Electroactive microorganisms synthesizing iron sulfide nanoparticles for enhanced hexavalent chromium removal in microbial fuel cells

Microbial fuel cells (MFCs) have been considered a promising technology for Cr⁶⁺ removal, but they are limited by Cr⁶⁺-reducing biocathodes with low extracellular electron transfer (EET) and poor microbial activity. In this study, three kinds of nano-FeS hybridized electrode biofilms, obtained through synchronous biosynthesis (Sy-FeS), sequential biosynthesis (Se-FeS) and cathode biosynthesis (Ca-FeS), were applied as biocathodes for Cr⁶⁺ removal in MFCs. The Ca-FeS biocathode exhibited the best performance due to the superior properties of biogenic nano-FeS (e.g., more synthetic amount, smaller particle size, better dispersion). The MFC with the Ca-FeS biocathode achieved the highest power density (42.08 ± 1.42 mW/m²) and Cr⁶⁺ removal efficiency (99.18 ± 0.1 %), which were 1.42 and 2.08 times as high as those of the MFC with the normal biocathode, respectively. The synergistic effects of nano-FeS and microorganisms enhanced the bioelectrochemical reduction of Cr⁶⁺, first realizing deep reduction of Cr⁶⁺ to Cr⁰ in biocathode MFCs. This significantly alleviated the cathode passivation caused by Cr³⁺ deposition. In addition, the hybridized nano-FeS as “armor” layers protected the microbes from toxic attack by Cr⁶⁺, improving the biofilm physiological activity and extracellular polymeric substances (EPS) secretion. The hybridized nano-FeS as “electron bridges” facilitated the microbial community to form a balanced, stable and syntrophic ecological structure. This study proposes a novel strategy through the cathode in-situ biosynthesis of nanomaterials to fabricate hybridized electrode biofilms with enhanced EET and microbial activity for toxic pollutant treatment in bioelectrochemical systems.