Transformation Of Antibiotics By Goethite And Manganese Dioxide:Kinetic,Mechanism And Antibacterial Property

Antibiotics and antibiotic resistance genes have been drawing a widespread attention due to their threats to environment and ecosystem.Understanding the environmental behavior of antibiotics is an important prerequisite for effective control of their environmental pollution and ecological risks.Iron or manganese oxide and hydroxide,which are widely distributed in soil and sedimentary environment,play a critical role in controlling the environmental transformation and fate of environmental pollutants due to their high specific surface area and redox activity.Hence,it is of great significance to study the effects of goethite and manganese dioxide on the environmental behavior and ecotoxicity of antibiotics.In this study,the transformation kinetics and transformation products of four kinds of commonly used antibiotics,including tetracycline（TCs）,sulfanilamide（SAs）,fluoroquinolone（FQs）and cephalosporin（CPs）,by goethite or birnessite,were systematically studied.The antibacterial toxicity of transformation products of each antibiotic was also evaluated.The results showed that goethite could transform TCs and CPs but had no obvious transformation effect on SAs and FQs,while birnessite could transform all target antibiotics including TCs,SAs,FQs and CPs.Besides,the inhibition of each target antibiotic towards E.coli was reduced significantly as transformed by either goethite or birnessite,indicating transformation products lost antibacterial toxicity.The transformation mechanism of birnessite for antibiotics is dominated by oxidation,while the mechanism of goethite for antibiotics remains unclear.In this study,the transformation of tetracycline（TTC）with goethite was further investigated.The results showed that the single coordination（≡Fe OH）and double coordination（≡Fe₂OH）hydroxyl groups on goethite surface were reactive sites for the adsorption and transformation of TTC,and≡Fe OH was more reactive than≡Fe₂OH.A kinetic model that developed based on this mechanism could accurately predict the transformation of TTC by goethite under different p H conditions（4.0–10.0）.The transformation pathway of TTC was dominated by N,N-dedimethylation.It was proved that the key reactive moiety in TTC with goethite was the structure of tertiary nitrogen connected with benzene ring.This study strongly implied that 102 kinds of drugs containing this structure are likely to undergo N-dealkylation by goethite.Considering the dynamic change of water content in the actual soil environment,the effect of water content on the interaction between manganese dioxide（γ-Mn O₂）and sulfamethoxazole（SMX）was studied by simulating evaporation-rainfall experiment.The results showed that the kinetic results in suspension system could not explain the transformation of SMX byγ-Mn O₂,when the water content（H₂O/γ-Mn O₂）decreased to 3.85 g/g.In evaporation process,Mn²⁺,the reduction product ofγ-Mn O₂,was more likely to gather on the surface ofγ-Mn O₂,followed by the formation of"γ-Mn O₂-Mn²⁺-SMX"ternary complex thus inhibited further transformation of SMX.Once the reaction ceased,it was difficult to continue even multiple rainfalls occurred.It was also proved that the aniline group in SMX molecule is the reactive moiety towardγ-Mn O₂.In suspension system,the major transformation pathway of SMX included the cleavage of sulfanilamide bond,leading to the significant decrease in its antibacterial toxicity,while in evaporation process,the antibacterial toxicity increased at first and then decreased,indicating that the intermediate products retained strong antibacterial activity.