Electrochemical and chemical enrichment methods of a sodic–saline inoculum for microbial fuel cells

In microbial fuel cells (MFCs) efficient extracellular electron transfer microbes, also known as anode-respiring bacteria, play an important role on cell performance. This type of microbes can be developed by application of enrichment procedures. The objective of this study was to compare a chemical (only C, final terminal electron acceptor Fe(III)), an electrochemical (only E), and a hybrid method (H, i.e., E followed by 3 serial transfers in iron (III) citrate medium) enrichment methods departing from a saline–sodic soil inoculum. In the electrochemical enrichment procedure in an electrolysis cell, the inoculum was subjected to a continuous electrical stress continually by posing the cell at −150 mV/SCE (+94 mV/SHE). The only C enrichment method delivered powers superior to the only E one (higher values of PAn,max = 49 mW m−2 and PV,max = 558 mW m−3 of C compared to 33 and 379 of only E). Interestingly, overall resistance as determined by EIS was lower for only E (1240 Ω) than for only C (1632 Ω). Yet, the hybrid H method, showed electrochemical characteristics consistently superior to both only C and only E methods (higher PAn,max and PV,max, lower internal resistance). Further detailed electrochemical studies of only E-method showed that the anodic resistance decreased with the time of operation of the electrolysis cell that would be consistent with the adaptability/enrichment purpose of the method. Also, Cyclic voltammetry peaks with values close to those reported for bacterial cytochromes appeared with time of cell operation.To the best of our knowledge, this is the first time that it is reported that serial transfers with Fe(III) as electron acceptor to an inoculum previously enriched in an electrolysis cell, leads to improved characteristics of MFC and increased Fe(III)-reducing capability of the inoculum.

► Evaluation of 3 enrichment methods of a saline–sodic soil for using in MFC.
► They were chemical (C), electrochemical (E), a hybrid method (E followed by C).
► E adaptability/enrichment of the inoculum associated to anodic resistance decrease.
► Hybrid method gave superior characteristics of the tested microbial fuel cell.
► First time a successful hybrid method of enrichment is reported in the literature.