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-.