Elucidating the electron flow in microbial electrochemical technology for broadening the application

Abstract

Microbial electrochemical technologies (MET) have received increasing attention due to the unique merits as environmentally friendly, carbon-neutral technologies for sustainable water treatment, resources recovery, environmental monitoring, and for production of biochemicals and biofuels. In MET, electroactive bacteria (i.e., exoelectrogens), which are able to conduct extracellular electron transfer, are the key engine driving the various applications. Thus, formation of dense, thick and electroactive biofilm on the electrode, is a determining factor for efficient MET processes. However, fundamental understanding on the electron transfer and electroactive biofilm formation is still missing, hindering dedicated development for progressing the state of MET applications in the years to come. The main objective of this thesis is to provide fundamental understanding of the electron flow driving the MET and simultaneously develop novel solutions for key MET applications. Firstly, efforts were made to develop a microbial electrolysis cell (MEC)-based ammonia sensor to broaden the MET application. An innovative electrochemical cell (EC)-nitrification system was developed for ammonia monitoring. The biosensor was composed of two stages: nitrification stage of ammonia oxidation to nitrate, and cathodic nitrate reduction in EC. Good linear relationship between ammonia levels (0 to 7.1 mM NH4+-N) and current signal was consistently obtained independent of the applied voltage and wastewater pH. Subsequently, the biosensor was improved to a novel MEC based sensor which had reduced energy consumption during biosensor operation. Likewise, linear relationship was established between current and ammonia levels (0 and 62.1 mg NH4+-N /L). There was no significant difference between the results obtained from biosensor and fast testing kits, indicating the reliable results. However, the anodic biocatalysts were found to be the key limitation to hinder its commercial application. Therefore, much attention has been paid to fundamental areas particularly the anodic biocatalyst. To obtain an efficient biocatalyst in MET, anaerobic granular sludge (AGS) was selected as potential biocatalyst due to the innate massive microbe concentrations and large surface area. Different strategies were investigated to transform methanogenic AGS into exoelectrogenic. Controlling the anodic potential at +20 mV was found as the optimal strategy to meet the goal of carbon removal along with electron generation. The analysis of microbial community dynamics proved that electroactive Desulfurmonadales spp. increased significantly after the potential control, whereas Methanosaeta concilii and Mesotoga infera decreased. This result was in good accordance with the increased electricity generation. Furthermore, the electron storage in exoelectrogenic AGS was studied to elucidate the biological way of electron generation, transportation and storage fundamentally. The results showed that the AGS-based system had a good capacity of electron storage. The optimized capacitance was obtained with 5 min charging and 10 min discharging cycle at +0.2 V. The formal potential observed from cyclic voltammetry analysis was associated to Geobacter bacteria. Peaks of cytochromes were also detected by Raman spectra. All these together proved that there was an effective electron flow from the granule to the conductive material (i.e., electrode). Thereof, a potential mechanism of electron storage in such system was disclosed: one contributor is double layer effect and the other one is the cytochromes in exoelectrogens. Overall, this study broadens the application niches of the technology and also provides novel solutions to the key challenges in MET (i.e. maintaining sufficient amount of biocatalysts). The conducted research work not only provides a capacious application view for MET technology in environmental field, but also brings a fundamental understanding of an effective electron flow in MET.