Reshaping bio-nano interfaces via dissolved humic substances: Metabolic priming and electron shuttling drive robust Cr(VI) detoxification by MR-1/nZVI.

The application of nanoscale zero-valent iron (nZVI) and dissimilatory iron-reducing bacteria for environmental remediation is restricted by nZVI passivation and reactive oxygen species (ROS)-induced bacterial inactivation. To address these challenges, a dual-enhancement strategy using natural dissolved humic substances (DHS) was developed, integrating metabolic priming and electron shuttling to establish a highly active MR-120/nZVI/DHS system. Under optimal conditions, this system completely eliminated 80 mg/L Cr(VI) within 48 h, demonstrating a 2.94-fold increase in the apparent reduction rate constant (kobs) versus the traditional MR-1/nZVI system. Mechanistically, DHS-induced metabolic priming stimulates substantial extracellular polymeric substances (EPS) secretion. EPS and exogenous DHS form an electroactive microlayer that physically shields cells from nanoparticles and Cr(VI) toxicity, and directly quenches extracellular hydroxyl radicals (·OH). Meanwhile, metabolic priming pre-activates intracellular antioxidant defense and enhances ROS scavenging, alleviating nZVI/Cr(VI) combined toxicity and greatly improving bacterial survival. Benefiting from the protective microenvironment, intracellular energy (ATP) and reductive power (NADH) levels enhanced by 1.74- and 2.78-fold, respectively. These surges triggered 8.88- and 48.64-fold upregulation of outer membrane cytochrome c (Cyt c) and endogenous riboflavin (RF), effectively activating a dual-channel electron transfer network and driving a 15.71-fold surge in overall electron transport system activity (ETSA). The enhanced electron flux accelerates dissolution of the nZVI surfaces passivation layer, continuously regenerating active biogenic Fe(II) to reduce Cr(VI) and drive its co-precipitation into stable FeCr2O4 spinels. This work presents an efficient approach for treatment of Cr(VI)-contaminated wastewater and offers new theoretical insights into synergistic detoxification at the bio-nanomaterial interface under heavy metal stress.