Cation-driven electrostatic modulation enhances Cr(VI) reduction by biochar

The coexistence of cations and anions critically influences contaminant behavior in various environments, yet their roles in redox-driven remediation remain underexplored. This study elucidates how cations (K(I), Mg(II), Zn(II), Cu(II), and Cr(III)) enhance biochar-mediated Cr(VI) removal through electrostatic modulation. By quantitatively decoupling the adsorption and reduction pathways, we demonstrated that cation co-presence promoted Cr(VI) adsorption (3.5–26.2-fold) and reduction (1.1–3.0-fold), with more than 63.9 % the overall Cr(VI) removal attributing to reduction. While superoxide radicals (·O₂ − ) contributed − to liquid-phase reduction, their efficacy depended on interfacial accessibility rather than abundance. A significant correlation between zeta potential elevation and Cr(VI) reduction ( P < 0.01) indicated electrostatic modulation as the key mechanism for the enhanced Cr(VI) removal in the presence of ions. Two-dimensional correlation spectroscopy (2DCOS) revealed that the preferential complexation of Cr(III) with carboxyl groups served as the primary electrostatic modulation pathway, retarding Cr(VI) adsorption. Subsequent reduction was driven by phenolic hydroxyl groups, which were oxidized to carboxylates during the conversion of Cr(VI) to Cr(III). This work emphasizes surface charge modification as a critical strategy for optimizing the redox functionality of biochar. • Cations amplify Cr(VI) adsorption (3.5–26.2×) and reduction (1.1–3.0×) in biochar systems. • Solid-phase reduction dominates over liquid-phase (38 % contribution) in Cr(VI) reduction. • Electrostatic modulation, not ROS abundance, governs Cr(VI)-biochar efficacy. • Cr(III)-carboxyl complexation reduces surface charge, enabling hydroxyl-driven Cr(VI) reduction.