Abstract
Technologies able to convert CO2 to various feedstocks for fuels and chemicals are emerging due to the urge of reducing greenhouse gas emissions and de-fossilizing chemical production. Microbial electrosynthesis (MES) has been shown a promising technique to synthesize organic products particularly acetate using microorganisms and electrons. However, the efficiency of the system is low. In this study, we demonstrated the simple yet efficient strategy in enhancing the efficiency of MES by applying continuous feeding regime. Compared to the fed-batch system, continuous operational mode provided better control of pH and constant medium refreshment, resulting in higher acetate production rate and more diverse bio-products, when the cathodic potential of -1.0 V Ag/AgCl and dissolved CO2 were provided. It was observed that hydraulic retention time (HRT) had a direct effect on the pattern of production, acetate production rate and coulombic efficiency. At HRT of 3 days, pH was around 5.2 and acetate was the dominant product with the highest production rate of 651.8 ± 214.2 ppm per day and a significant coulombic efficiency of 90%. However at the HRT of 7 days, pH was lower at around 4.5, and lower but stable acetate production rate of 280 ppm per day and a maximum coulombic efficiency of 80% was obtained. In addition, more diverse and longer chain products, such as butyrate, isovalerate and caproate, were detected with low concentrations only at the HRT of 7 days. Although microbial community analysis showed the change in the planktonic cells communities after switching the fed-batch mode to continuous feeding regime, Acetobacterium still remained as the responsible bacteria for CO2 reduction to acetate, dominating the cathodic biofilm.
Highlights
Adverse effects of CO2 emissions in the atmosphere on the environment such as climate change have raised the urgent demand on new technologies to capture CO2
We have shown in our previous study that compared with bicarbonate, gaseous CO2 leads to higher abundance of Acetobacterium, by improving the protein synthesis of bacteria.[16]
A er 14 days, operational mode was switched to continuous feeding regime with hydraulic retention time (HRT): 3 days
Summary
Adverse effects of CO2 emissions in the atmosphere on the environment such as climate change have raised the urgent demand on new technologies to capture CO2. Acetate is the major product of CO2 reduction via microbial electrosynthesis (MES) following the Wood–Ljungdahl pathway.[6] Production of longer chain organics than acetate through MES was reported in previous studies by controlling fermentation pathways.[3,7] chain elongation in BES and selectivity in production of C4 or C6 organics is still the major challenge. It has been reported in previous studies that operational conditions provided in BES have an important effect on enrichment of desired microbial communities and on selectivity of products.[8] pH is one of the key operational parameters affecting microbial metabolisms. In a previous study by Bajracharya et al.,[17] higher acetate production rate (149 mg per L per day) was achieved over batch experiments compared to that in continuous operational
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