| Due to the widespread use of antibiotics and the insufficient metabolism in animals,a variety of antibiotics have been frequently discharged in natural water bodies,which seriously threatens the ecological environment and human health.As an emerging green technology,photocatalytic oxidation shows broad application prospects in the field of antibiotic degradation.As the core target of photocatalytic technology,designing and developing highly efficient photocatalysts or improving the activity of photocatalysts through specific technical approaches are the current research focus.Carbon nitride(gC3N4),as an ideal non-metallic photocatalyst,has attracted extensive attention due to its abundance in raw materials,simple and easy preparation,excellent visible light response,and tunable energy band structure.However,g-C3N4 that directly synthesized by thermal polycondensation has the disadvantages of poor crystallinity,small specific surface area,low quantum efficiency and large forbidden band width,which seriously affect its photocatalytic activity.Moreover,the skeleton structure of g-C3N4 has a special π-π conjugated electron system,which will limit the separation of photogenerated electron-hole pairs.In this work,g-C3N4 was optimized by three different modification methods,the physicochemical structure and optical properties of the material were systematically investigated,the photocatalytic activity was evaluated by using typical antibiotics as pollutants,and the photocatalytic degradation ability was evaluated by free radical quenching experiments.Extinction experiments and band structure analysis helped discussing the possible charge transfer mechanisms.Based on this,the photocatalytic performance of the three modified materials was compared,and the best modification strategy in this system was obtained,and the influence of water quality parameters on the photocatalytic performance of the materials were further explored.Finally,the degradation pathways and toxicity changes of pollutants in the photocatalytic degradation process were clarified.The main contents and results of this study are as follows:(1)A new type of photocatalyst with silver/sulfur doped g-C3N4(Ag-S-C3N4)was synthesized and designed by double doping of metal and non-metal.The optimal doping dosage of S in S-C3N4 is 0.2 g,and the optimal doping dosage of Ag in Ag-S-C3N4 is 30%.Cefazolin was selected as the model pollutant,and after 20 min of simulated sunlight irradiation,the pollutants removal efficiency by Ag-S-C3N4 reached 100%,and its first-order degradation kinetic constant was 0.630 min-1.The reasons for the excellent photocatalytic activity are as follows:the doping of S increased the specific surface area of the material and obtained more catalytic active sites;the doping of Ag enhanced the light absorption ability of the material and obtained a narrower band gap width;Moreover,the double doping of S and Ag had a synergistic effect,which effectively inhibited the recombination of photogenerated electron-hole pairs and improved the transfer rate of charges.After 4 cycles of reusing experiments,Ag-S-C3N4 still maintained good photocatalytic degradation activity,and the chemical composition and element valence state were consistent before and after reusing.(2)A novel photocatalyst for 2,4,6-Triaminopyrimidine(TAP)polymerization of g-C3N4 was synthesized by using heterocyclic polymerization as a modification method.First,the optimal doping levels of TAP for three different precursors(urea,melamine,and thiourea)were determined to be 0.5%,1.0%,and 1.0%,respectively.Among them,u-0.5-TCN has the best photocatalytic degradation performance.Ciprofloxacin was selected as the model pollutant,the degradation efficiency of the pollutants reached 97.9%after 40 min of simulated sunlight exposure,and its first-order degradation kinetic constant was 0.623 min-1,which are 10.8 times and 7.5 times of m-1-TCN and t-1-TCN,respectively.This material still maintains good photocatalytic degradation activity under simulated visible light irradiation.The reasons for the excellent photocatalytic activity are as follows:the copolymerization of TAP changes the microstructure of carbon nitride.The u-0.5-TCN has a porous tubular structure,which increases the specific surface area of the material and obtains more planned sites,and TAP The doping of the material leads to the enhancement of the light absorption ability of the material,and the energy band structure with stronger redox ability is obtained.In addition,the polymerization of TAP reduces the Π electron defect in the g-C3N4 framework and accelerates the migration of photogenerated carriers.After 4 cycles of reusing experiments,u-0.5-TCN still maintains good photocatalytic degradation activity,and the chemical composition and element valence state are consistent before and after reusing.Therefore,it has good recyclability and stability.(3)A new type of photocatalyst with Ag/AgVO3@Carbon rich g-C3N4(AACCN)ternary heterojunction was synthesized by constructing a Z-type heterojunction as a modification method.First,the CCN photocatalyst was obtained by resonance polymerization of 1,3,5-cyclohexanetriol with an optimal doping amount of 90 mg.Then Ag/AgVO3 was compounded on CCN by hydrothermal method to obtain AACCN ternary heterojunction.The optimal mass ratio of Ag/AgVO3 to CCN was 10%.Sulfamethiazole was selected as the model pollutant,and the pollutants were completely degraded under 40 min of solar irradiation.The reasons for the excellent photocatalytic activity are as follows:the introduction of heterocycles inside the CCN to build a built-in electric field,which facilitates the migration of electron-hole pairs;the Ag/AgVO3 enhances the light absorption ability of the material and obtains a narrower band gap.The charge transfer mechanism of the Z-type heterojunction enables the spatial separation of electron-hole pairs,which inhibits the recombination of electron-hole pairs,and the construction of the heterojunction enhances the redox ability of the material,thereby improving the photocatalytic degradation activity.After 4 cycles of reusing experiments,AACCN still maintains good photocatalytic degradation activity,and the chemical composition and element valence state are consistent before and after reusing.Therefore,it has good recyclability and stability.(4)The photocatalytic activity of the three modified materials was evaluated with sulfamethiazole as the model pollutant.Comparing the three materials with different modification methods,10-AACCN has the highest photocatalytic activity,and the pollutants can be completely degraded by simulating sunlight exposure after 40 min,and its first-order degradation kinetic constant is 0.1504 min-1,which are 8.8 and 5.7 times than that of Ag-S-C3N4 and u-0.5-TCN,respectively.The effects of pH,SO42and HA on the photocatalytic system were explored,and the results showed that 10AACCN still maintained excellent photocatalytic degradation performance and could be used in practical antibiotic wastewater treatment.The photocatalytic degradation process of sulfamathiazole were clarified,and 4 possible degradation pathways were identified.Finally,the toxicological analysis of the intermediates in the photocatalytic degradation process was carried out,and the results showed a large number of intermediates became more toxic.Therefore,it is necessary to prolong the photocatalytic degradation time to achieve a high degree of mineralization,thereby ensuring the reduction of the toxicity of pollutants during the photocatalytic degradation process. |
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