Comparative Study for Interactions of Sulfate Radical and Hydroxyl Radical with Phenol in the Presence of Nitrite

Sulfate radical (SO4•–)- and hydroxyl radical (HO•)-based advanced oxidation processes (AOPs) are effective for the removal of organic pollutants in water treatment. This study compared the interactions of SO4•– and HO• for the transformation of phenol in UV/peroxydisulfate (PDS) and UV/H2O2 with the presence of NO2–, which is widely present in aquatic environments and transforms SO4•– and HO• to •NO2. By using laser flash photolysis, the products of phenol reacting with SO4•– and HO• were demonstrated to be phenoxy radical and phenol-HO-adduct radical, respectively. This result, along with density functional theory (DFT) calculations, indicate that the predominant reaction mechanisms of phenol with SO4•– and HO• with phenol are electron transfer and addition, respectively. The different mechanisms induced the much higher formation of nitrophenols by SO4•– than HO• in the presence of NO2– through the fast combination of phenoxy radicals and •NO2. The conversion yields of phenol to nitrophenols (including 2-nitrophenol and 4-nitrophenol), were 47.5% by SO4•– versus 5.3% by HO• at the experimental conditions. Increasing PDS/H2O2 dosages from 0.2 to 1 mM resulted in a 61.9% increase of nitrophenol conversion yield in UV/PDS/NO2– but a 35.4% decrease of that in UV/H2O2/NO2–. In addition, the significant formation of phenoxy radicals by SO4•– also induced many nitrated polymers in UV/PDS/NO2–, while those induced in UV/H2O2/NO2– were negligible. The significant formation of nitrophenols and nitrated polymers increased the mutagenicity by 860.5% when the removal rate of phenol was 98% by UV/PDS/NO2–. This is the first study to demonstrate the different mechanisms of phenol transformation by SO4•– and HO• in the presence of NO2–.