Effects of nitrate and nitrite on the UV/PDS process: performance and byproduct formation from the perspective of substituents

Abstract

Nitrate and nitrite are ubiquitous ions in wastewater that affect the performance of advanced oxidation processes such as UV-activated persulfate (UV/PDS) and lead to the formation of by-products. Three structurally similar compounds with different substituent compounds, namely phenol (Ph), benzoic acid (BA) and salicylic acid (SA), were selected as target pollutants in this study, to explore these issues from a new perspective: the effect of substituents on contaminants. The results showed that both NO₃⁻ and NO₂⁻ inhibited the removal of the three pollutants in the UV/PDS system. However, the varying substituents on the compounds influenced the electron density of their molecular structures, causing different responses to NO₃⁻ and NO₂⁻ during treatment. Reactive nitrogen species (RNS) played a more significant role in the oxidation of these compounds in the UV/PDS/NO₂⁻ system than in UV/PDS/NO₃⁻. Additionally, NO₃⁻ and NO₂⁻ seemed to affect the types of RNS that are most active in the process. The different substituents also influenced which positions on the molecule were attacked by reactive species, ultimately impacting the formation of N-containing byproducts. Although oxidation products were theoretically predicted and identified, many potential products remained undetected according to results from Fourier transform ion cyclotron resonance mass spectrometry. This study offers a new perspective by focusing on the characteristics of pollutants to evaluate the impact of NO₃⁻ and NO₂⁻ when UV/PDS is applied as an advanced wastewater treatment method.