Removal performance of norfloxacin from waters by advanced oxidation processes based on different free radicals

Because of their low occurrence concentration in water, the antibiotics are difficult to be effectively removed by the conventional municipal sewage treatment system. The removal efficiency of norfloxacin (NFX) by ultraviolet combined chlorine (UV/chlorine), hydrogen peroxide (UV/H₂O₂) and persulfate (UV/PS) was investigated. The results showed that the degradation of NFX by UV/chlorine was significantly more efficient than the other two systems. When the initial pH of solution was 7 and the concentration of chlorine was 30 μmol/L, the removal rate of NFX in 2 min could reach 98.67%. At the same oxidant concentration, the removal rate of NFX was 41.41% and 65.73% after 10 min by UV/H₂O₂ and UV/PS, respectively. For UV/chlorine system, the removal rate of NFX increased with the increase of chlorine concentration with the optimal concentration being 30 μmol/L. The pH had different effects on the removal rate of NFX by three systems. For UV/chlorine system, strong alkaline condition was beneficial for the removal of NFX, while neutral and weak alkalinity was more favorable for the removal of NFX for UV/H₂O₂ and UV/PS. Cl^-in water matrix inhibited the removal of NFX by UV/chlorine, but had little effect on UV/H₂O₂ and UV/PS, while ${\mathrm{HCO}}_3^{-}$ inhibited the removal of NFX by all three systems.