Effects of water matrices on the degradation of naproxen by reactive radicals in the UV/peracetic acid process

The UV/peracetic acid (UV/PAA) process as a novel advanced oxidation process has been reported to produce carbon-centered radicals (RC•) for Naproxen (NAP) degradation, which is a representative of naphthyl structure substances. Real water matrices, such as carbonate and bicarbonate ions (CO32-/HCO3-), humic acid (HA), and chloride ion (Cl-), may react with these reactive radicals and change their contributions to NAP degradation. The results showed that RC• contributed 60.8% and •OH contributed 39.2% to NAP degradation in pure water by a competition method. CO32-/HCO3- (0-20 mM) showed minimal effect on NAP degradation in the UV/PAA process, meanwhile, it has observable inhibition effect on NAP degradation in the UV/H2O2 process (mainly of •OH) and minimal effect in the UV/PAA process with tert-butanol (TBA) (mainly of RC•). Results suggested that CO32-/HCO3- could react with •OH yielding CO3•- with low reactivity to NAP, CO3•- could further react with PAA to produce RC•. This speculation was confirmed by the increased contribution of RC• to NAP degradation with the increase of CO32-/HCO3- concentration through the competition method. HA (0-5 mg/L) had a higher scavenging capacity for RC• than •OH because HA with naphthyl structure was likely to be attacked by RC•. Cl- (0-200 mM) had little effect on NAP degradation in the UV/PAA and UV/H2O2 processes, while exerted an observable inhibition on NAP degradation in the UV/PAA process with TBA. This finding suggested that Cl- could react with RC• to produce Cl•, which could further convert into HOCl•-, and then excess •OH was formed. The new knowledge on the conversion of reactive radicals obtained in this study provides an important basis for facilitating further research on the UV/PAA advanced oxidation.