Insights into the enhancement of trace level NO2− on 2,4,6-tribromophenol degradation in UV/PS process: Experimental study and theoretical calculation

Abstract

Sulfate radical (SO4−)-based UV/persulfate (PS) process is extensively studied to remove organic pollutants in water, while the wide presence of nitrite (NO2−) in natural water can quickly react with SO4− and hydroxyl radicals (OH) to form reactive nitrogen species (RNS). In this study, the degradation of 2,4,6-tribromophenol (TBP) in UV/PS process in the presence of low concentration of NO2− was systematically investigated through experimental studies and theoretical calculations. The introduction of trace NO2− enhanced the TBP degradation by UV/PS process due to the formation of RNS (such as NO2 and BrNO2) and both RNS and SO4− played the important role in the pH range of 6.0–8.0. TBP was more easily photolyzed under alkaline conditions due to higher photodegradation coefficient at pH 8.0. When TBP underwent deprotonation, the reaction energy barrier of TBP with SO4−, OH and NO2 significantly decreased and subsequent debromination may be more likely to occur. Additionally, NO2 attacked ionized TBP at relatively high rate mainly through electron transfer. The degradation pathways of TBP mainly involved in the debromination, hydroxylation and nitrosylation and the generated products all presented lower toxicity than TBP. Moreover, the effects of PS dosage, NO2− concentration, humic acid, bicarbonate and chloride on TBP elimination also were thoroughly assessed. The presence of low concentration of NO2− reduced the toxicity risk caused by the formation of trihalomethanes (THMs) and haloacetic acids (HAAs) during the chlorination process after UV/PS pre-oxidation. This study broadens the knowledge of trace NO2− enhancing organic pollutants degradation by UV/PS process.