| Groundwater is an important part of water resources and it is is one of the important water sources for agricultural irrigation,industry and city due to the characteristics of stable water quantity and good water quality.Groundwater resources not only promote the rapid development of human society and economy,maintain the benign cycle of ecosystem,but also play an important role in human life and health.However,due to factors like the leakage of oil products and organic solvents,the leakage of waste leachate,the abuse of chemical fertilizer and pesticides,the development of mineral and petroleum resources,as well as the factors such as the discharge of industry,agriculture and domestic sewage,the groundwater environment in urban and rural areas suffered serious pollution.In particular,some organic pollutants are difficult to degrade and form long-term pollution sources,and it seriously threaten the safety of groundwater resources.It is urgent to the pollution treatment and remediation for groundwater resources.Magnetite nanoparticles(MNPs)and zero-valent iron nanoparticles(nZVI)are two kind of iron-based nanoparticles,which can either directly degrade pollutants or activate persulfate to accelerate the oxidative degradation of a variety of refractory organic contaminants.They have great application potential in the in-situ chemical remediation of groundwater environment.However,the stability and mobility of MNPs and nZVI in the in-situ application are very poor.Therefore,it is difficult for the two iron-based nanoparticles to be injected to and effectively distributed in the designated contaminated sites,which results in inefficiency and even failure for in-situ remediation of groundwater based on these two iron-based nanoparticles.Dissolution of polymers and surfactants in water is an important way to enhance the dispersion stability and transport of nanoparticles.For example,xanthan gum is a kind of common water-soluble polymer,which integrates thickening,suspension,emulsification and stability,and has the best performance.Compared with other types of polymers or surfactants,xanthan gum may play a unique role in enhancing the stability and transport properties of nanoparticles.Therefore,to strengthen the mobility of MNPs and nZVI in the porous media for their effective distribution in the contaminated sites,the thesis combined experiments(e.g.,batch sedimentation experiment,steady-state rheological experiment,column transport experiment,micromodel transport experiment)and theoretical analysis to investigate the sedimentation,rheological and transport behavior characteristics of MNPs and nZVI suspensions.These works provide theoretical guidance for the efficient application of iron-based nanoparticles in in-situ remediation of groundwater environment.The specific research work and achievements of this thesis include the following three sections:In the first section,two surfactants(SDBS,Triton X-100)and five polymers(xanthan gum,sodium alginate,carboxymethyl cellulose,polyvinylpyrrolidone and polystyrene sulfonate)was selected as model dispersants and their effect on the stability of iron-based nanoparticles was investigated with sedimentation tests.The results showed that the surfactant had no long-term stabilization effect on the iron-based nanoparticles,while the polymer could significantly improve the stability of iron-based nanoparticles,and xanthan gum(XG)was the best dispersants among the five tested polymers.There was a critical concentration for XG stabilizing the iron-based nanoparticles and it was 2.0 g/L.At such a critical concentration,XG could significantly prevent the particle aggregation and sedimentation of iron-based nanoparticles for a long time,even if the particle concentration was as high as 20 g/L.The XG-enhanced stability of iron-based nanoparticles was through a combination of the network and electrostatic stabilization.Finally,the sedimentation behavior of the suspension of the highly concentrated iron-based nanoparticles of MNPs and nZVI was studied in the absence and presence of XG.The results showed that in the absence of XG,the highly concentrated MNPs and nZVI particle suspension would soon form an obvious solidliquid interface during sedimentation process.The interface settled down with time and it successively experienced the zone settlement and compression settlement stage.In the presence of XG,solid-liquid interface did not appear in the highly concentrated MNPs suspension until about 30 days.After that,the interface first made an acceleration settlement and then a deceleration settlement with time.In the second section,the rheological behavior of XG solution and and XG-based MNPs and nZVI suspensions was investigated with steady-state flow experiments.The results showed that XG concentration had a great effect on its rheological behavior.When XG concentration was low,the zero-shear viscosity of XG solution was linear with its own concentration;however,when the concentration of XG was high,the zero-shear viscosity increased rapidly and had a power function relationship with XG concentration.Both XGbased suspension also exhibited the rheological behavior of shear thinning.The presence of iron-based nanoparticles had a great impact on the rheological properties of XG solution,and such a change was positively related to the nanoparticles concentration.The iron-based nanoparticles weaken the stiffness of network structure of XG solution,with a result that it reduced the zero-shear viscosity of the solution and increased the shear thinning property of the solution.The rheological behavior of XG solution and XG-based suspensions can be well simulated with the Cross model.By analyzing the relationship between the particle concentration and the rheological parameters,the Cross model was fully parameterized,and it could give a good prediction for the rheological behavior of XG solution and XG-based iron-based nanoparticles suspensions.In the third section,the transport behavior and particle distribution characteristics of iron-based nanoparticles of MNPs and nZVI in porous media were studied with column trasport experiments and microfluidic transport experiment.The results form column transport tests showed that the both iron-based nanoparticles in the presence of XG had a good mobility in quartz sand,and the performance of XG in the particle transport enhancement was better than carboxymethyl cellulose and other polymers.After the transport,the iron-based nanoparticles were not only effectively distributed throughout the column packed with quartz sand,but also successfully loaded on the surface of quartz sand.The transport of nanoparticles in quartz sand in the presence of XG did not continuously increased with the increase of flow velocity;instead,it in a certain range of flow velocity first decreased and then increased with the increase of flow velocity.The results form mocromodel transport tests showed that the iron-based nanoparticles tended to form a particle retention layer on the back surface of porous media within the microfluidic pore model.The thickness and color of the retention layer increased first and then decreased with the increase of flow velocity,indicating that the particle retention increased first and then decreased with the increase of flow velocity,which is consistent with the results of column transport experiments.Finally,the polystyrene microspheres with excellent stability itself were selected for the control experiment and with a combination of model analysis,it was inferred that the non-flowing zone would be formed in the transport system in the presence of XG.The presence of nonflowing zone was the main reason that the particle transport decreased first and then increased with the increase of flow velocity in a certain range of flow velocity. |
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