Preparation Of Porous Carbon Nitride Based On Self-assembly And Degradation Mechanism Of Typical PPCPs

Carbamazepine（CBZ）and naproxen（NPX）are typical pharmaceuticals and personal care products（PPCPs）.It is continuously discharged into the water environment through the effluent of sewage treatment plants due to its wide use,which poses a great threat to the water environment.Photocatalytic technology is expected to be applied to the removal of refractory pollutants in water,which attributes to its"green"and"high efficiency"characteristics.Graphite-phase carbon nitride（g-C₃N₄）,as a non-metallic photocatalyst,is widely used in photocatalytic research due to its adjustable morphology,band gap width,and superior optical properties.However,the high carrier recombination rate and the small visible light absorption range of carbon nitride limit the photocatalytic activity.Therefore,it is necessary to modification to enhance the photocatalytic activity of carbon nitride.In this paper,two different methods are used to modify carbon nitride:A novel tubular graphitic carbon nitride（g-C₃N₄）modified with carbon quantum dots（CQDs）was fabricated and employed for the elimination of carbamazepine（CBZ）under visible light irradiation.The as-fabricated metal-free catalysts exhibited tubular morphologies due to the preforming of tubular protonated melamine with CQDs surface adsorption as the polymerization precursors.The surface bonded CQDs did not alter the band gap structure of g-C₃N₄,but greatly inhibited the charge recombination.Therefore,the CBZ degradation kinetics of tubular g-C₃N₄were increased by over 5 times by the incorporation of CQDs.The main active species for CBZ degradation were found to be superoxide radical（·O₂^-）and photo-generated holes（h⁺）,which were further confirmed by electron spin resonance（ESR）analysis.In addition,the degradation pathways of CBZ were clarified via intermediates identification and quantum chemical computation using density functional theory（DFT） and wave function analysis.The olefinic double bond with the highest condensed Fukui index（f0=0.108）in CBZ molecule was found to be the most preferable sites for radical attack.Moreover,good stability of the as-prepared photocatalysts was observed in the consecutive recycling cycles,while the slight decline of photo-catalytic activity was attributed to the minimal surface oxidation.Silver-modified porous carbon nitride materials（MHAs）were further synthesized by adsorption-thermal polymerization method and used to degrade naproxen under visible light.According to the NPX photocatalytic degradation experiment,it is known that the photocatalytic degradation efficiency of MHAs is more than 17 times that of pure g-C₃N₄.The reason is that the modification of nitric acid and silver has formed more pore structure on the surface of the material,and the specific surface area has increased.The surface plasmon resonance effect（SPR）and electronic bridge effect of the particles make the material’s ability to absorb visible light enhanced,the band gap narrowed,the recombination of photo-generated carriers is suppressed,charge transfer is promoted,and more active sites are generated.The main active substances in the photocatalytic degradation of NPX were measured by ESR and capture experiments,which were h⁺and·O₂^-.It can be known from the cycle experiments and p H experiments that MHA2 has strong photocatalytic activity,reusability,chemical stability,and high adaptability to different p H.LC-MS was used to identify the intermediates of photocatalytic degradation and proposed possible degradation pathways.