Coexposure Degradation of Purine Derivatives in the Sulfate Radical-Mediated Oxidation Process

Purines are the most widely occurring heterocyclic-N compounds. The degradation behaviors of purine derivatives, theophylline (TPL) and adenine (ADN) as representatives, in both single-component and mixture systems during UV/peroxydisulfate (PDS) treatment were explored. In the mixture system when the concentrations of SO₄^•- and HO^• were reduced by more than half in comparison to a single-component system, the observed first-order rate constant of TPL was reduced by 11%, whereas ADN degradation was almost completely inhibited. An ADN "revert" pathway, that is, back transformation of ADN(-H)^• to ADN by TPL via single electron-transfer reactions, was found. The second-order rate constant of ADN(-H)^• with TPL was determined to be (1.94 ± 0.21) × 10⁸ M^-1 s^-1. A kinetic model was developed, which successfully quantifies the contribution of each reaction pathway at various target compound concentrations. In the copresence of 0.1 μM TPL, 58% ADN (3.0 μM) was reduced back to ADN at the PDS dose of 290 μM. The ADN revert pathway's effectiveness is governed by the relative reduction potentials of the reactants. Purines and phenols with lower reduction potentials are able to react via the ADN revert pathway. These findings improve the understanding of the removal of mixture pollutants from real water media in advanced oxidation processes.