Utilization of photochemical circulation between NO₃⁻ and NO₂⁻ in water to degrade photoinert dimethyl phthalate: Influence of organic media and mechanism study

Photoconversion of NO₃⁻/NO₂⁻ at wastewater relevant concentrations as an advanced oxidation approach to degrade dimethyl pthalate (DMP), a relatively photoinert endocrine disruptor, were examined. Three different wavelengths (350 nm, 300 nm, 254 nm) were involved. The influence of NO₃⁻ or NO₂⁻ was found to be wavelength-dependent. The 254 nm UV light could decompose DMP efficiently, but photolysis of DMP was slow at λ =300 nm and noneffective at λ =350 nm, which could be catalyzed by the presence of NO₃⁻ or NO₂⁻. Both ^[rad]OH and O₂^[rad]− were detected, while ^[rad]OH was identified as the primary contributor to DMP decomposition. NO₂⁻ plays a dual role as both a source and sink of ^[rad]OH, depending on the relative abundance between NO₂⁻ and DMP. NO₃⁻ was more efficient than NO₂⁻ for treating low-level DMP. However, higher organic content could effectively inhibit the quenching role of NO₂⁻, making NO₂⁻ more efficient for catalyzing DMP decomposition. For irradiation at λ =350 nm, NO₃⁻ was completely ineffective, while self regeneration of NO₂⁻ enabled Δ[NO₂⁻]:Δ[^[rad]OH] <<1. For irradiation at λ =300 nm, cycling between NO₂⁻ and NO₃⁻ occurred, and the transformation from NO₂⁻ to NO₃⁻ proceeded much faster. Complete decomposition of DMP at concentrations higher than those of NO₂⁻ or NO₃⁻ was observed, and mineralization was also achieved. Based on the identification of the intermediates, ^[rad]OH addition to the aromatic ring and hydrogen atom abstraction by ^[rad]OH were the dominant pathways, while nitration products were detected at low levels.