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.
Keywords: Adenine; Cytosine; Disinfection byproducts; UV irradiation; UV/Chlorine; Water treatment.
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