis MR-1 was investigated. Results show that DMSO promoted HFO reduction by both wild type and ΔdmsE, but had no effect on the HFO reduction by ΔdmsB, indicating that such a promotion was dependent on the DMSO respiration. With the DMSO dosing, the levels of extracellular flavins and omcA expression were significantly increased in WT and further increased in ΔdmsE. Bioelectrochemical analysis show that DMSO also promoted the extracellular electron transfer of WT and ΔdmsE. These results demonstrate that DMSO could stimulate the HFO reduction through metabolic and genetic regulation in S. oneidensis MR-1, rather than compete for electrons with HFO. This may provide a potential respiratory pathway to enhance the microbial electron flows for environmental and engineering applications.

Exclusive Extracellular Bioreduction of Methyl Orange by Azo Reductase-Free Geobacter sulfurreducens.

Azo dyes are a class of recalcitrant organic pollutants causing severe environmental pollution. For their biodecolorization, the azo reductase system was considered as the major molecular basis in bacteria. However, the intracellular localization of azo reductase limits their function for efficient azo dye decolorization. This limitation may be circumvented by electrochemically active bacteria (EAB) which is capable of extracellular respiration. To verify the essential role of extracellular respiration in azo dye decolorization, Geobacter sulfurreducens PCA, a model EAB, was used for the bioreduction of methyl orange (MO), a typical azo dye. G. sulfurreducens PCA efficiently reduced MO into amines. Kinetic results showed that G. sulfurreducens PCA had the highest decolorization efficiency among the currently known MO reducing bacteria. Electrons from acetate oxidization by this strain were transferred by the respiratory chain to MO. The mass and electron balances, fluorescent probing and proteinase K treatment experimental results indicate that the biodecolorization of MO by G. sulfurreducens PCA is an exclusive extracellular process. OmcB, OmcC and OmcE were identified as the key outer-membrane proteins for the extracellular MO reduction. This work deepens our understanding of EAB physiology and is useful for the decontamination of environments polluted with azo dyes. The contribution of extracellular respiration to pollutants reduction will broaden the environmental applications of EAB.

Anaerobic reduction of 2,6‐dinitrotoluene by Shewanella oneidensis MR‐1: Roles of Mtr respiratory pathway and NfnB

Dinitrotoluene (DNT) is a widely present pollutant in aquatic environments, and its biodegradation is an economically attractive way to effectively removal. In aquatic environments, the presence of electrochemically active bacteria (EAB) could contribute to the anaerobic bioreduction of DNT. However, the mechanism behind such a biodegradation process at gene level remains to be further elucidated. In this work, the anaerobic reduction of 2,6‐dinitrotoluene (2,6‐DNT) by Shewanella oneidensis MR‐1, a typical EAB in aquatic environments, was investigated. S. oneidensis MR‐1 was found to be able to obtain energy for growth through the anaerobic respiration on 2,6‐DNT. Experimental results show that the Mtr respiratory pathway, a transmembrane electron transport chain, was involved in the 2,6‐DNT bioreduction. Knockout of cymA or nfnB resulted in a substantial loss of its 2,6‐DNT‐reducing ability, indicating that both CymA and NfnB were the key proteins in the microbial electron transfer chain. The genetic analysis further confirms that the Mtr respiratory pathway and NfnB are mainly responsible for the anaerobic reduction of 2,6‐DNT by S. oneidensis MR‐1. This work is useful to better understand the anaerobic bioreduction of nitroaromatic compounds in aquatic environments and remediate the environments contaminated by nitroaromatic compounds. Biotechnol. Bioeng. 2017;114: 761–768.

Algal biomass derived biochar anode for efficient extracellular electron uptake from Shewanella oneidensis MR-1

The development of cost-effective and highly efficient anode materials for extracellular electron uptake is important to improve the electricity generation of bioelectrochemical systems. An effective approach to mitigate harmful algal bloom (HAB) is mechanical harvesting of algal biomass, thus subsequent processing for the collected algal biomass is desired. In this study, a low-cost biochar derived from algal biomass via pyrolysis was utilized as an anode material for efficient electron uptake. Electrochemical properties of the algal biochar and graphite plate electrodes were characterized in a bioelectrochemical system (BES). Compared with graphite plate electrode, the algal biochar electrode could effectively utilize both indirect and direct electron transfer pathways for current production, and showed stronger electrochemical response and better adsorption of redox mediators. The maximum current density of algal biochar anode was about 4.1 times higher than graphite plate anode in BES. This work provides an application potential for collected HAB to develop a cost-effective anode material for efficient extracellular electron uptake in BES and to achieve waste resource utilization.