Individual and combined degradation of N-heterocyclic compounds under sulfate radical-based advanced oxidation processes

Although the inhibitory effect of purines and phenols on the degradation of N-heterocyclic compounds under UV/peroxymonosulfate (PMS) has been reported, the understanding of contaminants' underlying mechanisms and kinetics is still very limited. In this study, five omnipresent nitrogenous bases (adenine, guanine, cytosine, thymine, and uracil) in the aquatic environment were used to study the inhibitory effect of N-heterocyclic compounds under UV/PMS by quantum chemical calculations. We found that HO center dot and SO4 center dot- initiate base degradation with second-order rate constants of (0.11 - 15.6) x 10(9) M-1 s(-1) and (7.25 - 13.5) x 10(9) M-1 s(-1), respectively, and the primary products contain hydroxyl derivatives (HO-P), intermediate radicals (P(-H)(center dot)) and cationic radicals (P center dot+, only for purines). Because of the lower oxidation potentials of HO-P than their parent compounds (P), we first proposed a self-inhibition pathway in which HO-P can revert P(-H)(center dot) and P center dot+ back to P through H atom abstraction and single electron transfer reactions, respectively. A more significant self-inhibition was found in the degradation of uracil than that of adenine under UV/PMS because the former has a higher yield of hydroxylated derivatives. The main hydroxylated product (6-HO-U) of uracil repairs the uracil radical (U (-4H)(center dot)) at the reaction rate constants of 1.01 x 10(9) M-1 s(-1). The combined degradation of bases showed that the reduction rate constants of purine cationic radicals have a linear relationship with the oxidation potentials of reductants. In the adenine-uracil mixture system, adenine inhibits uracil degradation by repairing U(-4H)(center dot) (2.33 x 10(5) M-1 s(-1)). This work revealed the mechanisms and kinetics of self-inhibition and joint-degradation of N heterocyclic compounds, which is of great significance for understanding the collective removal of contaminants in the real water environment.

Automated summary

In this study, the inhibitory effect of five omnipresent nitrogenous bases on the degradation of N-heterocyclic compounds under UV/peroxymonosulfate (PMS) was investigated using quantum chemical calculations. The study revealed the mechanisms and kinetics of self-inhibition and joint-degradation of N-heterocyclic compounds, which is significant for understanding the collective removal of contaminants in the real water environment.