Bioelectrochemical Nitrogen fixation (e-BNF): Electro-stimulation of enriched biofilm communities drives autotrophic nitrogen and carbon fixation

Large-scale environmental applications of microbial electrochemical technologies (MET), such as wastewater treatment, bioremediation or soil improvement, would be more feasible if bioelectrodes could be fabricated with simpler materials. Biochar with potentially improved electroactive properties (e-biochar) can be an ideal candidate for this scope, being at the same time widely available, biocompatible and fully recyclable at end-of-life as soil amendment. Here we review the application of biochar to MET, to set benchmarks aimed at tuning the electroactive properties of such materials from the point of view of MET. The precursor biomass, thermochemical process conditions, and pre-, in-situ-and/or post-treatments should tailor optimized combinations of electrical conductivity, capacitance, superficial redox-active and electroactive functional groups, porosity distribution and capacity to host electroactive microbial communities. We also discuss methods to rigorously characterize e-biochar properties and the most relevant multidisciplinary research challenges towards its application in large-scale MET.

Recently, terracotta has attracted interest as low-cost and biocompatible material to build separators in microbial fuel cells (MFCs). However, the influence of a non-conductive material like terracotta on electroactive microbiological communities remains substantially unexplored. This study aims at describing the microbial pools developed from two different seed inocula (bovine and swine sewage) in terracotta-based air-breathing MFC. A statistical approach on microbiological data confirmed different community enrichment in the MFCs, depending mainly on the inoculum. Terracotta separators impeded the growth of electroactive communities in contact with cathodes (biocathodes), while a thick biofilm was observed on the surface (anolyte-side) of the terracotta separator. Terracotta-free MFCs, set as control experiments, showed a well-developed biocathode, Biocathode-MFCs resulted in 4 to 6-fold higher power densities. All biofilms were analyzed by high-throughput Illumina sequencing applied to 16S rRNA gene. The results showed more abundant (3- to 5-fold) electroactive genera (mainly Geobacter, Pseudomonas, Desulfuromonas and Clostridia MBA03) in terracotta-free biocathodes. Nevertheless, terracotta separators induced only slight changes in anodic microbial communities.

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