N-doped graphitic C3N4 nanosheets decorated with CoP nanoparticles: A highly efficient activator in singlet oxygen dominated visible-light-driven peroxymonosulfate activation for degradation of pharmaceuticals and personal care products

Highlights: • Graphitic N atoms doping modulates the electronic properties of g-C₃N₄ nanosheets. • ¹O₂ is the predominant ROS over the CoP/N-g-C₃N₄ nanosheets. • The electrons transferring via Co‒N bond dramatically promote the degradation. • CoP/N-g-C₃N₄ show convincible potential for the purification of actual sewage. CoP nanoparticle-loaded N-doped graphitic C₃N₄ nanosheets (CoP/N-g-C₃N₄) were fabricated via a facile three-step method to degrade pharmaceuticals and personal care products (PPCPs) via a visible-light-driven (VLD) peroxymonosulfate (PMS) activation system. 2 ppm carbamazepine (CBZ) can be removed completely within 10 min by the VLD-PMS system with a kinetic constant of k = 0.29128 min⁻¹, as 25.8 times compared to that under dark conditions (k = 0.01128 min⁻¹). The experimental and theoretical results showed that the doped graphitic N atoms could modulate the electronic properties of the g-C₃N₄ nanosheets. Subsequently, the Density Functional Theory (DFT) explained that CoP showed preference to bonding with the nitrogen atoms involved in the newly formed NN bond, and the Co‒N bond dramatically enhanced the transfer of photo-generated electrons from the N-g-C₃N₄ nanosheets. Electron paramagnetic resonance (EPR) tests show that singlet oxygen (¹O₂) plays a leading role in this case. Moreover, PMS molecules are also tended to be absorbed onto the electron-deficient carbon atoms near the newly formed NN bonds for PMS reduction, synergistically enhancing the degradation efficiency for CBZ and benzophenone-3 (BZP). The newly established VLD-PMS activation system was shown to treat the actual sewage in Hong Kong sewage treatment plants (STPs) very well. This work supplements the fundamental theory of radical and non-radical pathways in the sulfate radical (SO₄^{• -})-based advanced oxidation process (SR-AOP) for environmental cleanup purposes.