Ethylene Glycol (EG)-Derived Chlorine-Resistant Cu0/TiO2–x for Efficient Photocatalytic Degradation of Nitrate to N2 without Sacrificial Agents at Near-Neutral pH Conditions: The Synergistic Effects of Cu0 and EG Radicals

Abstract

The selective photoreduction of nitrate to nontoxic nitrogen gas has emerged as an energy-efficient and environmentally friendly route for nitrate removal. However, the coexisting high-concentration chloride ions in wastewater can exert a significant influence on nitrate reduction due to the competitive adsorption and corrosion of Cl^– on photocatalysts. Herein, we prepared ethylene glycol-Cu/TiO_2–x (EG-Cu/TiO_2–x) through a solvothermal reaction of Cu-doped TiO₂ in an EG solution. The photodegradation of nitrate using EG-Cu/TiO_2–x without adding sacrificial agents can efficiently occur in near-neutral pH solutions containing 50 mM Cl^– with 95.26% of NO₃^– removal and 76.52% of N₂ selectivity. Moreover, the photocatalyst performance remained at a high level after 8 cycles. In this work, NO₃^– was first converted to NH₄⁺ by Cu⁰ and Ti³⁺, followed by the NH₄⁺-to-N₂ conversion by photogenerated chlorine free radicals. Compared to HO^•, Cl^•, and Cl₂^•–, ClO^• is proved to play the predominant role in transforming NH₄⁺ to N₂. The EG radicals produced by UV light impede Cl^– adsorption on Cu, protecting Cu⁰ from being corroded. What’s more, photoelectrons can reduce Ti⁴⁺ to Ti³⁺ and protect Cu⁰ from being oxidized, enabling the stability of reactive sites. This work provides novel insights and understanding on designing photocatalysts for NO₃^– removal in solutions containing chloride ions, highlighting the significance of eliminating Cl^– by EG radicals and adjusting the conversion process of NO₃^– for the efficient removal of NO₃^–.