PANI-functionalized Cu/TiO₂ photocatalysts for selective Cl^•-mediated nitrate-to-N₂ conversion with minimal toxic...

Strict global nitrogen discharge standards target nitrate (NO₃^-) pollution, yet advanced removal from industrial secondary effluents remains challenging. Biological denitrification struggles with insufficient electron donors (low organic content), while electrochemical reduction faces high conductivity dependence. Photocatalytic NO₃^--to-N₂ conversion, a promising alternative, also faces bottlenecks: intermediate (NO₂^-/NH₄⁺) re-oxidation by photogenerated h⁺/HO^•, HO^•-induced toxic byproducts (e.g., NO₂^-, ClO^-, Cl₂), and traditional selectivity-enhancement strategies (pH adjustment, sacrificial agents) incurring secondary pollution or high costs. To address these, we developed a polyaniline (PANI)-functionalized ethylene glycol-derived Cu/TiO₂ (PEGCT) photocatalyst. Cu^0/+ sites efficiently reduce NO₃^- to NH₄⁺, which is oxidized to N₂ by Cl^•. PANI's protonated surface enhances NO₃^- adsorption in Cl^--containing solutions, while its moderate highest occupied molecular orbital (HOMO) level facilitates h⁺ migration from TiO₂ to PANI, driving Cl^-→Cl^• conversion and suppresses HO^• to reduce toxic byproducts (3.8 % NO₂^- selectivity, free chlorine<0.13 mgCl₂·L^-1, below WHO guidelines). PANI also reduces Cu leaching, stabilizing catalytic efficiency. Consequently, the PEGCT/UV/Cl^- system achieved 95.3 ± 4.7 % NO₃^- removal and 78.4 ± 2.8 % N₂ selectivity over 8 cycles without pH adjustments and sacrificial agents' addition, and maintained ∼70 % nitrate removal/N₂ selectivity in real wastewater (initial 7.07 pH). This work highlights the critical role of synergistic effects in treating nitrate effluents: (1) positive charged surface for NO₃^- adsorption^, (2) radical modulation for nitrate-to-N₂ conversion/toxic byproducts suppression, (3) active sites immobilization for catalytic stability.