Surface Deposition And Transport In Porous Media Of Graphene Oxide And Its Reduced Product

Derivatives of graphene such as graphene oxide(GO)and reduced graphene oxide(RGO)are widely used for industrial application in last decade,and their environmental risks are inevitably increasing.The existence of natural reducer in the environment can reduce GO to RGO,which changes the surface chemical properties of GO and its transport behavior.In this study,restore GO to RGO using L-ascorbic acid(L-AA)removed the most part of hydroxyl and epoxy groups on GO,which was revealed by Fourier transform infrared spectra(FTIR)and X-ray photoelectron spectroscopy(XPS).Raman spectra of RGO showed lower ID/IG than GO,suggesting the restoration of lattice defects on GO.This thesis focused on the particle aggregation,surface deposition and the retention in porous media of GO and RGO.1.Particle aggregation.The aggregation of GO/RGO affects the colloidal stability of dispersion,and the subsequent surface deposition and retention in porous media.The electrophoretic mobilities(EPMs)measurements revealed a negatively charged GO and RGO at pH?7.00 and higher stability of the GO suspension than RGO.With increased electrolyte concentration,their hydrodynamic diameters increased faster and achieved maximum(attachment efficiency is 1).A smaller critical coagulation concentration(CCC)of RGO(45 mM)than GO(230 mM)was identified in NaCl solution,but their CCCs were similar in CaCl2(1.3 and 1.1 mM).Since Ca2+ can form cation bridge with carboxyl and hydroxyl on GO/RGO,which results slightly lower CCCGO than CCCRGO.2.Surface deposition.GO/RGO may deposit on the mineral surface due to the interactions.The deposition experiments on silica surface deployed with the quartz crystal microbalance with dissipation monitoring(QCM-D).The inhibited deposition at high NaCl concentration was identified.DLVO calculation explains faster deposition in relative low concentration,since the energy barrier between particle-surface recedes with NaCl increases.However,high energy barrier between GO/RGO-SiO2 lead to the similar minimal deposition rates(rf)of GO and RGO with increased CaCl2 concentration,revealing that cross-linking of Ca2+ ions with GO/RGO,which favored the formation of larger aggregates and inhibited deposition diffusion to oxide surface.Further deposition study on alumina surface confirmed obviously increases in deposition rates of GO and RGO due to the electrostatic interactions.3.Retention in porous media.The retention of GO/RGO in porous media influences their transport when they pass through the porous media with natural waters.Two-dimension plane flow experiments of GO and RGO was conducted using a micromodel pore net work to evaluate the mechanism of their transport and retention in porous media(with two common environmental oxide surfaces:silica and aluminum oxides).The transport and retention of GO and RGO aggregates in pore network was dominant by interception,diffusion,attachment and straining processes.Electrostatic attraction between Al2O3 and GO/RGO(at low ionic strength)can lead to additional retention in downstream region and the surface of collectors.However,the electrostatic repulsion between SiO2 and GO/RGO againsts the attachment of aggregates transported by diffusion,hence the interception domiant the retention in upstream region.In general,high aggregation efficiencies at 300 mM facilitated interception and straining of GO/RGO in Al2O3 and SiO2 pores due to the inertia.Moreover,the transporting aggregates can adhere to pre-deposited aggregates and result in further interception,while RGO trend to strain larger aggregates than GO in pores and lead to clogging.The retention efficiencies of GO(?T4 and ?T2)increased with ionic strength.Electrostatic attraction facilitated ?T4 in Al2O3 pores at 100 mM,while no visible distinction between in Al2O3 and SiO2 pores at 200,300 mM.Since the larger size of aggregates,the ?T4 of RGO increased first then droped with ionic strength increased.The larger aggregates of RGO trend to intercept and stain at the enterance of pore network.Transformations of GO alter its aggregation,deposition kinetics and transport performance due to the changes of physicochemical properties.Therefore this study provide experimental evidences for the potential environmental implications of GO and RGO,and theoretical basis for researching the changes of ecological impact mechanism from GO transformations.