Читать книгу Sustainable Solutions for Environmental Pollution - Группа авторов - Страница 18

Due to low values of short-chain carboxylates, there has been significant interest in upgrading short-chain carboxylates to high-value chemicals, such as medium-chain carboxylates (C6-C12; caproate, heptanoate, caprylate, etc.). Particularly, chain elongation has emerged as an innovative approach for manipulating carbon chain length of the products. The biological upgrading of short-chain carboxylates (e.g., acetate, propionate, butyrate) and alcohols (e.g., ethanol) via chain elongation can be used to synthesize medium-chain carboxylates. Chain elongation can be defined as an anaerobic open-culture secondary fermentation process that converts short-chain volatile fatty acids and an electron donor into medium-chain carboxylates (Angenent et al., 2016). The chain elongating microbes, such as Clostridium kluyveri can use the reverse β-oxidation pathway to convert short-chain volatile fatty acids to medium-chain carboxylates with ethanol as an electron donor (Roghair et al., 2018). For every five chain elongation reactions, one additional mole of ethanol is oxidized into acetate (Roghair et al., 2018; Seedorf et al., 2008).

Among various medium-chain carboxylates, caproate (C6) has received increasing interest due to its application as a precursor for aviation fuels and various commodity chemicals (Roghair et al., 2018). Jiang et al. (2020) studied the impact of cathodic EF on the mixed culture chain elongation for caproate production from acetate+ethanol. Integrating EF with the chain elongation process could increase the caproate specificity by ~28% with a fresh carbon felt cathode compared to a control reactor (open circuit without electrodes). However, EF with an acclimated cathode failed to increase the caproate specificity; caproate specificity was highly varied with the substrate concentrations. Thus, their results suggested a direct interaction between chain elongating microbes and the fresh electrode. Overall, their results suggested that deploying EF can provide an excellent opportunity to tune the chain elongation process for achieving higher efficiency.