The internal and external reverse-reduction pathway involved in the degradation of free DNA bases during advanced oxidation processes.

The degradation kinetics and mechanism of four free bases were comparatively investigated during UV/HO and UV/PDS water treatment processes. The UV/PDS system demonstrated superior removal efficiency for all four bases compared to the UV/HO process. Pyrimidine bases (cytosine and thymine) exhibited notably faster degradation rates than purine bases (adenine and guanine) in both advanced oxidation systems. Guanine (G) was then selected as a representative free base to investigate the internal and external reverse-reduction process. The time-resolved transient spectra shows that key intermediate product G(-H) was formed in both processes, where this radical play critical role in initiating reverse-reduction process. During UV/HO process, the G's antioxidant intermediates (e.g. 8-oxoG) trigger an internal reverse-reduction process, which can reduce G(-H) back to their parent compounds. The second-order rate constant for the reaction between of G(-H) and 8-oxoG was quantitatively determined to be (2.60 ± 0.12) × 10 M s. Conversely, during the UV/PDS process, the reverse-reduction process of G(-H) is predominantly governed by external inhibition mediated by dissolved organic matter (DOM), with no competing internal pathway observed. Specifically, intrinsic intermediate products generated during UV/HO treatment initiate an internal reverse-reduction process, effectively suppressing DOM-driven external reduction. This may also explain why the degradation rates of free bases were lower in the UV/H₂O₂ system than in the UV/PDS system, and why DOM exhibited a stronger inhibitory effect on their degradation in the UV/PDS process compared to UV/H₂O₂. DFT calculations were conducted to verify the proposed mechanisms, which were applicable to the four studied bases. Studying both internal and external reverse-reduction processes can significantly enhance the understanding of compounds transformation pathways and enhance the accuracy of predicting degradation rates in real-world aqueous systems.