Emerging investigators series: comparative study of naproxen degradation by the UV/chlorine and the UV/H2O2 advanced oxidation processes

The UV/chlorine advanced oxidation process (AOP), which forms HO and reactive chlorine species (RCS such as Cl, ClO and Cl-2(-)), is being considered as an alternative to the UV/H2O2 AOP for the degradation of emerging organic contaminants. This study compared the kinetics and pathways of the degradation of a recalcitrant pharmaceutically active compound, naproxen (NPX), by the UV/chlorine and UV/H2O2 AOPs. The degradation of NPX by both AOPs followed pseudo first-order kinetics, and, at pH 7, the first-order rate constant (k) in UV/chlorine was 4.9 times higher than that in UV/H2O2. At pH 7, in the UV/chlorine process, the HO and RCS contributed to 15.9% and 76.3%, respectively, of the NPX degradation. Radical scavenging tests indicated that ClO and CO3- were important to the NPX degradation by UV/chlorine. A higher efficiency was observed in UV/chlorine than in UV/H2O2 at the pH range of 6-9, but as the pH rose from 6 to 9, k decreased from 6.10 x 10(-3) s(-1) to 2.98 x 10(-3) s(-1) in UV/chlorine. However, in UV/H2O2, k was only slightly affected by pH. In both AOPs, k increased linearly with increasing dosages of oxidants (chlorine or H2O2) from 20 M to 200 M. The UV/H2O2 process was less affected by the water matrix than the UV/chlorine process. Compared to pure water, k in tap water was reduced by 9% and 23.2% by UV/H2O2 and UV/chlorine, respectively. The degradation by both AOPs was associated with hydroxylation and demethylation. Decarboxylation was particularly observed in UV/H2O2, and chlorine substitution was observed in UV/chlorine. During the UV/chlorine process, the acute toxicity to Vibrio fischeri increased and then decreased in the system.