Study On The Transport Behavior Of Silver Nanoparticles In Saturated Porous Media

Silver nanoparticles（Ag NPs）are the most commonly used engineered nanomaterials in consumer products,serving primarily as antimicrobial agents in water treatment devices,fabrics,medical devices and food and beverage packaging.The rapidly expanding use of Ag NPs inevitably leads to their release to the environment during production,storage,use and disposal processes,Ag NPs enter the water,soil,and sediment via multiple pathways,including atmospheric deposition,stormwater runoff,wastewater irrigation and land application of wastewater treatment biosolids.The potential ecological risk of Ag NPs is of wide concern,and Ag NPs entering the surface environment can be harmful to fish,algae,plants,soil microorganisms and invertebrates.Soil is considered as one of the major sinks for Ag NPs which may also transport through soil into groundwater,causing human health risks via drinking groundwater.Thus,it is of critical significance to thoroughly understand the transport of Ag NPs in soils and to assess their environmental behaviors,fate,and ecological effects.Silver nanoparticles（Ag NPs）are usually capped with stabilizing agents to protect their activities and improve stability.Polyvinylpyrrolidone（PVP）is one of the most used capping agents of Ag NPs,and may affect the transport of Ag NPs in porous media.The transport and retention of Ag NPs capped with PVPs of different molecular weights（PVP10-Ag NP,PVP40-Ag NP and PVP360-Ag NP）in uncoated,and humic acid（HA）-,kaolinite（KL）-and ferrihydrite（FH）-coated sand porous media were investigated.Among the three Ag NPs,PVP360-Ag NP exhibited the highest mobility and eluted from all types of porous media.This is because PVPs of higher molecular weight provided stronger steric effect and electrostatic repulsive forces among PVP-Ag NPs,inducing stronger blocking and shadow effects.The transport of the PVP-Ag NPs increased in the HA-Sand columns,while decreased in the KL-and FH-Sand columns,especially for PVP10-Ag NP and PVP40-Ag NP.The simulation results using one-site kinetic model indicated that HA-Sand reduced the maximum retention capacity(S_max),while KL-and FH-Sand increased the S_max as well as the first-order attachment rate coefficients(k_att),particularly at high ionic strength.To predict the transport of Ag NPs more accurately in both natural and engineered environments,more attention should be paid on the characteristics of capping agents and the surface properties of the porous media.The effect of four monovalent cations,Li⁺,Na⁺,K⁺,and Cs⁺,on the transport of PVP-Ag NPs in quartz sand porous media was also investigated.The interaction between the four monovalent cations and the nanomaterials should differ due to the difference in hydration capacity,However,there is no difference in theζof PVP-Ag NPs in the four different monovalent cation solutions due to the effect of PVP coated on the Ag NPs surface.Forζof quartz sand media,the four monovalent cations show ion-specific effects,with the effect of quartz sand ranging from weak to strong:Li⁺<Na⁺<K⁺<Cs⁺.We observed that the effects of monovalent cations on the transport of PVP10-Ag NP and PVP40-Ag NP in saturated quartz sand obeyed the ion-specific effects,that is,transport-inhibition effects of cations followed the order of Li⁺<Na⁺<K⁺<Cs⁺.The highest value ofζwas found on quartz sand at 100 mmol/L Cs⁺,and the transport of PVP10-Ag NP and PVP40-Ag NP in the porous media was greatly inhibited,this is due to the increased ofζon quartz sand and the decreased of electrostatic repulsion between PVP-Ag NPs and porous media,in addition,Cs⁺on the surface of quartz sand may form inner-sphere complexes,thus increasing the retention of PVP-Ag NPs via cation bridging between quartz sand and PVP-Ag NPs.PVP360-Ag NP exhibited the highest mobility,the ion-specific effects and increasing cation concentration had no significant impact on the transport due to the stronger steric effect,and the C/C₀ was slightly reduced under 100 mmol/L Cs⁺.Dissolved black carbons（DBCs）are ubiquitous in soil as a result of large-scale application of biomass-derived black carbons as soil amendments and remediation agents.In this study,two DBCs with different functional groups were prepared at 300and 500℃（DBC300 and DBC500）,and their impacts on the transport of uncoated Ag NPs（Bare-Ag NP）and polyvinylpyrrolidone-coated Ag NPs（PVP-Ag NP）in saturated quartz sand were investigated.The transport of PVP-Ag NP was much higher than Bare-Ag NP under the same conditions because of the increased steric hindrance provided by PVP surface coating.The transport of two kinds of Ag NPs was both enhanced by the DBCs under all the experimental conditions.DBC500 displayed stronger enhancement effect than DBC300 on PVP-Ag NP transport,but that of Bare-Ag NP was more facilitated by DBC300.The higher aromaticity and stronger hydrophobicity of DBC500 drove it to be adsorbed on the surface of PVP-Ag NP,thus providing stronger steric hindrance and promotion effect on PVP-Ag NP transport.However,DBC300 contained surface sulfhydryl groups,which bound with the Bare-Ag NP tightly,therefore it greatly promoted Bare-Ag NP transport via enhanced steric hindrance.