Bio-electrocatalytic reduction of CO2: Enrichment of homoacetogens and pH optimization towards enhancement of carboxylic acids biosynthesis

Abstract

The microbial catalyzed electrochemical reduction of CO2 is gaining significant attention in the field of energy and environment as it provides dual benefits of product recovery with simultaneous CO2 neutrality. Specific reduction of single carbon unit (C1-CO2) to two-carbon (C2-CH3COOH) carboxylic acids was studied in a bio-electrochemical system (BES) via a three stage experimental design using BESA treated and untreated (control) anaerobic consortia. During stage-I, enrichment of homoacetogenic culture was carried out by supplementing H2 and CO2 in the reactors containing BESA treated and control cultures respectively. Optimization of pH was carried out in stage-II to enhance the carboxylic acids production at diverse pH range (acidic to alkaline viz., pH 5, 6.5, 8.5 and 10), where pH 10 was found to be optimum for maximum carboxylic acids (VFA: volatile fatty acids) generation in BESA treated (3500mg/l) and control cultures (1200mg/l) respectively followed by pH 8.5 utilizing bicarbonate. Interestingly, reduction in VFA concentration was observed after 24h which can be attributed to its consumption by other groups of bacteria that co-exist along with the enriched culture. During stage-III, bioelectro-catalytic production of acetate was evaluated by considering the optimized pH 10 and under applied potential of -0.8V vs Ag/AgCl (S) in two BES viz., BESB (BES with BESA treated consortia) and BESC (BES with parent consortia as control) using CO2 and bicarbonate respectively. Maximum acetate production of 1.7g/l (2.88mmol/d)/2.1g/l (3.55mmol/d) was recovered in BESB through the bio-electrochemical reduction of CO2/bicarbonate, which correlated well with the observed higher reduction currents and columbic efficiency.