Modified Carbon Nitride-Based Photocatalysts For Degrading Typical Antibiotics In Wastewater

Environmental pollution and energy crisis have become the two stumbling blocks to achieve sustainable development in modern society.Among them,the problems related to the antibiotic wastewater pollution are arrived an urgent level which threaten the survival of plants,animals and even human health.Tetracycline（TC）,as a typical antibiotic,was regarded as an essential medicine by the World Health Organization（WHO）,which has been widely applied in animal husbandry and aquaculture.However,TC and its metabolites to continuously enter surface water and even drinking water,resulting in the continuous proliferation and spread of antibiotic resistance genes,which will pose a huge threat to public health.Therefore,the development of convenient,non-toxic and effective antibiotic wastewater treatment methods is of great significance to the sustainable development of society.In recent decades,because of the economical and environmentally friendly characteristics,solar-driven semiconductor photocatalysis technology has become a research hotspot among numerous antibiotic wastewater treatment technologies.From previous studies,choosing a“suitable”photocatalyst plays a crucial role in the photo-degradation reaction.Among them,graphitic carbon nitride（g-C₃N₄）,as a conjugated polymer semiconductor,has attracted extensive attention due to its excellent physicochemical stability,attractive electronic structure,and appropriate band gap of 2.7 e V.Unsatisfactorily,the pristine g-C₃N₄ also faces the shortcomings arising from the limited visible light response,few active sites and swift recombination of photo-induced carriers.Based on these problems,a lot of modification methods have been applied to conquer above limitations and enhance the capability of bulk g-C₃N₄,such as morphological control,element doping,structural design,and construction of heterojunctions.In this study,the modified g-C₃N₄ based photocatalytic material was applied to the degradation of TC antibiotics,and the structure,optical properties,electrical properties and photocataly tic reaction mechanism of the modified material were analyzed through a series of characterization measures.Therefore,it provides basic work and guidance for the construction of modified g-C₃N₄-based photocatalysts to remove antibiotics in wastewater.The main research contents and results are as follows:（1）A novel potassium ion（K⁺）-doped ultrathin porous g-C₃N₄（denoted as KMCN）photocatalyst was prepared by a simple hydrothermal recalcination method,which exhibits efficient catalytic performance,environmental friendliness,and good stability.The obtained KMCN photocatalysts were applied to the photodegradation of TC under different reaction conditions to simulate the actual wastewater treatment.It was found that the ultrathin porous structure an d K⁺doping of g-C₃N₄ increased its specific surface area and pore size,thereby increasing the light absorption range and photocatalytic reaction active sites.The optical and electrochemical properties of KMCN photocatalysts were systematically character ized by UV-DRS,PL,photocurrent,EIS and other techniques.The results show that the optimal sample KMCN（0.05）has a narrow band gap,lower photo-generated charge carrier recombination and higher electron and hole transport efficiency.Benefiting from these advantages,KMCN（0.05）photocatalysts demonstrated excellent photocatalytic performance for TC degradation（85.13%）,which was approximately 2.88-fold and1.40-fold increase compared to bulk g-C₃N₄（29.60%）and porous ultrathin g-C₃N₄（60.84%）,respectively.In addition,the KMCN（0.05）photocatalyst also exhibited good chemical stability and mineralization ability,which were confirmed by TOC,3D EEMs fluorescence spectroscopy and cycling experiments.Meanwhile,the possible intermediate products formed during the photodegradation of TC were analyzed by LC/MS-MS technique.Active species capture experiments and ESR analysis showed that h⁺and·O₂^-played a leading role in the photocatalytic degradation of TC,while·OH played an auxiliary role.（2）A novel photocatalyst（BPTCN）with one-dimensional hollow tubular carbon nitride（TCN）trapped black phosphorus quantum dots（BPQDs）was prepared by a simple ice bath-assisted ultrasonic method,and its morphology,structure and physicochemical properties were analyzed in detail.In this composite,BPQDs（with an average size of 3.32 nm）were uniformly distributed on the TCN,and the removal of various pollutants（oxytetracycline hydrochloride（OTC-HCl）,TC,Rhodamine B and heavy metal Cr（Ⅵ）ions）.Meanwhile,the electronic band structures and strong electronic interactions of the composites were obtained by density functional theory（DFT）calculations.Finally,based on the theoretical and experimental results,we propose the degradation mechanism of BPTCN photocatalysts and emphasize the roles of BPQDs and tubular structures in the photocatalytic process.On the one hand,the one-dimensional hollow porous tubular structure reduces the diffusion distance of photo-generated charge carriers to the reaction interf ace,thus effectively separating photo-generated electron-hole pairs.On the other hand,the loading of BPQDs can greatly improve the photon absorption of BPTCN composites.And BPQDs（acting as electron mediators）are uniformly loaded on the surface of TCN to obtain a unique0D/1D structure（connected by P-C bonds）,which retards the recombination rate of photo-generated charge carriers.The P-C bond established close interactions between BPQDs and TCN,which facilitated cyclic photostability and electron t ransfer.This study provides promising applications for designing various non-metallic nanomaterials that play a role in solar-chemical energy conversion and environmental remediation.（3）Sulfur doped carbon quantum dots（S-CQDs）/hollow tubular g-C₃N₄photocatalyst（HTCN-C）,prepared via ultrasonic assisted synthesis strategy.The successful preparation of HTCN-C and its excellent photocatalytic performance were demonstrated by structural characterization,morphology characterization and various performance characterizations.The hollow tubular g-C₃N₄ prepared by molecular self-assembly between melamine and cyanuric acid has a larger specific surface area,which provides more active sites for the reaction.Compared to bulk g-C₃N₄,the optimal photocatalyst（HTCN-C（2））displayed superior photocatalytic activity for TC degradation and photocatalytic inactivation toward Escherichia coli,with the 82.67%（60 min）removal rate of tetracycline and the inactivation of 6.88 log₁₀ cfu·mL^-1（about 99.99%destruction rate,40 min）of bacterial cells under visible-light irradiation,respectively.In addition,the characterizations of UV-Vis diffuse reflectance spectroscopy,photoluminescence technique,transient photocurrent response and electrochemical impedance spectroscopy also verified the good optical and electrochemical properties of the obtained samples.Eventually,possible reaction mechanism for enhancing the photocatalytic activity of antibiotic degradation and bacterial inactivation was raised.