One-step synthesis of a novel natural mineral-derived Fe@BC for enhancing Cr(VI) bioreduction: Synergistic role of electron transfer and microbial metabolism

Iron minerals, which exert excellent biocompatibility and reactivity with redox-active microorganisms, have attracted attention as a precursor to synthesizing composite materials with higher catalytic efficiency in driving redox-active microorganisms to reduce Cr(VI). However, researches on the effective preparation method of composites, the interaction between bacteria and composite materials and the mechanism of electron transfer are still scarce. In this work, Fe-complex@BC prepared by a one-step method using goethite was used for chromium treatment together with soil microorganisms. The composite was the best-performing in promoting Cr(VI) bioreduction (up to 3.48 mg (L·h)^-1) than Fe-complex (2.26 mg (L·h)^-1) and biochar (0.5 mg (L·h)^-1), even about 19 times higher than that of bioreduction without materials. Specifically, Fe-complex@BC shortened the electron transfer distance due to its excellent adsorption properties for bacteria and Cr(VI). Its high redox activity also promoted Cr(VI) bioreduction by directly enhancing electron transfer. In addition, the presence of the Fe(III)/Fe(II) cycle proved that the active sites of composite could be regenerated to reduce Cr(VI) persistently by receiving extracellular electrons from bacteria. High-throughput 16 S rDNA gene sequencing indicated the composite could promote the proliferation of electrochemically active bacteria, which directly enhanced bioreduction. This study developed the low-cost Fe@BC material prepared by a one-step co-pyrolysis method, which exerts a synergistic effect with soil microorganisms and presents a promising potential for chromium pollution treatment.