Graduate Thesis Or Dissertation
 

Electrical Conductivity in Mixed-Species Biofilms for Enhancing Energy Generation in Anaerobic Microbial Systems

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  • Further enhancement of energy generation is desired for practical application of anaerobic microbial systems such as microbial fuel cells (MFC) and anaerobic digesters (AD). A possible approach is to enhance the ability of microbial communities to transfer electron extracellularly in the form of electrical current. Critical to perform direct extracellular electron transfer (DEET) is the establishment of electrical connections between electron donors and acceptors in microbial communities. These connections can be facilitated through conductivity of microbial assemblages (e.g. biofilms and granules). Current understanding towards the microbially constructed conductivity is limited and two distinct theories of conductive mechanisms have emerged: an incoherent redox conduction model and a coherent metallic-like conduction model. While single species biofilms of Geobacter sulfurreducens have been thoroughly studies in terms of conductivity, conductive behaviors, and conductive mechanisms, these conductive features of mixed-species communities remain unexplored. The present dissertation aims to evaluate the electrical conductivity of mixed- species exoelectrogenic and methanogenic biofilms and develop strategies to enhance the energy generation in anaerobic microbial systems. Conductive behaviors of these mixed- species biofilms were investigated during the initial growth period, over various nonconductive distances, and at different metabolic modes such as deprivation of substrate. Conductive mechanisms were elucidated by examining the conductance of these mixed-species biofilms over a range of redox potentials and at different intensities of magnetic field. Correlations between biofilm conductivity, microbial community, strength of magnetic field, and power/current generation were also investigated in mixed- species MFCs. Results demonstrated that both mixed-species exoelectrogenic biofilms and methanogenic biofilms exhibited conductivities (2651 and 71.8 μS/cm, respectively) across non-conductive gaps. Electrochemical gating analysis indicated that electron transfer in exoelectrogenic and methanogenic biofilms occurs through incoherent redox conduction. For exoelectrogenic biofilms, increase of biofilms conductance correlates with the increase of power output of MFCs during the startup period. Deprivation of substrate decreased the biofilm conductivity by an order of magnitude. Exoelectrogenic biofilms are capable of extending their conductive structures on the millimeter scale. The redox conductivity of exoelectrogenic biofilms was also confirmed by the dependence of conductance on magnetic field, in which biofilm conductance increased along with the increase of magnetic field intensity. This conductive behavior resembles the negative magnetoresistance in certain conductive polymers that has ever been observed in biofilms before. The strong correlation observed between biofilm conductivity and Geobacter spp. in the metabolically diverse anaerobic communities suggests that the efficiency of direct extracellular electron transfer may provide pressure for microbial communities to select for species that can produce electrical conduits. Application of static low intensity magnetic field to MFC can increase the conductivity of exoelectrogenic biofilms on anode of MFC and thereby enhance the power/current generation of MFC. These results propose feasible approaches by which energy generation and start-up times may be improved in engineered anaerobic microbial systems such as MFC and AD.
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  • 2017-08-16 to 2018-03-23

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