Oxygen/methyl-modified carbon nitride as an efficient metal-free photocatalyst for phenolic pollutant removal via peroxymonosulfate activation.

Metal-free photocatalysts for peroxymonosulfate (PMS) activation offer a sustainable approach to water purification, though achieving high efficiency remains challenging. Herein, we reported an oxygen/methyl-modified carbon nitride (OMCN) as efficient PMS activator for phenolic pollutant removal. Through facile one-pot thermal copolymerization of urea and dimethyl malonate, the electronic structure of carbon nitride was engineered to generate a strong internal electric field that significantly enhances charge separation. OMCN exhibited a stronger built-in electric field compared to oxygen-modified carbon nitride (OCN) and methyl-modified carbon nitride (MCN) individually. The optimized OMCN2 photocatalyst featured a narrowed bandgap, enhanced PMS adsorption energy, and reduced activation barrier for PMS cleavage. Consequently, the visible-light-driven PMS (Vis-PMS)/OMCN2 system achieved completely phenol degradation within 90 min, substantially outperforming Vis-PMS/CN system. The catalyst demonstrated excellent stability across multiple cycles, retaining 88.1 % of its initial catalytic activity after four consecutive runs while maintaining broad pH tolerance. Mechanistic studies revealed that sulfate radicals dominate the degradation process. When integrated into continuous-flow membrane reactors, the system maintained complete pollutant removal at a water flux of 205 L m h over 270 min. Comprehensive toxicity assessments confirmed effective detoxification of treated effluents. These findings establish a rational design framework for metal-free photocatalysts that efficiently combines solar energy utilization with persulfate activation for sustainable water treatment.