Effects Of Molecular Weight-dependent Heterogeneous Properties Of Natural Organic Matter On The Aggregation And Transport Of Graphene Oxide In Aquatic Environment

In recent years,engineered nanomaterials(ENMs)have been widely used in industrial and household products due to their smaller particle size,larger specific surface area,different morphologic characteristics and size effect from that of macroscopic substances with the same chemical composition.The ever growing production and wide application of ENMs make it inevitable that ENMs will be released into the environment and become a group of potential pollutants.Therefore,the species,source,distribution,aggregation,transformation and biotoxicity of ENMs in aquatic environment have become research hotspots in the field of environmental science in recent years.The environmental behavior of ENMs is not only affected by its own properties,but also influenced by a variety of environmental factors,such as natural organic matter(NOM),pH,ionic strength and sunlight irradiation.Among them,metal cations with different valence may affect the stability of colloid and surface chemical properties of NOM and ENMs through many interactions such as compressing double electron layers and binding with oxygen-containing functional groups.Then,the environmental behavior and trend of ENMs are affected.In addition,NOM is an important factor in the water environment.The concentration of NOM wins several orders of magnitude in natural water than that of ENMs,and has complex chemical composition and rich functional groups.Thus,NOM is easy to react with ENMs in water,and change the surface properties and environmental fate of ENMs.However,molecular weights(MW)of NOM vary widely in natural water environments.Thus,MW-dependent optical properties,functional group composition,surface charge and chemical composition of NOM would be altered greatly.Therefore,the study on NOM heterogeneity's influence on ENMs' environmental behavior is of great significance for evaluating its bioavailability,toxicity and environmental risks.Graphene oxide(GO)is a new kind of two-dimensional nanomaterial,which has been widely used in production and life in recent years.Thus,in this study,the coupling effects of different valence metal cations and NOM components with different molecular weights on the aggregation behavior and transport behavior of GO were discussed.The study and findings mainly includes the following parts:First,we used ultrafiltration centrifugation to classify the pristine Suwannee River NOM(pristine-SRNOM)to five parts as SRNOM>100 kD,SRNOM 30-100 kD,SRNOM 10-30 kD,SRNOM 3-10 kD and SRNOM<3 kD by MW.They are collectively called MW fractionations of SRNOM(Mf-SRNOM),the characterization results showed that with the change of MW,the functional group content,chromophore abundance and spectral properties all changed significantly.Zeta potential determination model and dynamic light scattering method of Zetasizer Nano ZS were used to determine the electrophoretic mobility(EPM)and aggregation kinetics of GO,and the effects of electrolytes and Mf-SRNOM on the EPM and aggregation kinetics of GO in NaCl/CaCl2 solution were investigated.The results show that:(1)In the system without SRNOMs,the higher the concentration of electrolyte in the solution,the faster the aggregation of GO,no matter in the solution of monovalent or divalent electrolyte;Moreover,the effect of divalent electrolyte on the aggregation promotion of GO is more significant than that of monovalent electrolyte.(2)In the system with SRNOMs:In either the whole concentration range of monovalent or low concentration range of divalent electrolyte solution,the addition of either pristine-or Mf-SRNOMs enhanced the stability of GO in solution.And with the increase of Mf-SRNOM MW,the stability of GO in water was improved.In the presence of high concentration of Na+,the stability of GO was positively correlated with Mf-SRNOM MW.However,in the presence of high concentration of Ca2+,due to the bridging effect of-COOH/-OH in Mf-SRNOM with Ca2+,Mf-SRNOM with higher molecular weight can enhance the aggregation of GO in a high concentration of bivalent electrolyte solutionThen,we use the quartz sand column to simulate the saturated porous media in natural environment.From the results of GO,and GO-SRNOMs' breakthrough curves,we can see that:(1)In the system without SRNOMs,GO-NaCl system,with the increases of NaCl concentration,the highest value of C/C0 in the breakthrough curve of GO and total mass recovery of GO bothgradually reduced.(2)In the system with SRNOMs:GO-NOM-NaCl systems.With the addition of pristine-/Mf-SRNOM,the mobility of GO were all significantly enhanced.And the greater the MW of Mf-SRNOM,the greater the promotion of GO's transportIn order to better illustrate the influence of NOM-based physicochemical heterogeneity on the aggregation and transport behavior of GO in the environment,the micro-and macro-morphology,hydrodynamic diameters and polydispersion index of GO under various electrolyte condition and pristine-/Mf-SRNOMs' existence condition was investigated by various characterization methods.The conclusion is:(1)In the presence of NaCl or low concentration of CaCl2,the larger the MW of Mf-SRNOM,the richer of their aromatic structure,thus inhibited the aggregation and enhanced the transport of GO by stronger force of the steric hindrance effect.(2)In the situation of Mf-SRNOM with large MW in high-concentration CaCl2,the dominant factor leading to the aggregation of GO changed to the bridging effect among Ca2+with Mf-SRNOM with large MW and GO.In summary,this study illustrates that,when exploring the influence of NOM on ENMs'environmental behavior in water,the effects of NOM's heterogeneity and the coupling effect of water electrolyte valence concentration should be highly evaluated on ENMs' aggregation and transport behavior and even their bioavailability.