Enhanced CO2 Conversion to Acetate through Microbial Electrosynthesis (MES) by Continuous Headspace Gas Recirculation

Raúl Mateos,Antonio Morán,Ana Sotres,Raúl M Alonso,Adrián Escapa

doi:10.3390/en12173297

Abstract

Bioelectrochemical systems (BESs) is a term that encompasses a group of novel technologies able to interconvert electrical energy and chemical energy by means of a bioelectroactive biofilm. Microbial electrosynthesis (MES) systems, which branch off from BESs, are able to convert CO2 into valuable organic chemicals and fuels. This study demonstrates that CO2 reduction in MES systems can be enhanced by enriching the inoculum and improving CO2 availability to the biofilm. The proposed system is proven to be a repetitive, efficient, and selective way of consuming CO2 for the production of acetic acid, showing cathodic efficiencies of over 55% and CO2 conversions of over 80%. Continuous recirculation of the gas headspace through the catholyte allowed for a 44% improvement in performance, achieving CO2 fixation rates of 171 mL CO2 L−1·d−1, a maximum daily acetate production rate of 261 mg HAc·L−1·d−1, and a maximum acetate titer of 1957 mg·L−1. High-throughput sequencing revealed that CO2 reduction was mainly driven by a mixed-culture biocathode, in which Sporomusa and Clostridium, both bioelectrochemical acetogenic bacteria, were identified together with other species such as Desulfovibrio, Pseudomonas, Arcobacter, Acinetobacter or Sulfurospirillum, which are usually found in cathodic biofilms. Moreover, results suggest that these communities are responsible of maintaining a stable reactor performance.

Highlights

Anthropogenic greenhouse gas emissions—among which CO2 occupies a preeminent position—are widely considered as the main contributors to the global rise in temperature [1]
Both replicate cells developed a similar electrical behavior during the start-up and acclimation period, as revealed by the low deviation between them in terms of the current consumption and product generation (Figure 3). This trend continued during the first three batch cycles; in addition, both microbial electrosynthesis (MES) showed little evolution between cycles. This behavior can be expected from a well-established biofilm enriched in important families that can undergo the proposed process: early colonizers in both biofilm and supernatant were dominated by Arcobacter, Acinetobacter, Pseudomonas, and Sulfurospirillum [26] with a high relative abundance, which are responsible for the current consumption from the first three batches
The inoculum enrichment procedure proved to be effective in developing a homoacetogenic community that is capable of producing a stable, replicable, and selective biofilm

Summary

Introduction

Anthropogenic greenhouse gas emissions—among which CO2 occupies a preeminent position—are widely considered as the main contributors to the global rise in temperature [1]. Not surprisingly, increasing worldwide CO2 emissions and their impact on climate change have become one of the main public concerns, and great efforts and investments are being made in the fields of science and engineering to reverse this situation [2]. In the last few years, the concept of carbon capture and utilization has become of great interest for the chemical industry as it represents a technological solution that aims to prevent CO2 emissions by converting them into value-added chemicals [3,4]. An electrode (cathode) can serve as an electron donor, avoiding the need for a chemical reductant [6]. This is the case for Energies 2019, 12, 3297; doi:10.3390/en12173297 www.mdpi.com/journal/energies

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Discussion

Conclusion