Formation Process Of Environmental Corona On GO At Trace Level In Aquatic Media Based On Hollow-fiber Flow Field-flow Fractionation

Graphene oxide（GO）has a relatively high surface area and rich polar oxygen-containing functional groups.When GO is discharged into water,it can adsorb natural organic matter（NOM）in water through electrostatic,hydrogen bonding and hydrophobicity,thus changing the surface morphology and charge distribution of GO.NOM with different molecular weights is adsorbed on the surface of GO,forming a canopy structure,which is called"environmental corona".The toxicity of GO is closely related to its size,so it is of great significance to effectively separate and quantify GO with different sizes.However,most of the commonly used materials separation and analysis techniques have many shortcomings,which have limitations in realizing the direct separation of GO in the environment,which limits the research on the size distribution of GO and its biological toxicity in water environment.Field flow fractionation（FFF）has gradually developed into an important method to separate GO,while atomic emission spectrometry（AES）has become an important method for quantitative detection of carbon materials because of its simple structure,low energy consumption and mild working conditions.In this study,hollow-fiber flow field-flow fractionation（HF5）and point dicharge-optical emission spectrum（PD-OES）were used for the first time,and the“environmental corona”formed by the reaction of GO and NOM with different molecular weights in water was separated and quantified.It provides a scientific basis for evaluating the environmental risks and biological effects of GO in water.The specific reaearch results are as follows:（1）Characterization of GO,NOM and"environmental corona".GO was characterized by SEM,TEM,AFM and UV-Vis,and NOM was characterized by TOC,UV-Vis and FT-ICR MS.The results show that GO is a two-dimensional lamellar structure with a diameter of about 1μm and a thickness of about 1 nm.GO has a large surface area and contains many functional groups,including hydroxyl,carboxyl and epoxy groups.TOC determination of NOM concentration found that NOM<3 kD accounts for the majority.With the increase of molecular weight,the concentration gradually decreased,that is,the proportion of molecular weight gradually decreased.The aromaticity and hydrophilicity of NOM with different molecular weights are different,and the components are complex,mainly lignin,accounting for more than 50%of all components.SEM measurement show that the surface of GO is adsorbed with NOM moleculers,and NOM reacts with GO by electrostatic action to form an environmental corona through.（2）The separation and determination system of HF5-PD-OES was established,and the optimal experimental conditions of HF5 and PD-OES were obtained by GO optimization,and the stability of the method was verified.The optimum experimental conditions of HF5 are as follows:the hollow fiber membrane is polyethersulfone（PES）membrane,radial flow rate is 0.8 mL/min,total flow rate is 1.2 mL/min,and focusing time is 8 min.The optimaum experimental conditions of PD-OES are as follows:the flow rate of carrier liquid is 0.4mL/min,the flow rate of carrier gas is 150 mL/min,the injection amount is 1 mL,and the ratio of oxidant is as follows:every 100 mL of oxidant contains 1mL 500 mM Ti O₂,1mL 50mM Fe³⁺（ferric nitrate solution）and 2 mL of 30%H₂O₂ solution,the discharge voltage is 75V,and the distance between electrodes is 3.In the best state of PD-OES,sodium persulfate and potassium hydrogen phthalate（KHP）with different concentrations were used as standard substances to calibrate the PD-OES system.The results show that the response signal of the system showed a good linear range,and its standard curve was I=8493.5C+8182.8（C represents the concentration of KHP in mg/L）,and the correlation coefficient is 0.999.（3）Using the established HF5-PD-OES system,we can effectively separate and measure the size change of"environmental corona"formed by the reaction of NOM and GO of different grades.The"environmental corona"was formed by the reaction of different grades of NOM and GO.NOM and GO with different concentrations and different grades were mixed to form an"environmental cap",which was separated by HF5,and the samples flowing out of the detector were measured by PD-OES.When measuring NOM alone,with the increase of NOM concentration,the solvent peak and sample peak of NOM detected in HF5 will increase,and the areas of solvent peak and sample peak will gradually increase by PD-OES.When different concentrations of NOM are added to GO,the solvent peak and small molecule peak of NOM detected in HF5 gradually increase with the increase of NOM concentration,which is consistent with the results of measuring NOM by HF5 alone.The results of PD-OES show that the areas of the solvent peak,small molecule peak and sample peak gradually increase with the increase of NOM concentration.Compared with GO solution without NOM,the peak of the sample tends to move backward gradually,and the area of PD-OES added with NOM solution is larger than that without NOM,which indicates that some NOM is bound to the surface of GO,which increases the area of PD-OES.SEM characterization proves the existence of"environmental corona".（4）Using the established HF5-PD-OES system to verify the formation of"environmental corona"in real water environment.In this study,standard addition experiment was carried out in real water to verify the feasibility of the separation and detection system in real water.It was found that with the extension of the reaction time,the area of the sample peak gragually increased,indicating the formation of the environmental corona,and with extension of time,two peaks appeared,indicating that more NOM and GO formed the“environmental corona”.Then,the change of size was measured.The size was consistent with the that of HF5,which showed that the particle size gradually increased.The change of Zeta potential indicated that NOM transferred the negative charge to the surface of GO,and it was bound to the surface of GO by electrostatic action.Therefore,this study first used different methods to characterize GO,NOM and environmental corona.Secondly,the separation and determination system of HF5-PD-OES inorganic carbon was constructed,and the experimental conditions of HF5 and PD-OES were optimized,respectively,and GO with different sizes was separated and determined from environmental coronas.Finally,the established separation and determination system was used to effectively separate and determine the environmental corona in the actual water environment.