Sustainable Solutions for Environmental Pollution. Группа авторов
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1.3.1 Carboxylates
Carboxylates (organic acids having a carboxyl group) are primarily categorized as (1) short-chain carboxylates, containing 2–5 carbon atoms, such as acetate, propionate, butyrate, valerate, and (2) medium-chain carboxylates, containing 6–12 carbon atoms, such as caproate, heptanoate, caprylate (Nzeteu et al., 2018). They can be used as valuable platform chemicals and as precursors for the synthesis of a wide variety of marketable products, including fertilizers, pharmaceuticals, polymers, and personal care products (Mohan et al., 2016). The synthesis of these chemicals via microbial fermentation of organic waste and waste biomass provides a biorefinery framework for sustainable waste management for a greener future. However, a high-rate and robust fermentation processes must be achieved to ensure the same and lower prices of these chemicals, as compared to their petroleum-based production pathways. Recent studies have shown that EF could be a potential approach to achieve this goal.
1.3.1.1 Short-Chain Carboxylates
Various short-chain carboxylates, such as acetate, propionate, butyrate, etc., can be produced via microbial fermentation of organic feedstocks, including waste biomass. The acidogenic fermentation of complex organic substrates with mixed consortia can produce a mixture of various short-chain carboxylates (also called volatile fatty acids) in the fermentation broth (Paiano et al., 2019). These short-chain carboxylates can be either extracted or incorporated within other bioprocesses. However, a standalone acidogenic fermentation process is yet to be developed for industrial-scale synthesis of short-chain carboxylates (Paiano et al., 2019). Over the past few decades, significant research efforts have been dedicated towards developing strategies for long-term operation of acidogenic fermentation, optimization of process parameters (e.g., retention time, pH, temperature, substrate concentration, etc.), tuning the product spectrum (i.e., composition of carboxylates) (Arslan et al., 2016; Paiano et al., 2019).
Recently, EF has been investigated for various short-chain carboxylates production (see Table 1.1). These studies showed promising results in enhancing the yield and productivity of short-chain carboxylates with pure and mixed-culture. For example, Villano et al. (2017) studied cathodic EF of single and mixtures of substrates (glucose, ethanol, and acetate) with undefined mixed culture. Their results showed an applied potential of -700 mV vs. SHE could increase iso-butyrate production over the control (open-circuit reactor) by 20-fold (0.43 vs. 0.02 mol/mol glucose) for a mixture of glucose, ethanol, and acetate. Interestingly, iso-butyrate yields were quite comparable for both reactors when operated with glucose as a sole substrate and a combination of glucose plus ethanol or glucose plus ethanol. Thus, their results suggested that in the presence of applied potential and mixture of substrates could stimulate the growth of iso-butyrate producing microbial population. Although no exogenous redox mediator was provided in the fermenter, cyclic voltammetry (CV) of microbial culture showed a reduction peak at -700 mV vs. SHE (same as operating working electrode potential of EF system). Thus, the authors concluded that applied potential altered the redox status of the cells by shifting intracellular NADH/NAD+ ratio, which