Riboflavin-mediated reactivation of aged nano zero-valent iron by Shewanella oneidensis MR-1 for sustained Cr(VI) detoxification

Aging-induced surface passivation of nano zero-valent iron (nZVI) significantly diminishes its reactivity for Cr(VI) reduction, thereby impeding its practical application in environmental remediation. Herein, we propose a novel system integrating aged nZVI (AnZVI), Shewanella oneidensis MR-1, and the endogenous electron shuttle riboflavin (RF) to overcome the passivation limitations and enhance Cr(VI) removal. Remarkably, the AnZVI/MR-1/RF system achieved complete removal of 20 mg L⁻¹ Cr(VI) within 3 h, outperforming AnZVI/MR-1 and AnZVI alone by 23.8 % and 36.6 %, respectively. Mechanistic investigations revealed that RF mediated direct interfacial contact between AnZVI and MR-1, promoting microbial reduction of the AnZVI passivation layer. Electrochemical analyses confirmed RF’s critical role as an electron mediator in accelerating charge transfer between AnZVI and MR-1. Three-dimensional fluorescence mapping (3D-EEM) analysis revealed microbial extracellular polymeric substances (EPS) contributions to chromium immobilization processes. The immobilization of chromium is primarily attributed to the reduction of Cr(VI) to insoluble Cr(III) species, in conjunction with adsorption and surface complexation mechanisms. TEM validated a significant reduction in the thickness of the iron passivation layer, resulting in the exposure of the underlying Fe⁰ core and a resultant enhancement in reductive capacity. This work provides a robust strategy to overcome AnZVI’s limitations by coupling microbial activity with electron shuttling, offering new insights into sustainable remediation via biotic-abiotic synergies.