Cotransport Of Graphene Oxide With Nanoparticles In Saturated Porous Media

As a typical carbon-based nanomaterial,graphene oxide（GO）has shown increasing more potential applications in a number of fields due to its unique physicochemical characteristics,such as energy,medicine,chemistry and polymer composite materials.The rapid increasing production and use of GO will lead to its environmental release.Once released into the environment,GO may co-present and interact with other nanoparticles.Such interaction is crucial in modifying their transport and retention in environmental media and thus likelihood of fates and risks.Therefore,to explore the cotransport of GO with other nanoparticles in saturated porous media is closer to the real situation in natural soils and sediments.In this study,column experiments were conducted to deeply explore the cotransport behavior of GO and different types of nanoparticles（metal oxide nanoparticles and organic polymer nanoparticles）in saturated porous media.The influence mechanisms of the physicochemical characteristics and the solution chemistry on the cotransport were detailedly investigated and discussed.The main conclusions of this study are as follows:1. Due to the different physicochemical properties of the three types of nanoparticles, their transport and retention in saturated porous media are significantly different,and the response mechanisms to different solution chemistry are also different.Typical carbonaceous nanomaterial,GO,exhibited high mobility in saturated porous media.GO was negatively charged and well dispersed under the examined conditions.Thus,the strong electrostatic repulsion between the GO sheets and quartz sand（which was even more negatively charged than GO）made GO easily pass through the sand columns.In addition,the high content of surface O-functional groups also makes GO highly mobile in porous media.RGO exhibited lower mobility than the pristine GO,particularly at relatively high ionic strength,which might be attributed to the changes in surface charge and particle size after photo-irradiation.The transport of typical metal oxide nanoparticles（n Ti O₂）in saturated porous media was highly related to p H values.Under favorable conditions,the positively charged n Ti O₂preferred to retain on the negatively charged sand surface,thus the mobility of n Ti O₂was very weak.Both of the n Ti O₂nanoparticles and quartz sand were negatively charged at p H8.0.The electrostatic repulsion between n Ti O₂and sand surface led to unfavorable particles retention.The transport of typical organic polymer nanoparticles polystyrene nanoplastic（PSNP）in saturated porous media was highly related to the electronegativity and hydrophobicity.There was strong hydrophobic interaction between the PSNP and sand.The less negative charge of PSNP also led to weaker electrostatic repulsion between PSNP and the quartz sand,therefore promoted PSNP retention onto sand surface and inhibited their transport.2. The co-presence of GO and n Ti O₂affected their transport and retention in saturatedquartz sand,which made the cotransport behaviors of the two kinds of nanoparticles more intricate than the individual ones.Moreover,the cotransport and retention of GO and n Ti O₂were largely dependent on the solution chemical factors,such as cation types,ionic strength and p H.Generally,the presence of n Ti O₂significantly inhibited the transport of GO,especially at lower p H,higher ionic strength and in electrolyte solutions contained divalent cation Ca²⁺.The formation of GO-n Ti O₂agglomerates（which were less negatively charged and more prone to aggregate compared with pure GO）and the addition of retention sites on sand surface by previously covered n Ti O₂decreased the transport of GO in quartz sand.Comparably,the co-presence of GO slightly enhanced the transport of n Ti O₂in quartz sand in Na⁺electrolyte solutions,which was benefit from the negative charge and high mobility of GO.However,GO had virtually no impact on n Ti O₂transport in Ca²⁺electrolyte solutions,mainly because of the attachment of n Ti O₂on sand via strong electrostatic attraction or the likelihood of particles straining.3. The cotransport and retention of GO and n Ti O₂were largely dependent on thesolution chemical factors and the physicochemical characteristics of nanomaterials.In the Na⁺saturated porous media,PSNP preferred to interact with（R）GO relative to the highly negatively charged quartz sand,thus（R）GO carried PSNP to break through the sand column.However,in Ca²⁺saturated porous media,the transport of both（R）GO and PSNP was depressed,attributed to the particle-collector and particle-particle bridging effects between Ca²⁺and the metal-complexing moieties of the nanoparticles and sand grains.Moreover,GO influenced the cotransport of PSNP to a larger extent than RGO,especially at relatively high ionic strength,because the more abundant surface O-functional groups of GO than RGO provided more complexion sites with Ca²⁺.