Transport And Fate Of Nanoparticles In The Saturated Porous Medium And Its Numerical Simulation

Nanoparticles(NPs)are used in a variety of industrial,medical and life applications due to its unique physical and chemical properties(small size effect and large specific surface,high reaction activity,quantum effects).Because of the prevailing productions and uses,it is expected that NPs will inevitably be released into the environment including soil and groundwater systems,thus bring potential risks to human and ecological system.This work studied the effects of water chemistry conditions(humic acid(HA)concentration,ionic strength(IS)and pH)on the stability of NPs in suspension and the effects of water chemistry conditions(HA concentration,IS and pH)?sand size and medium heterogeneity on the retention and transport of NPs in saturated porous media under environmental relevant conditions by using two NPs(Nano-CeO2 and Nano-TiO2)as the main experimental materials.The main conclusions are as follows:1.The stability of both NPs in aqueous matrices were effected by the water chemistry conditions(HA concentration,pH and IS).The presence of HA significantly promoted the stability of nanoparticles(NPs)in suspension as evidenced by increased NPs stability with increasing HA concentration(0?5 mg L"1).The increased stability of NPs with HA mainly attributed to the introduced steric effect and increased electrostatic repulsion between NPs.Although increasing IS(1?50 mmol L-1 NaC1)could decrease NPs stability in suspension,HA(5 mg L-1)coated on NPs could weaken this effect by introducing great steric repulsion.Similarly,the presence of 5 mg L-1 HA decreased the effect of pH increasing from 6.01 to 10.02 on NPs stability.In addition,all the experimental data matched well with the predictions based on extended Derjaguin-Landau-Verwey-Overbeek(XDLVO)theory.2.The transport of NPs in saturated porous media were effected by the water chemistry conditions(HA concentration,pH and IS).HA and IS showed remarkable effect on the retention and transport of CeO2 NPs in the porous media.Even at low concentrations,HA stabilized CeO2 NPs in the suspensions by introducing both negative surface charge and steric repulsion and thus enhanced their mobility in the porous media.When solution HA concentration increased(0?10 mg L-1)or IS decreased(1?100 mg L-1 NaCl),mobility of CeO2 in the porous media enhanced dramatically.Under conditions of higher IS,the diffuse layers of the sand grains and the CeO2 NPs were suppressed by the added ions,and thus resulting in a decrease in the electrostatic repulsive interactions and steric repulsion between them.Solution pH,however,had little influence on the mobility of the CeO2 NPs under the tested experimental conditions in the presence of 5 mg L"1 HA,and increasing solution pH only slightly increased the transport of the NPs.Mathematical models were applied to describe the experimental data.Predictions from the XDLVO theory and advection-dispersion-reaction(ADR)model,matched the experimental data well.3.Grain size strongly affected the retention and transport of the Nano-TiO2 in the columns.The mobility of Nano-TiO2 in the sand columns reduced with decreasing grain size(275?920?m)and almost all Nano-TiO2 were retained in fine sand columns for all of the tested conditions.This effect was attributed to an increasing rate of mass transfer to the collector surface as the grain size decreases.When solution IS increased,mobility of Nano-TiO2 in the porous media descended dramatically under the conditions of same grain size.4.In the heterogeneous saturated porous media,the heterogeneity of hydraulic conductivity is one of the physical factors that determine the priority flow and the transport behavior of Nano-TiO2.For 920&390 ?m quartz sand system,the difference in permeability coefficient is relatively large,the breakthrough curves of both tracer and Nano-TiO2 had two separate breakthrough peak.For 920&550 ?m and 550&390?m quartz sand system,the difference in permeability coefficient is relatively small,the breakthrough curves of both tracer and Nano-TiO2 had superimposed flow.When the sand size of the medium is 920&550 ?m,longer matrix length resulted in shorter time for the solute appearing in the column effluent and longer time for the BTCs returning to base line levels.Through a combination of laboratory experiments and numerical simulation method,this work systematically studied the transport of NPs in the groundwater system,and established the corresponding mathematical model which were applied to describe the experimental data.The research could help to gain insight into understanding the environmental behavior of NPs in the groundwater system,and provide scientific basis for the accurate prediction of NPs pollution in the groundwater systems,thus laid a solid foundation for the ecological environmental risk warning and pollution remediation research,and also had important potential application value.