Extensive incorporation of carboxyl groups into g-C3N4 by integrated oxygen doping and HNO3 oxidation for enhanced catalytic ozonation of para-chlorobenzoic acid and atrazine

Carbon nitride (CN), oxygen-doped carbon nitride (CNO), HNO 3 oxidized CN (CN-10) and CNO (CNO-10), were synthesized and used to catalyze ozonation of para -chlorobenzoic acid ( p -CBA) and atrazine in aqueous solution. Pre-doping oxygen atom into CN could create extra defects for grafting more carboxyl groups during HNO 3 oxidation , which was verified by much lower formation energy of CNO-10 than that of CN-10. Density functional theory and room temperature electron paramagnetic resonance (EPR) analysis signified the changes of electronic structure afforded by extensively grafted carboxyl groups. The incorporation of carboxyl groups distorted the carbon nitride framework and changed the spatial dimensions of CN and CNO, which attenuated the characters of delocalized π-electrons in conjugated structure. Moreover, the introduction of carboxyl groups reduced the pore diameter and nano-size distributed in aqueous solution for CN-10 and CNO-10 relative to those of CN and CNO. Results showed that the elimination efficiency of p -CBA by catalytic ozonation and R ct (standard reactivity metric for catalytic ozonation process) with CNO-10 improved 1.12- and 1.48-fold relative to those with CN-10, respectively, far surpassing non-catalytic ozonation. The removal of ozone-resisted compounds by CNO-10 catalyzed ozonation was not significantly affected by bicarbonate and NOM, and it was promoted by natural surface water resulting in 87.4% p -CBA removal in 30 min. Mechanistic evaluation demonstrated that carboxyl groups were the main active sites, catalyzing ozone decomposition into hydroxyl radical through radical chain reaction. This work provides fundamental supportive of pre-doping oxygen atom into CN before HNO 3 functionalization, which could incorporate more carboxyl groups on surface compared to conventional strategy, further enhancing catalytic activity in ozonation.