Experimental And Theoretical Study On The Adsorption And Removal Of Tetracycline,Norfloxacin And Sulfadiazine In Aqueous Solution By Graphene-Based Composites

The extensive use of antibiotics has brought a series of negative impacts on the environment and human health.Due to the special antibacterial properties,the removal of antibiotics in water has aroused widespread public attention,thereby attracting the interest of numerous researchers.Compared with other traditional treatment processes,such as biological treatment,advanced oxidation process and membrane technology,adsorption has been broadly applied to the removal of antibiotics on account of high efficiency,low-cost and environmental friendliness.Graphene-based composites are a new kind of materials,in which graphene is functionally modified and composited to obtain new active groups and topological structures.These composites usually greatly improved the limited properties of graphene,such as difficult in separation,while its excellent physical and chemical properties are retained.In this work,the adsorption and removal of several antibiotics(tetracyclines,norfloxacin and sulfadiazine)in water by graphene-based materials(graphene oxide,graphene oxide/chitosan composites,etc.)were studied by means of experiments and theoretical calculations.The specific contents are as follows:(1)Adsorption and removal of tetracycline(TC),norfloxacin(NOR)and sulfadiazine(SDZ)by graphene oxide/chitosan composite.The fine dispersion of Graphene oxide(GO)makes it difficult to separate from water.Moreover,the traditional magnetic modification will significantly decrease the functional sites of GO,thus reducing the adsorption efficiency.Chitosan(CS)is a kind of cellulose,which contains abundant hydroxyl,amino and other functional groups.In this study,a CS-linked modified GO composite(GO-CS)was synthesized by a modified Hummers and solvothermal method.It can be effectively separated from the solution by low-speed centrifugation,thereby improving the separation efficiency of GO in water.Through characterizing by scanning electron microscopy(SEM),Transmission electron microscopy(TEM),X-ray diffraction(XRD),Fourier transform infrared spectroscopy(FT-IR)and 13C nuclear magnetic resonance(NMR),it was found that the functional groups and the layered structure of GO were retained in the GO-CS material.In the meantime.GO was proved to be composited with CS by an amide bond(-CONH-).The adsorption performance.pH effect,adsorption isotherm and the mechanism of three antibiotics adsorbed by GO-CS in aqueous solution were studied through adsorption batch experiments and X-ray photoelectron spectroscopy(XPS)analysis.The batch experiments showed that the optimal adsorption pH values for TC,NOR and SDZ were pH=4,5,and 4,respectively.Furthermore,the adsorption capacities of TC and NOR were greatly affected by the pH value,indicating that the molecular existing forms and charges under different pH values may affect their adsorption behaviors on GO-CS.At 298 K and the optimal pH,the saturated adsorption capacities of TC,NOR and SDZ fitted by Langmuir models were 597.77 mg/g,388.99 mg/g and 136.37 mg/g,respectively,which were obviously higher than that of pristine graphene oxide.It was found that hydrogen bonding played significant roles in the adsorption of three antibiotics by analyzing XPS spectra.On the basis of experimental observations,the existing forms of antibiotics as well as the functional groups of adsorbents may influence the adsorption of antibiotics on GO-CS.Thus,in order to explore their initiate effects and the underlying mechanism,density functional theory(DFT)method was employed to model the adsorption process of TC,NOR and SDZ on GO-CS.Based on the analysis of optimized adsorption configuration,adsorption energy,differential charge density,etc.,we found that the adsorption mechanism of GO-CS for three antibiotics was mainly through hydrogen bonds and electrostatic interaction.The calculated adsorption energy follows the order of TC>NOR>SDZ.Specifically,for TC molecules,positively charged TC+was more conducive to the stability of the adsorption configuration,moreover,the adsorption process was mainly conducted by electrostatic interaction and hydrogen bonding.While different molecular forms had no.obvious influence on the adsorption of NOR molecules.The hydrogen bond was the main driving force in the adsorption process of NOR,so does the SDZ molecules.At the same time,the computational results showed that the carboxyl functional group on GO was more effective than the epoxy functional group during the adsorption process.(2)The competitive adsorption process of GO for three tetracyclines antibiotics(tetracycline(TTC),oxytetracycline(OTC)and chlortetracycline(CTC))was studied by molecular dynamics(MD)simulations.MD simulations showed that GO had superior adsorption performance towards above three TC molecules,especially for CTC.This research will provide theoretical reference for graphene-based composite materials in interface interactions and the removal of antibiotics and interface,thus promoting them to be more widely applied in the environmental field.