Electro-Oxidation for Advanced Treatment of Recalcitrant Organics in Limestone-Gypsum Desulfurization Wastewater

Limestone-gypsum desulfurization wastewater contains recalcitrant organic pollutants that are difficult to remove by conventional processes. This study investigates an electrochemical oxidation (EO) system for advanced treatment after chloride and ammonia pre-removal, with a focus on elucidating the contributions of multiple reactive species, particularly reactive chlorine species, and evaluating the inhibitory effects of key coexisting ions, aspects that have been rarely systematically investigated. Using naphthol as a model pollutant in batch experiments with simulated and actual wastewater, the EO system operated at 50 mA cm^-2 achieved high removal efficiencies: 96.3% for naphthol and 81.2% for total organic carbon in simulated wastewater, though performance was moderately lower in actual wastewater due to its complex matrix. Quenching experiments revealed the significant roles of ^•OH, ^•Cl, ¹O₂, and chlorine-based oxidants in the degradation process, while coexisting ions (SO₄ ^2-, CO₃ ^2-, NO₃ ^-, and NH₄ ⁺) exhibited notable inhibition, with NH₄ ⁺ and CO₃ ^2- having the strongest effects. Electrochemical analysis indicated enhanced mass transfer and higher oxygen evolution potential in simulated wastewater, which improved current efficiency. GC-MS and ultraviolet-visible (UV-vis) analyses identified key intermediates, demonstrating that degradation proceeds through hydroxylation, chlorination, ring cleavage, and eventual mineralization, accelerated by the synergistic action of ^•OH and ^•Cl. This work confirms EO as a promising technology for advanced desulfurization wastewater treatment and provides essential insights for both theoretical understanding and practical application.