Dissolved humic substances-mediated electron shuttling unlocking nanoscale zero-valent iron reactivity in Shewanella oneidensis MR-1 systems for enhanced Cr(VI) removal

Dissimilatory iron-reducing bacteria (DIRB) offer an eco-friendly approach to address nanoscale zero-valent iron (nZVI) surface passivation problem, yet their efficacy is limited by the slow direct extracellular electron transfer (EET) between DIRB and Fe(III) oxide passivation layers (Fe( III )OPL). This study demonstrates that natural dissolved humic substances (DHS)-mediated electron shuttling unlocking nZVI reactivity in Shewanella oneidensis MR-1 (MR-1) system for enhance Cr(VI) removal. Seven DHSs from diverse sources were evaluated, revealing that Cr(VI) removal rate constants ( k obs ) correlate strongly with DHS's quinone content (C=O) and electron transfer capacity (ETC), quantified as k obs = 1.66 C C = O − 9.39 ( P  < 0.0007) and k obs = 19.37 ETC + 0.74 ( P  < 0.0005). DHS-mediated systems achieve 2.0–12.1-fold higher k obs than nZVI/MR-1 alone. The enhancement mechanism of DHS-mediated Cr(VI) removal by nZVI/MR-1 is that DHS effectively facilitates Fe( III )OPL dissolution on the nZVI surface via electron shuttling, maintaining nZVI reactivity and amplifying Fe(II) generation by 1.65-fold. Moreover, DHS stimulates MR-1 to secrete more extracellular polymeric substances (EPS), enhancing Cr(VI) adsorption and reduction. This work elucidates DHS's dual role in maintaining nZVI activity and amplifying Cr(VI) removal by nZVI/MR-1, while establishing a predictive framework linking DHS molecular traits parameters to Cr(VI) removal kinetic, offering sustainable strategies for heavy metal-contaminated water bodies remediation by integrating biogeochemical and material engineering principles.