Monodispersed Co-N3 loaded carbon nitride mediated peroxymonosulfate activation for rapid degradation of trace urea via singlet oxygen

Abstract

The removal of trace urea presents a significant challenge in the production of electronic-grade ultra-pure water (UPW). This study proposed a simple strategy to regulate the low-coordination of a single-atom cobalt catalyst (RyCo-CN0.20) for the efficient removal of trace urea via peroxymonosulfate (PMS) activation. The optimal R0.50Co-CN0.20 with low-coordination Co-N3 sites was constructed by in-situ reduction of NaBH4 and significantly enhanced the PMS activation. The removal efficiency of trace urea achieved 94 % within 5 mins. The kobs of trace urea degradation in R0.50Co-CN0.20/PMS system reached 0.4576 min−1, which was much higher than previous reported systems. Experiments of free radical quenching and electron paramagnetic resonance demonstrated that singlet oxygen species were the dominant free radical. The corresponding steady concentration of singlet oxygen species in the R0.50Co-CN0.20/PMS system was enhanced to be 16.361 × 10–6 μM, which was twice of Co-CN0.20/PMS. Density functional theory calculations revealed that the optimized electron distribution of the Co center in the Co-N3 sites processed a higher d band center (−1.08 eV) and richer electron transfer number (0.75 e−) compared with Co-N4 sites, hence realizing the stronger PMS activation. Continuous operation verified that R0.50Co-CN0.20/PMS could maintain the removal rate of trace urea above 80 % after 150 h operation. Moreover, R0.50Co-CN0.20/PMS in the pilot UPW production process effectively reduced the pollution caused by trace urea and ensured the quality of the terminal UPW (Total organic carbon <1 μg/L). This study provides a new approach for the removal of trace urea through PMS activation by low-coordination single-atom catalysts.