Synergistic Coupling Mechanism Of Mesoporous G-C₃N₄ Photocatalysis With Persulfate To Oxidize β-lactam Antibiotics

β-lactam antibiotics are widely used in clinic because of their superior antibacterial properties.However,due to a variety of excessive abuse,they are continuously transported to the ecological environment through human disease treatment,animal husbandry and aquaculture.And the excessive antibiotics in the environment will lead to microbial resistance and resistance genes,thus posing a potential threat to human health and the ecological environment.Therefore,how to effectively remove such antibiotics from the environment is of great significance.Sevenβ-lactam antibiotics were selected as model pollutants in this study,including amoxicillin（AMX）,benzylpenicillin（BEN）,cefotaxime（CFX）,cephalexin（CEF）,meropenem（MER）,aztreonam（AZT）and sulbactam（SUL）.By preparing the photocatalytic material Mesoporous g-C₃N4（MCN）,the MCN photocatalytic synergistic persulfate（Persulfate,PS）oxidation system was constructed.This study systematically analyzed the synergistic coupling effect of the MCN/PS system,revealed the characteristic and origins of selective differential oxidation ofβ-lactam antibiotics with different structures,and clarified the oxidative degradation laws ofβ-lactam antibiotics.Furthermore,the environmental practicability of the cooperative oxidation system was evaluated.The main research contents and conclusions are as follows:（1）Performances and synergistic coupling effect of MCN/PS driven by visible-light in the treatment ofβ-lactam antibiotics.The template-free method was used to successfully prepare MCN photocatalyst.TEM,XRD,XPS and UV-vis characterization analysis showed that the formation of mesoporous structure improved the utilization rate of visible light,promoted the separation of electron-hole pair and increased the active site of photocatalytic reaction.Therefore,MCN has excellent photocatalytic oxidation ofβ-lactam antibiotics.Subsequently,an efficient‘1+1>2’synergistic coupling system driven by visible light consisting of MCN and PS was constructed.The synergistic coupling effect of the system was systematically elucidated and visualized from three aspects:the transformation of reactive oxidative species（ROSs）,electron transfer channels and non-covalent interaction.The results show that typical adsorption process(E_ads=-8.924 e V)and non-covalent interaction dominated by van der Waals force exist in MCN/PS system.A large number of electrons could be transferred from MCN layer to the surrounding of PS for its activation,resulting in stretching and cracking of the O-O bond of PS form 1.496（?）to1.505（?）.This study provides a theoretical basis for the development and application of photocatalytic synergistic persulfate oxidation technology.（2）Characteristic and origins of the differential oxidation ofβ-lactam antibiotics with different structures.In the MCN/PS system,the differential oxidation characteristic of theβ-lactam antibiotics with different structures was discovered,and the origins was further revealed in this study.Firstly,based on the optimized geometric structure by Gaussian 09 and the simplified Fukui calculations,the consistencies and differences of the seven antibiotics were summarized from three aspects:three-dimensional structures,electron cloud distributions,and the vulnerable sites.Notably,the selective differential degradation ofβ-lactam antibiotics in the MCN/PS system was speculated to be related with the molecular ionization potential（MIP）.Subsequently,the distribution relationship between MIP and the oxidation kinetic constant（K）was explored and showed the following trend:a smaller MIP index（MER,6.329 e V）indicates a stronger ability of theβ-lactam antibiotic to provide electrons in the oxidation system,and this leads to a lower oxidation difficulty.On the contrary,a larger MIP index（SUL,10.240 e V）indicates a weaker ability of theβ-lactam antibiotic to provide electrons and a greater resistance to oxidation.In addition,the LSV result confirms that MER can be a better electron donor in the system,whereas SUL does not function as an electron donor,which further confirmed the above conclusion.（3）Degradation mechanism ofβ-lactam antibiotics in MCN/PS system.Combining the HPLC-QTOF-MS and simplified Fukui function calculation,the degradation mechanism ofβ-lactam antibiotics was clarified including the unconventional and conventional oxidation channels.In the study,the novel in-situ-chemical oxidation mechanism of SUL was firstly discovered,which is different from other sixβ-lactam antibiotics.The asymmetry of PS protonation（HS₂O₈^-）under acidic conditions leads to the rupture of the S-O bond rather than the traditional O-O bond.Subsequently,the HSO₃^-intermediate state specifically bind with SUL thus generating SUL-O-SO₂H as a byproduct,which could be further oxidized by ROSs.Meanwhile,the conventional oxidation paths ofβ-lactam antibiotics have been explored mainly including:（1）fracture or hydrolysis of theβ-lactam ring;（2）direct cracking of the C-N or C-C bond;（3）shedding of the branch chain.Finally,the toxicity of intermediates of sevenβ-lactam antibiotics was evaluated by using the database of T.E.S.T of USEPA,and the results showed that the environmental toxicity of these antibiotics could be effectively reduced in the oxidation process.（4）Environmental practicability analysis of the MCN/PS system.The environmental practicability of the MCN/PS synergistic system was evaluated through investigating the oxidative degradation performance ofβ-lactam antibiotics in three complex environments（Indi-Mix-WWTP）.With the change of complex environment,the oxidation degradation of antibiotics maintained more than 95%within60 mins,indicating that the high efficiency of the system remained well.Meanwhile,the photocatalyst MCN had superior stability by comparing the apparent morphology and microstructure of MCN before and after use,and no material poisoning was found.Subsequently,in the treatment of actual wastewater,the biodegradability of wastewater has been effectively improved,and the intensity of dissolved organic matter has also been effectively reduced.In summary,the MCN/PS system has excellent stability in complex reaction environment and has powerful application potential.Figures,58;Tables,14;References,203.