| Antibiotics as a kind of emerging micropollutants have raised concerns due to the ecological risks caused by their ubiquitous presence in the aquatic ecosystem.However,the conventional treatments(such as sedimentation and biological processes)are unsuitable for the degradation of antibiotics,leading to the consideration of various oxidation processes to remove them.Since complex degradation pathways by various oxidants,are easily affected by many factors(such as pH and coordination of metal ion),the degradation mechanisms cannot be thoroughly elucidated only by means of experimental approches.Thoretical calculation as an effective tool can elucidate the degradation mechanism of pollutants,which is helpful for better understanding of the reaction mechanism and developing new oxidation technologies.Thus,sulfonamides and fluoroquinolones which have been frequently detected in aquatic environment were adopted for case studies.The oxidative mechanisms of various oxidants and the effects of metal ion(Cu2+ and Zn2+ coordination as well as different dissociation speices of antibiotic have been investigated.The main research contexts and results are as follows:(1)Ozonation as a kind of selective oxidants could directly oxidate the biochemically essential moieties of antibiotics.A DFT study has been carried out herein for exploring ozonation pathways and degradation mechanisms of sulfamethoxazole(SMX).The results show that the primary oxidation of its amino and methyl groups firstly occurs via ozone-mediated hydrogen-atom abstraction,and the oxidation of the aromatic rings of SMX follows the direct electrophilic addition mechanism.According to this result,it turns out that the favorable ozonation site of sulfonamides could be theoretically predictable through the information of their frontier molecular orbitals,and the dissociation energies of N-H and C-H bonds in SMX positively correlate with their energy barriers of hydrogen atom abstractions.The results reveal the behavior of the ozonation of sulfonamides,which could provide theoretical guidance for predicting the degradation of other antibiotics in ozone oxidation.(2)Ferrate(Ⅵ)(HFeO4-)is an efficient and environmentally friendly oxidant for the degradation of antibiotics,owing to its efficiency and precipitation character of production.Herein,DFT calculations were performed to unveil the mechanism of ferrate(Ⅵ)-mediated degradation,taking SMX as a model compound.The results show that nucleophilic attack(rather than electrophilic attack)of HFeO4-on the isoxazole moiety of SMX initiates the subsequent degradations,and ferrate(VI)rather than the water molecule provides O atoms for the oxidation of the nitroso group and isoxazole moiety.Unpaired electron delocalization from the Fe atom to the isoxazole moiety is crucial for the ring-opening of isoxazole.Moreover,the intermediates having Fe-SMX bond suggested previously are not the necessary ones in the oxidation of SMX by ferrate(Ⅵ).and introducing a water molecule in reaction has little effect on the reaction mechanism.This study enriches the mechanism of ferrate(VI)-mediated degradation,and provides new insights of this kind of degradation.(3)The stability constants(β)of the complexes of seven fluoroquinolones(Fluoro)binding with metal ion(M = Cu2+,Zn2+)have been investigated by means of fluorescence titration and DFT calculations.The results indicate that the ketonic and carboxylate oxygen atoms of Fluoro bind with Cu2+ and Zn2+.The equation of Fluoro+ M(H2O)62+(?)[M(Fluoro)(H2O)4]2+ +(H2O)2 and(SMD)M06-2X/6-31+G(d,p)theoretical method have shown better performance in predicting β of M-Fluoro complexes.Using this computational protocols,a linear fitting equation y = 0.8662 x-1.1171 has been obtained on the basis of experimental and computational data,in which x and y represent computational and experimental β values,represently,and R2 is 0.853.This relationship has been verified by the β value of complex between Cu2+ and marbofloxacin which indicated that this theoretical equation can give reasonable results of β value.(4)The hydroxyl radical(·OH)is the main oxidative species in aqueous environment.Herein,the mechanism of ·OH mediated degradation of the frequently detected antibiotics,ciprofloxacin(CIP),has been carried out by DFT calculations.And the effects of various dissociation species of CIP and the complexation of Cu2+ and Zn2+ with CIP on the degradation mechanism have also been considered in this study.The results indicate that the oxidation of the quinolone ring of CIP follows the direct addition mechanism,and the oxidation of the piperazine and cyclopropyl rings occurs via hydrogen atom abstraction.The dissociation energies of C-H bonds in CIP also positively correlate with their energy barriers of hydrogen atom abstraction.The various dissociation species of CIP have effect on the favorite active-site and favorite reaction mechanism.As is the same with the results of experiment,the favorable reaction is degradation of H2CIP+ between various dissociation species of CIP,which could be in accordance with the smallest absolute hardness in H2CIP+.The result shows that Cu2+ and Zn2+ have little effect on the degradation in kinetics.The coordination of Cu2+ and Zn2+ with H2CIP+ destabilized the degradation product of quinolone ring which could alter the final distribution of degradation products. |
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