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

Abstract

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 OH and O2− were detected, while OH was identified as the primary contributor to DMP decomposition. NO2− plays a dual role as both a source and sink of 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[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, OH addition to the aromatic ring and hydrogen atom abstraction by OH were the dominant pathways, while nitration products were detected at low levels.