Nitric acid treated graphite granular cathode for microbial electro reduction of carbon dioxide to acetate

Abstract

In the present study, nitric acid treated graphite granular three-dimensional (3D) cathode was utilized for microbial reduction of carbon dioxide into acetate. Though graphite granules were previously used in few microbial electrosynthesis (MES) studies, nitric acid treated graphite granules were never utilized in MES. Nitric acid treatment was done to ensure the presence of nitrogen in the elemental composition of graphite granules, for further improving their properties, and to study their effect on the MES system performance. Treated graphite granules have shown better electron transfer resulted in better carbon dioxide reduction in the MES. Acetate volumetric production rate obtained from nitric acid treated graphite granular system (0.17 g L−1 d−1) was 1.4 times higher than the system having untreated graphite granules (0.12 g L−1 d−1) and 4.2 times higher than the system having no graphite granules in the cathode chamber (0.04 g L−1 d−1). Maximum acetate titer obtained in the study was 4.4 g L−1, with a current density of −5.12 A m−2 and columbic efficiency of 65.5% within 28 days of study. Modified graphite granules enhanced MES performance which could be associated with indirect (via H2) electron transfer coupled with direct electron transfer pathway. Electrochemical study, Cyclic voltammetry (CV) and Linear sweep voltammetry (LSV), also depicted better performance of nitric acid treated graphite granular system. Scanning electron microscopy (SEM) images have shown thicker biofilm formation in the modified graphite granular system. Thus, nitric acid can be used as a pre-treatment step to enhance the performance of microbial electrosynthesis and the production of acetate in MES having graphite granules.