Morphology And Structure Regulation Of Carbon Nitride And Mechanism Of Photocatalytic Degradation Of Typical PPCPs

In recent years,pharmaceuticals and personal care products(PPCPs)have been continually discharged into ambient water bodies through various anthropogenic sources.More seriously,due to the low biodegradability of PPCPs,conventional wastewater treatment plants are unable to completely remove these compounds.The accumulation of PPCPs in natural aquatic environments pose a range of potentially adverse effects on human health and ecosystems.Consequently,the development of efficient and environmentally-compatible strategies for the removal of recalcitrant PPCPs from water has emerged as a global challenge.Photocatalytic technology can directly convert solar energy into chemical energy,which is one of the ideal technologies to alleviate energy crisis and solve environmental pollution problems.Graphitic carbon nitride(g-C₃N₄)is a kind of nonmetallic polymer semiconductor material,which has been widely used in environmental remediation due to its advantages of simple preparation method,suitable energy band,high stability and visible light response.However,g-C₃N₄ still has some disadvantages,such as small specific surface area,low utilization rate of visible light,high carrier recombination rate,and low yield of strongly oxidizing active radicals,which leads to its unsatisfactory photocatalytic activity.In view of these disadvantages of g-C₃N₄,in this thesis,the morphology and structure of g-C₃N₄ were regulated,and then combined with other technologies(including morphology control,defect engineering and composite with other non-metallic materials)to build a highly active g-C₃N₄-based photocatalytic system,so as to achieve the purpose of efficient removal of PPCPs.Furthermore,the relationship between the morphology,surface composition,photoelectric properties of photocatalyst and the performance of photocatalytic degradation of PPCPs was systematically studied,and the photocatalytic mechanism was elucidated.The main research contents and experimental results of this thesis are as follows:(1)Two-dimensional ultrathin g-C₃N₄(UCN)nanosheets were successfully prepared by one-step thermal polymerization and applied to the photocatalytic degradation of diclofenac sodium(DCF).UCN had higher specific surface area,stronger carrier migration ability,so it showed better photocatalytic activity than bulk g-C₃N₄.UCN coupled persulfate(PDS)could produce more strong oxidizing radicals,which was confirmed by electron spin resonance(ESR)test.After 150 min of reaction,the mineralization rate of DCF reached 78%.Combined quenching experiments showed that superoxide radical(O₂^·-)played a dominant role in the degradation process.It was further found that the photocatalytic degradation of DCF in UCN/PDS system was in accordance with the first-order kinetics and Langmuir-Hinshelwood model,and it still maintained good photocatalytic activity in the cyclic experiments.Interestingly,UCN/PDS system exhibited higher photocatalytic activity in acidic/alkaline p H and in actual river water;especially,the kinetic rate of DCF degradation in river water was 3.4times higher than that in ultrapure water.(2)Hydroxyl radicals(·OH)have robust non-selective oxidizing properties to effectively degrade organic pollutants.However,graphitic carbon nitride(g-C₃N₄)is restricted to directly generate·OH due to its intrinsic valence band.In order to essentially improve the oxidation capacity of g-C₃N₄ system,in this study,we reported a facile environmental-friendly self-modification strategy to synthesize reduced graphitic carbon nitride(RCN),with nitrogen vacancies and C?N functional groups.The incorporation of nitrogen vacancies and C?N enabled to downshift the valence band level,which endowed RCN with the capacity to directly generate·OH via h⁺.Experimental results and instrumental analyses revealed the critical roles of nitrogen vacancies and C?N groups in the modification of the RCN band structure to improve its visible light absorption and oxidizing capacity.With these superior properties,the RCN was significantly enhanced for the photocatalytic degradation of DCF under visible light irradiation.The self-modification strategy articulated in this study has strong potential for the creation of customized g-C₃N₄ band structures with enhanced oxidation performance.(3)To synergistically enhance the efficiencies of carrier production-separation and oxidation capacity of g-C₃N₄ to improve its catalytic performance in environmental remediation.Herein,we prepared a novel boron nitride quantum dots-modified reduced ultrathin g-C₃N₄(BNRU)photocatalyst,which was intended to achieve the above objective through defect engineering and boron nitride quantum dots(BNQDs)loading.By introducing defects(nitrogen vacancies and C?N group),the optimized band structure could absorb more photons and provide stronger oxidation driving force(+2.15 e V).Meanwhile,nitrogen vacancies and BNQDs constructed the isolated electron-hole transfer channels to facilitate carrier separation.Compared with UCN,the optimal 2BNRU could obtain 1.7 times higher carrier density under visible light,and the photocurrent density increases from 1.33 to 9.31?A/cm~2.The results of ESR and molecular probe spectroscopy showed that these enhanced properties endowed 2BNRU with superior generation ability of free radicals.When used to degrade norfloxacin,2BNRU could degrade drug molecules at a kinetic rate of 0.3744 min^-1and the mineralization rate reached 46.9%under visible light irradiation for 30 min.This study not only demonstrates a highly oxidized performance of g-C₃N₄ photocatalytic system for environmental remediation,but also opens the way for the design of non-metallic photocatalysts with double carrier transfer mechanism.(4)To further improve the utilization efficiency of solar light and radical generation ability of g-C₃N₄,we synthesized a novel carbon quantum dot-decorated reducing ultrathin g-C₃N₄(RUCN/CQD)photocatalyst with strong redox ability and broad-spectrum photoresponse ability,which exhibited an excellent photocatalytic degradation performance for various typical PPCPs.In particular,RUCN/CQD showed a 100%removal rate of 5 mg/L DCF within 6 min under actual sunlight exposure.This remarkable performance was attributed to a customized band structure with a thermodynamic driving potential for the generation of·OH,CQD with an effective electron transfer capacity for promotional formation of O₂^·-,and ultrathin porous structure with abundant reaction sites.Simultaneously,the up-converted fluorescent properties of the CQD endowed the photocatalyst with a broad-spectrum response,thus effectively improving the utilization rate of sunlight.The RUCN/CQD system also exhibited superior photocatalytic activities under simulated natural conditions,which implied an immense potential for the remediation of PPCPs in ambient waterways.