Mechanism of degradation of a nitrogenous heterocycle induced by a reductive radical: decomposition of a sym-triazine ring
Cyanuric acid is a major component of many materials and chemicals, and is the most important intermediate in the degradation processes of sym-triazine compounds in the natural environment, as well as being used for water treatment. However, the degradation mechanism of cyanuric acid is still unclear in various advanced oxidation processes (AOPs), where ˙OH is usually regarded as the dominant radical. Here, using a combination of density functional theory calculations and experimental observations, we unexpectedly show that the sym-triazine ring structure is broken efficiently by reductive free radicals – hydrogen radicals (˙H), rather than traditional ˙OH. The energy barrier of cyanuric acid reacting with ˙H to form the –NH2 group and break the sym-triazine ring is only 4.96 kcal mol−1, which is clearly lower than that of cyanuric acid reacting with ˙OH (13.32 kcal mol−1). Our theoretical predictions are further confirmed by γ photon irradiation experiments, which show that when ˙H is present in the reaction, the nitrogen in cyanuric acid (or other nitrogenous compounds including primidone and bezafibrate) rapidly degrades into NH4+. In contrast, when ˙H is scavenged, cyanuric acid stops degrading into NH4+. Our results provide new insight for understanding the decomposition of nitrogenous materials, and we are the first to shed light on the key role of ˙H in organic transformation processes.