Simultaneous Adsorption Of Tetracycline And Norfloxacin By Amphoteric And Cationic Modified Bentonite

In recent years,with the widespread use of antibiotics,different types of antibiotics entered and combined into the environment.Compound pollution of antibiotics is more complex and harmful compared with the single one,which has the increasing attention of public concern.Clay minerals modified by organisms have striking features with cheap,convenient,efficient and pollution-free.Researching the organical clay minerals adsorbents with high adsorption capacity of antiobics and defining the effects,factors and mechanisms of the antibiotic compound polution by various adsorbents have a great significant to respond the antibiotic compound pollution existing in aquatic environment..We modify bentonite by long carbon chain modifier octadecyl dimethyl betaine（BS-18）and long carbon chain cation modifier octadecyl trimethyl ammonium chloride（OT-18）.This paper focus on two typical antiobics with different structures,i.e.tetracycline（TC）and norfloxacin（NOR）and studies the adsorption characteristics and interactions of TC and NOR by bentonite modified with BS-18 or OT-18 into the single or compound systems.Meanwhile,we explore the influence of different conditions（temperature,ionic strength and p H）and highlight the regeneration experiments of different modified bentonites into the single and compound systems.Scanning electron microscope（SEM）,total carbon,total nitrogen,cation exchange capacity（CEC）,anionic exchange capacity（AEC）,Zeta potential,specific surface area(S_BET),infrared spectra（FTIR）,X-ray diffraction（XRD）are used to analyze the surface characteristics of different types of modified bentonites.We have gained the difference of adsorption mechanisms to antibiotics with different structures by various modified bentonites,which would provide experimental basis and theoretical support for the application of organism modified bentonites to compound antibiotic pollution.The main results are as follows:（1）Surface characteristic experiments showed that BS-18 and OT-18 modifiers have been successfully inserted into bentonite layers.Compared with unmodified bentonite（CK）,BS-18 modified bentonite increased the content of total carbon and total nitrogen,layer spacing and Zeta potential,whereas CEC,AEC and S_BET decreased.The content of total carbon and total nitrogen as well as AEC,layer spacing and Zeta potential in OT-18modified bentonite increased,but CEC and S_BET decreased.With the increase of the modification proportion,the modification mechanism of BS-18 and OT-18 on the surface of bentonites changed,and hydrophobic adsorption mechanism appeared at the 50%CEC modification proportion.The difference of modifier structures（for example,modifier chain length BS-18<OT-18）resulted in different properties of organism modified bentonites.XRD results showed that both BS-18 and OT-18 were single-layer oblique arranged at50%CEC,and hydrophobic mechanism had appeared.,and hydrophobic mechanism plays a dominant role on the quasi-triplex oblique 100%CEC.OT-18 is more prone to hydrophobic adsorption mechanism than BS-18.（2）The adsorption of TC by BS-18 amphteric modified bentonite and OT-18 cationic modified bentonite conforms to Langmuir model,while the adsorption of NOR conforms to Henry model.In the single/composite system,the adsorption capacity of BS-18 amphoteric mo dified bentonite for TC showed a trend of CK>100BS-18>25BS-18>50BS-18,CK>25BS-18>50BS-18>100BS-18 respectively.The adsorption capacity of NOR showed a tr end of 25BS-18>50BS-18>CK>100BS-18,CK>50BS-18>25BS-18>100BS-18 respectiv ely.In the TC and NOR composite system,the synergistic effect played a main role.The improvement times of BS-18 modified bentonite on TC and NOR were both 50BS-18>25BS-18>100BS-18.In the single/composite system,the adsorption capacity of TC of OT-18 cationic modified bentonite was CK>25OT-18>100OT-18>50OT-18;The adsorption capacity of NOR was CK>25OT-18>50OT-18>100OT-18.In the TC and NOR composite system,the synergistic effect was also observed,but the improvement ratio of OT-18 to TC was 100OT-18>25OT-18>50OT-18;the improvement of NOR was 25OT-18>50OT-18>100OT-18.（3）With the increase of temperature,BS-18 and OT-18 modified bentonite showed a positive warming effect on the adsorption of single/composite TC.BS-18 showed a decreasing trend for single/composite NOR with the increase of temperature,while OT-18showed an increasing trend for single NOR and a decreasing trend for composite NOR.When the ionic strength increased from 0.001 mol·L^-1 to 0.5 mol·L^-1,the adsorption of TC and NOR in the tested soil samples was inhibited.With the increase of solution p H,the inflection point of the adsorption capacity of TC and NOR was related to p K_a.In the composite system,the compound of TC and NOR will weaken the influence of their existing forms on the adsorption capacity.（4）The adsorption of TC by BS-18 modified bentonite was mainly caused by electric charge attraction,whereas the adsorption of NOR was mainly caused by the combination of electric charge attraction and the hydrophobic effect.The adsorption of TC and NOR by OT-18 modified bentonite showed the combination of charge attraction and surface adsorption,but the strength of the two forces were different.The charge attraction effect on NOR was stronger than TC,and the surface adsorption effect on TC was stronger than NOR.In the composite system,TC and NOR formed a compound and formed a“bridge”to promote the adsorption of antibiotics to soil samples,showing synergistic adsorption（5）The results of the regeneration experiment showed that HCl was the best desorption solution,and CK,25BS-18,25OT-18 soil samples showed good regeneration performance against single（TC,NOR）and compound（TC+NOR）antibiotics when HCl was selected as the desorption solution.In the combined system,the existence of NOR can improve the reusability of TC in modified soil samples.