Effects Of Soil Humic Substances On The Degradation Of Chloramphenicol By Shewanella

Since the last decades,the majority of synthetic organic compounds such as pharmaceuticals and personal care products have swept the world.They are alarming our natural resources;their occurrences have been increased steadily.Antibiotics such as chloramphenicol（CAP）are considered as the most emerging contaminants and must to be urgently conducted for investigation.Biodegradation is a green,sustainable and low-cost process for attenuating chloramphenicol amounts.In the soil,Humic substances（HS）could interact with microorganisms through various mechanisms and affect the biodegradation of organic matter.In this study,Shewanella,a typical metal reducing model bacterium in the environment,and soil humic substances extracted from two types of cultivated soils were selected as experimental materials.On the same lines,chloramphenicol was choosed as the target contaminant for this research.Chloramphenicol degradation process in the presence of humic substances was studied by different instrumental analysis,and the effects of humic substances on the degradation of chloramphenicol by Shewanella was clarified.The main conclusions are as follows:1.After 36 h of reaction,the degradation rate of chloramphenicol by Shewanella was 75%,and in the presence of JHA,JFA and JHM,the degradation rate of chloramphenicol was 100%.In the presence of HHA,HFA,and HHM,degradation rates of chloramphenicol were:100%,90%,and 62%.The sequence of promoted chloramphenicol degradation by the same HS from different soils was:HHA>JHA,JFA>HFA,JHM>HHM.This may be ascribed to the humification of JHA,JFA,and JHM.2.Since the concentration of both JHA and HHA increased from 0.01 g/L to1g/L,the degradation rate of chloramphenicol increased from 75%to 100%.When the JHM and HHM concentrations were increased from 1 g/L to 20 g/L,the degradation of chloramphenicol by Shewanella increased slightly and then decreased.In order to promote chloramphenicol degradation,the optimal concentrations of JHM and HHM were 10 g/L and 5 g/L,and the degradation rates of Shewanella to chloramphenicol were 96%and 97%,respectively.With the increasing of the JFA concentration from 3 mg/L to 60 mg/L,the removal rate of chloramphenicol increased from 70%to 100%.When the HFA concentration was up from 3 mg/L to 30 mg/L,the attenuation of chloramphenicol increased,while the degradation rate of chloramphenicol decreased with increasing HFA concentration from 30 mg/L to 60mg/L.3.Conducting SEM technique,it was found that Shewanella aggregated successfully with HS,this complex contributed effectively to the attenuation of the chloramphenicol toxicity and creating a favorable way for the extracellular electrons transfer to HS.In addition,Zeta potential showed that negative charges on the surface of JHA、HHA、HFA、HHM decreased after addition of Shewanella,which could also indicated that Shewanella adsorbed to the surface of humic substances.4.In the presence of riboflavin and AQDS,the reaction rate constants of chloramphenicol degradation by Shewanella increased from 0.039 h^-1to 0.181 h^-1and0.929 h^-1,indicating that the degradation of chloramphenicol was related to the extracellular electron transfer of Shewanella.The addition of electron shuttles enhanced the electron transfer rate.After Shewanella interacted with humic substances,the cyclic voltammetric scans showed a higher redox peak current in the complex systems.With adding HHA,HFA and HHM,the current magnitude peak in these systems were:HHA>HHM>HFA,which also corresponded to the degradation rate of chloramphenicol.5.The fluorescence intensity of benzoquinone structures in JHA and HHA weakened during the time changes,which indicated the reduction of these components.JFA and HFA were partially decomposed into low molecular substances during the time changes,the humification of JHM and HHM deepened.The persistent free radicals in JHA,JFA,JHM,HHA,HFA,and HHM were oxygen-centered radicals.The type of PFRs of humic substances did not change after reaction with Shewanella,but the g-value factors all decreased,indicated that persistent free radicals had a deoxidation reaction.The intensity of PFRs signals in HHA,HFA,HHM,and JFA weakened,while the intensity of PFRs signal peaks in JHA and JHM increased.