Degradation and DBP formations from pyrimidines and purines bases during sequential or simultaneous use of UV and chlorine

Purine and pyrimidines are present an important pool of dissolved organic nitrogen in aqueous medias and also precursors of disinfection byproducts. The degradation kinetics of cytosine and adenine-model pyrimidine and purine compounds-were investigated along with their transformation pathways leading to the formation of disinfection byproducts during two typical multi-barrier disinfection processes: UV irradiation and UV/chlorine pretreatment followed by post-chlorination. UV irradiation followed by post-chlorination enhanced the degradation of cytosine and adenine (by 17.1 and 26.1%, respectively), but it also generated more byproduct precursors compared to chlorination alone. The presence of reactive species in the UV/chlorine treatment greatly enhanced cytosine and adenine degradation (by 61.8 and 123.0%) but generated even more disinfection byproducts. Compared to 24 h chlorination, the concentrations of byproducts increased by up to 361.6% for cytosine and 85.1% for adenine with longer UV/chlorine treatment (from 2 to 30 min). Thirty minutes of combined UV/chlorine treatment decreased the total organic chlorine produced from cytosine by 34.4% (from 233.8 to 153.3 μg Cl L-1) but it increased byproduct generation by 68.3% compared with 24 h of simple chlorination. The TOCl from adenine increased by 50.0% (from 9.2 to 18.4 μg Cl L-1) but byproduct generation was 11.0% less after 30 min of UV/chlorine pretreatment followed by 24 h of chlorination. The intermediates generated were analyzed in detail and multiple transformation pathways leading to byproduct formation are proposed.