Study On The Photo-Transformation And Environmental Effects Of Graphene Oxide Nanomaterials In Water

Graphene oxide(GO),an important derivative of graphene,is a two-dimensional nanomaterial made of sp2-hybridized carbon with a large number of oxygen-containing functional groups on its surface and edges,including epoxy,hydroxyl and carboxyl.The presence of these functional groups makes it more hydrophilic and disperses in water easily.With the continuous and rapid growth of GO production,GO will inevitably leak into the water environment in the whole cycle of production,storage,transportation,use,disposal and recycling.And its environmental risk has attracted increasing attention.GO can also absorb ultraviolet light and undergo the photochemical reaction because of the unique disrupted π-bond structure.In the process of photo-transformation,the changes of chemical characteristics of GO surface produced toxic effects on microorganisms,the migration behavior of GO and the migration trend of organic pollutants in water environment.It is important to investigate the effect of irradiation on the transformation of GO nanomaterials for evaluating the migration,bioavailability and toxicity of GO nanomaterials in water environment.This paper investigated the transformation process of GO in water with four different oxidation degrees under irradiation.The effect of GO oxidation degree on photochemistry reaction was researched and the changes of different active oxygen radical trapping agents in solution before and after the photo-transformation were measured.The mechanism of GO photo-transformation was also analyzed.The effects of GO oxidation degree before and after photo-transformation on two typical microorganisms growth behavior(Gram-positive bacteria:Staphylococcus aureus;Gram-negative bacteria:Escherichia coli)were studied.The morphological changes during the growth of bacteria were monitored and the toxicity mechanism of GO to the two bacteria was determined.The effects of GO oxidation degree on its migration in simulated porous media before and after photo-transformation were studied through penetration experiments.The mechanism of its influence on the migration behavior of antibiotic pollutants in porous media was clarified.The main conclusions are as follows:(1)Four kinds of GO(GO-1:3,GO-1:6,GO-1:9,GO-1:12)with different oxidation degree were prepared by Hummers method and then irradiated for 4 hours under UV-light.The surface chemical structure was studied by a series of characterization methods.The results showed that the higher the oxidation degree of GO,the more oxygen-containing functional groups on the surface.During photochemistry experiments,the color of GO solution changed from light yellow to black as the UV-irradiation time.Furthermore,the oxygen content and a large number of hydroxyl,carboxyl and epoxy groups were decreased after photo-transformation.AFM results demonstrated that the GOs lamellar structures were damaged after irradiation and forming relatively small fragments,which lead to the decreased thickness.This phenomenon proved that the higher the oxidation degree,the more obvious the lamellar damage degree.Raman spectroscopic showed that the ID/IG ratio decreased significantly after photoreaction,and the electronic conjugation structure on GO surface was partly improved.The higher the oxidation degree of GO,the more reactive oxygen radicals were produced during photoreaction,and the final stable concentration was linearly positively correlated with the oxygen content of GO.The more oxygen-containing functional groups on the surface of GO can make the energy transfer process easier,which was conducive to the formation of reactive oxygen species.(2)The toxicity of GO before and after irradiation was tested.Escherichia coli and Staphylococcus aureuswere used as bacterial models.The growth inhibition of GO and(RGO)on bacterial cells was studied by measuring bacterial growth curve.The results showed that the higher the oxidation degree of GO and RGO,the lower OD600 value and more obvious inhibitory effect on this two bacteria growth behavior.Oxygen-containing functional groups on GO surface enhanced its dispersion in aqueous solution and was conductive to interact with bacterial cells.The results of bacterial cell viability experiments indicated that RGO has stronger growth inhibition than GO after photoreaction.Firstly,RGO had higher conductivity and stronger oxidative ability to glutathione in bacterial cells,which lead to more severe oxidative stress.Faster charge transfer between bacteria and RGO edges lead to further damage of bacterial cell membranes.Secondly,the relatively small size of RGO was conducive to its interaction with bacterial cells,resulting in stronger toxic effects.SEM results illustrated that GO and RGO could cut the cell membranes through their sharp edges and enter the cells,which leading to physical damage of bacterial cells.(3)The migration behavior of GO in water environment before and after photoreaction showed that the oxidation degree of GO affected its migration in saturated porous media of quartz sand.With the increased oxidation degree of GO,the penetration rate of GO increases from 80%to 99%.For RGO,the surface negative charge decreased due to the reduction of oxygen-containing functional groups resulting in the reduction of repulsion between RGO and dielectric particles.Simultaneously,The penetration rate of RGO was increased from about 50%to about 80%.The migration of LEV was difficultly in the sand column.However,When GO suspension was introduced,the retained LEV in the sand column flowed out along with GO.With the increasing oxidation degree of GO suspension,the release of retained LEV increased from about 20%to about 27%,which was caused by the enhanced combination of LEV by π-π conjugation and electrostatic adsorption generated from the more active oxygen-containing functional groups on GO surface.The more active oxygen-containing functional groups on GO surface,the more LEV was combined byπ-π conjugation and electrostatic adsorption.For RGO,the LEV release increased from 5%to 18%.