A sustainable way to reuse Cr(VI) into an efficient biological nanometer electrocatalyst by Bacillus megaterium

The remediation of heavy metal is facing the great challenge of failing to achieve valuable transformation. Therefore, the development of a sustainable technology for heavy metal recycling and reuse is essential. The present study proposed a new way to convert Cr(VI) into value-added biological Cr₂O₃ nanoparticles (bio-Cr₂O₃ NPs) with B. megaterium-secreted tryptophan residues proteins (TPN). In this process, Cr(VI) was reduced extracellularly to Cr(III) by B. megaterium without additional reductant and electron donors. This study overcomes the difficulty of separation of NPs and biomass, and realizes the recovery of bio-Cr₂O₃ NPS from biomass. The conversing efficiency of bio-Cr₂O₃ NPs reached the highest level (96.56%) at the concentration of 10 ppm Cr(VI). In particular, bio-Cr₂O₃ NPs exhibited excellent catalytic activity for both hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) in 1 M KOH, outperforming chemically synthesized Cr-base catalysts. Three-dimensional matrix fluorescence (EEM), verification of tryptophan reduction and computation chemistry fully confirmed that TPN was responsible for the bio-Cr₂O₃ NPs formation. This comprehensive approach to bioremediation, synthesis NPs and recovery, as well as application will open a window for sustainable energy development and heavy metal pollution remediation.