Synthesis Of G-C₃N₄ Photocatalytic Materials And Their Application In The Degradation Of Sulfamethexazole

Graphite carbon nitride（g-C₃N₄）,as a visible light-responsive organic polymer semiconductor,has the advantages of good chemical stability,high abundance,low cost,and environmental friendliness.However,g-C₃N₄ prepared by the traditional thermal polymerization of nitrogen-rich precursors has the disadvantages of small specific surface area,low visible light response,and high carrier recombination rate,which severely restricts its photocatalytic performance.In recent years,studies have found that g-C₃N₄ nanostructures with higher specific surface area can be prepared by the method of precursor self-assembly.Previous studies have proved that MA-CA aggregates formed by hydrogen bond interaction between melamine（MA）and cyanuric acid（CA）are an ideal precursor for g-C₃N₄ synthesis.Therefore,in this paper,the porous tubular g-C₃N₄（MNCA-75）and Fe doped g-C₃N₄ crumpled nanosheets（MFC-CN）were prepared by introducing 1,5-naphthalenedisulfonic acid（NA）and 1,1’-ferrocene dicarboxylic acid（FDA）during the formation of MA-CA aggregates.The as-synthesized MNCA-75 and MFC-CN were used as photocatalysts and photo-responsive persulfate activated catalysts to degrade sulfamethoxazole（SMX）.The main research contents and conclusions are as follows:（1）After the introduction of dibasic sulfonic acid,namely NA,during the self-assembly of MA and CA precursors,MA and CA formed uniform rod-shaped precursor aggregates,which transformed into porous g-C₃N₄ nanotubes after pyrolysis.The BET surface area of MNCA-75nanotubes prepared with the aid of 75 mg/L NA is as high as 100.40 m²/g,which is 1.4 times higher than that of MCA,and has good morphology uniformity.Compared with MCA,the band gap of MNCA-75 was reduced from 2.45 e V to 2.38 e V,and the visible light absorption capacity was significantly improved.The photocatalytic degradation of SMX in MNCA-75 system followed pseudo first order kinetics,and the apparent rate constant(k_obs)was 0.035 min^-1,which was three times of that in MCA system.（2）The Vis-SR-AOP system was constructed using MNCA as catalyst and peroxymonosulfate（PMS）as oxidant.Compared with the pure photocatalytic system,the degradation efficiency of SMX in the Vis-SR-AOP system was significantly higher.In the Vis/PMS/MNCA-75 system,the degradation rate of 10 mg/L SMX reached 99.6%within 100minutes of reaction.The optimal reaction conditions for SMX degradation are MNCA-75dosage 0.2 g/L,PMS concentration 1.0 m M and initial p H 7.0.The system has wide p H applicability（3.0～9.0）.Background substances in water matrix,Cl-,NO₃-,HCO₃-and HA,all inhibited the degradation of SMX,and HCO₃-and HA had a greater impact on the degradation of SMX.The main active species in the degradation process of SMX include SO₄·-,·OH,·O₂-,¹O₂ and h⁺,of which·O₂-,¹O₂ and h⁺contributed more to the degradation of SMX.（3）FDA was introduced during the formation of MA-CA aggregates,and Fe-doped g-C₃N₄crumpled nanosheet aggregates were obtained through thermal polymerization.The S_BET of the as-synthesized MFC-CN reached 193.28 m²·g^-1.In the Vis/PMS/MFC-CN system,SMX with an initial concentration of 10 mg/L was completely removed within 6 minutes,and the k_obs was0.961 min^-1,which was 45.8 times higher than that of the Vis/PMS/MC-CN system.After used for 5 continuous cycles under light and dark conditions,the percentages of Fe（II）on the MFC-CN surface were 55.02%and 43.89%,respectively,indicating that visible light irradiation promoted the Fe（II）/Fe（III）cycling and improved the degradation efficiency of SMX.The main active species produced by PMS activation in the Vis/PMS/MFC-CN system include h⁺,SO₄·-,·OH,·O₂-and ¹O₂,among which h⁺,·O₂-and ¹O₂ had a greater contribution to the degradation of SMX.There are 11 main intermediate products in the degradation of SMX.There may be three degradation pathways,namely,SMX hydroxylation,amino oxidation and S-N bond cleavage.