BIO-ELECTRORECYCLING OF CARBON DIOXIDE INTO VALUABLE PRODUCTS
Carbon dioxide (CO2) is largely emitted during the combustion of fossil fuels for transportation, electricity generation, and other industrial processes. CO2 is an inorganic compound that naturally exists on Earth, but its concentration has increased exponentially since the industrial revolution, leading to the acceleration of global warming and, consequently, the climate change. Bioelectrochemical systems (BES) are among several platforms that have been proposed to capture and convert CO2. The primary system widely applied for this purpose is microbial electrosynthesis (MES), in which microorganisms serve as catalysts in the reduction of CO2 to target compounds, without the need for expensive and precious materials. In this regard, this PhD thesis addresses the challenges of improving microbial electrosynthesis of acetate (HA) from CO2 by testing different sources of inoculum in various reactor setups. Biocharbased electrodes were fabricated through the pyrolysis of waste feedstocks (woody chips (WC) and organic wastes (OW)). Their performances, after being applied to a carbon cloth cathode, were tested in MES systems. Additionally, a review of the state of the art of carbon capture technologies will provide an initial understanding of the context. My PhD project was divided into four different work packages: 1) selection and enrichment of bacterial consortia, 2) production of biochar-based electrodes, 3) production of HA by testing different MES setups, while modulating the applied potential and changing cathode materials, 4) analysis of possible applications in a pilot-scale scenario. In MES reactors equipped with a carbon cloth electrode (CC) and an inoculum from brewery wastewater, the best applied potential was -1.0 V with a conversion of 60.37% of CO2 in HA with a daily production rate of 97.05 mg HA L-1 d -1 . Considering these results, it has been calculated that, 8 using all the CO2 produced by the biogas plants in Italy after upgrading of biogas could approximately recycled 6.1·106 tons of CO2 in HA. Regarding the comparison of biochar-based electrodes, MES reactors equipped with WC-cathodes achieved better performance in all considered parameters than the OW-based ones. Moreover, the HA produced by WC-cathodes was ten times higher than in OW reactors (0.3 g L-1 d -1 ), and the moles of CO2 converted into acetic acid every day reached 65%. In the last MES experiment, the WC electrode was compared to the CC one in terms of energy demand, and it proved to be a better alternative than carbon-based electrodes in terms of Coulombic efficiency (57%), specific energy consumption (21.5 kWh kg-1HA), and HA surface production rate (13 g HA m-2 d -1 ). It was also demonstrated that its three-dimensional spatial distribution enhanced microbial growth, leading to a higher number of microorganisms adhering to its surface.
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