Alkaline UV/PAA synergistic degradation of iron-cyanide complexes: UV-driven ligand disruption and radical oxidation pathways.

The advanced degradation of ferricyanide ([Fe(CN)₆]³⁻) in industrial wastewater faces dual bottlenecks of self-acidification-induced hydrogen cyanide (HCN) release and inefficient decomplexation. This study innovatively constructs an alkaline UV/Peracetic Acid (PAA) synergistic system and systematically elucidates its triple action mechanism: (1) UV irradiation at 254 nm directly drives ligand-to-metal charge transfer (LMCT) excitation of ferricyanide, achieving efficient Fe-CN bond breaking (Φ₂₅₄ = 0.235-0.267 mol·Einstein⁻¹); (2) hydroxyl radicals (•OH) and organic radicals from PAA photolysis dominate the deep oxidation of free cyanide (contribution rate > 90 %); (3) The alkaline UV/PAA system possesses a self-stabilization mechanism: the oxide radical ion (O•⁻) derived from photolysis of deprotonated PAA and H₂O₂ release alkalinity during their conversion to •OH, neutralizing the acidification effect caused by ferricyanide decomplexation and maintaining pH fluctuations < 0.17 units. Addressing the kinetic mismatch between ferricyanide decomplexation and PAA pyrolysis (where kPAA/k[Fe(CN)₆]³⁻ ranges from 14.95 to 1.56), a delayed addition strategy is proposed: UV irradiation of ferricyanide is performed first, followed by PAA introduction. This strategy reduces total cyanide in actual wastewater to < 0.004 mg·L⁻¹, while shortening total treatment time by 50 % and avoiding HCN risk. This research provides a sustainable solution for safe treatment of cyanide-containing wastewaters with reduced chemical consumption and enhanced process stability.