Efficient electron transfer between Cr(VI) and organic acids mediated by wide bridges of Fe/CN

Various organic acids commonly coexist with Cr(VI) in wastewater. However, direct electron transfer between Cr(VI) and electron-rich organic acids is typically slow. Current Fe-based mediators primarily operate through the aqueous Fe(III)/Fe(II) cycle, which faces challenges in achieving efficient Fe recycling. To address the efficient removal of Cr(VI) and sodium citrate (Cit) composite pollutants (Cr(VI)-Cit), iron-loaded carbon nitride composites (Fe/CN) were synthesized via a combined loading-calcination approach. Crucially, coordination between carboxylate groups of Cit and undercoordinated Fe sites on Fe/CN formed photoactive complexes, effectively stimulating ligand-to-metal charge transfer (LMCT). This significantly enhanced the solid-phase Fe(III)/Fe(II) cycling efficiency, leading to a markedly increased Cr(VI) reduction rate constant (k) of 0.213 mg/(L·min). Furthermore, coordination with the N-rich matrix effectively inhibited Fe leaching, resulting in a post-reaction total Fe concentration below 0.5 mg/L. The formation of a Cr(VI)-Fe-Cit ternary cyclic complex effectively lowered the activation energy for Cr(VI) reduction by 81.3 % compared to pristine CN. In the dark, intramolecular electron transfer within this complex generated Fe(II)-Cit species with a low electrode potential. Under irradiation, the photoactive Fe(III)-Cit complex generated Fe(II) via LMCT to reduce Cr(VI); concurrently, photogenerated electrons (e^-) and superoxide radicals (·O₂^-) also contributed to Cr(VI) reduction. Reusability tests showed Fe/CN retained ∼90 % Cr(VI) removal efficiency from actual wastewater over five cycles, demonstrating good stability under realistic conditions. This study provides valuable insights for the cost-effective and efficient remediation of Cr(VI)-organic acid complexes in real wastewater.