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Establishing a sustainable methanogenic carbon dioxide reduction in bioelectrochemical systems and identification of kinetic and thermodynamic constraints.

Subject Area Hydrogeology, Hydrology, Limnology, Urban Water Management, Water Chemistry, Integrated Water Resources Management
Term from 2020 to 2024
Project identifier Deutsche Forschungsgemeinschaft (DFG) - Project number 445401355
 
Final Report Year 2024

Final Report Abstract

Herein, a new concept that integrates electrochemical and biological processes to produce renewable fuel gas was implemented and investigated, along with the development of novel methods for process analysis. This concept was based on a novel zero-gap electrolyser, which was comprehensively designed and improved for an efficient, stable and long-term operation by working with a poisoning catholyte derived from wastewater treatment streams. This wastewater catholyte was also the growth medium for carbon dioxide (CO2)-reducing strains of methanogens, providing an abundant and free source of nutrients. The use of CO2-rich flue gases, which are also available at wastewater treatment plants (e.g. from biogas combustion and sludge incineration), allows this power-to-gas approach to be implemented using only waste substrates. In addition, the here selected sulphide cathode catalyst is based on abundant minerals. The system was able to operate for over 120 days with the same electrodes, achieving average energy efficiencies of over 40% and daily production of up to 1700 L N CH4 per m2 of electrode. In this way, this proposed novel sustainable power-to-gas route that couples of the energy and water sectors achieved performance parameters that are very close to practical applications demands. The development of this concept involved a thoughtfully investigation of different processes and phenomena by an iterative conduction of experiments and model-based data analysis. This led to a novel electrochemical cell design with a pentlandite as the cathode catalyst for the hydrogen evolution reaction. A porous transport layer directly above the electrode provided a partial separation between biological and electrochemical processes. This avoided a cathodic biofilm formation but allowed for the transport of hydrogen (H2) to the liquid phase, which was continually exchanged with a bioreactor. Different designs for this biological unit were tested, whereby a column reactor packed with activated carbon allowed for the maximal CH4 production rates. Thereby, the utilization of packed bed reactors shifted the microbiome to a predominance of the genus Methanospirillum instead of the genera Methanobacterium and Methanobrevibacter, which were the most abundant methanogens at reactors with suspended biomass design. A comprehensive understanding of the H2 production-consumption cycle is fundamental for further improving this developed system. Therefore, a model-based estimation of electron donor (i.e., H2) availability at cell level was developed on basis of analysis of stable carbon isotope abundances (δ13C). This model includes a thermodynamic-dependent selectivity factor for the heavy stable carbon isotopes biological uptake with addition of isotope effects for ionic equilibria and mass-gas transfer. Hence, H2 concentrations available for biological uptake are estimated from Gibb’s free energy determined by the simulations, which used δ13C measurements in the system headspace for calibration. For a thoughtful analysis of the experimental system, this mechanistic model included also hydrogen evolution as well as oxygen evolution reactions using a multi-compartment implementation. Thereby, the main reactions as well as diffusive, convective and ion migration mass-transport processes of the experimental system were represented.

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