| With the rapid development of industrialization,the application of heavy metals in various products is in great demand.It is the most common pollutant in groundwater due to improper storage and disposal.Heavy metal ions are difficult to degrade and only accumulated in organisms,seriously threatening the ecological environment and human health.Remediation of soil and groundwater contaminated with toxic heavy metals has been a major environmental challenge for decades.Cr（Ⅵ）is considered to be one of the most common heavy metal pollutants in soil and water.The in-situ reaction zone technology based on nanoscale zero-valent iron（nZVI）has been widely used in the remediation of Cr（Ⅵ）contaminated groundwater due to its advantages of economy and high efficiency.In recent years,there are some problems such as easy inactivation,easy passivation,easy agglomeration,poor stability and poor mobility.These problems limit the further development and application of this technology.However,graphene has many excellent physical and chemical properties,such as great theoretical specific surface area,excellent electrical conductivity,high strength,strong thermal conductivity,etc.,which can be combined with nZVI to greatly enhance some aspects of composite material properties.Therefore,a new type of graphene oxide-supported nanoscale zero-valent iron composite（nZVI/rGO）with high reactivity,passivation resistance and durability was prepared in this paper.Xanthan gum（XG）was further used to stabilize nZVI/rGO,Cr（Ⅵ）was used as the target pollutant,and the reactivity,migration and remediated ability of Cr（Ⅵ）were investigated.Meanwhile,the removal mechanism of Cr（Ⅵ）,the stabilization mechanism of XG stabilized nZVI/rGO（XG-nZVI/rGO）,and the transport and deposition mechanism of XG-nZVI/rGO in saturated porous media were clarified.In addition,a priori model for predicting transport and deposition parameters is developed to more accurately predict the transport and deposition behavior of polydisperse colloidal particles.Based on the above systematical research,it provides theoretical basis and technical guidance for actual in situ engineerinig remediation of Cr（Ⅵ）contaminated groundwater.The main conclusions of this study are as followed:（1）nZVI/rGO was successfully synthesized,and nZVI/rGO with 9%GO addition had the best Cr（Ⅵ）removal efficiency.The graphene can be thus used as an effective mediator to effectively transfer electrons from the nZVI because the planar graphene has perfectπ-electron systems,Therefore,the reaction center of the nZVI/rGO would move to the region of the graphene so that the surface passivation of the nZVI is retarded efficiently.Within the pH range of 69 in the groundwater environment,nZVI/rGO had a high Cr（Ⅵ）removal efficiency,which was beneficial for application of rGO-nZVI in underground environments.NO3-and HCO3-had inhibitory effects on the removal of Cr（Ⅵ）,and the inhibitory effect of HCO3-was greater than that of NO3-.（2）Most of the nZVI/rGO remained near the injection location of the simulated column.The deposition mass of nZVI/rGO decreases as function of distance,and a negative linear correlation between the logarithm of the deposition mass and the distance was obtained.In addition,there were obvious segmental deposition phenomena.Injection velocity and particle size were the limiting factors for the application of nZVI/rGO in the actual contaminated site.Within the given injection velocity,medium particle size and nZVI/rGO concentration range,the deposition rate coefficient(Kdep)and maximum transport distance(Lmax)can be calculated according to model equations for predicting and evaluating the remediation scope.（3）XG enchanced stability and transport behavior of nZVI/rGO.Adsorbed XG played a significant effect on electrostatic stability of XG-nZVI/rGO,while free XG hindered aggregation and sedimentation of XG-nZVI/rGO.Moreover,mass concentration ratio of saturated adsorption phase XG to nZVI/rGO(CAds XG/CnZVI/rGO)was 0.133.With 0.83 g/L nZVI/rGO injection concentration,there existed a critical injection velocity 1.39 cm/min.Aggregation would be promoted and lead to transport inhibition when injection velocity（v）was below this critical value,but disagglomeration and transport would be enhanced by high hydrodynamic shear stress when v was above the critical value.（4）The depostion mechanism of polydisperse XG-nZVI/rGO in saturated porous media was controlled by the combination of DLVO,Brownian diffusion,hydrodynamic shear,and interception.The deposition control mechanism was different for different XG-nZVI/rGO particle sizes.Specifically,there existed two critical threshold value of XG-nZVI/rGO diameter（V1-0.1737μm and V2-1.2080μm,V2>V1）.XG-nZVI/rGO with particle diameter exceeding V2（1.2080μm）was retained in the porous medium by interception.XG-nZVI/rGO with particle diameter smaller than V1（0.1737μm）deposited in the porous medium when it possessed kinetic energy（Ek）less than the depth of the secondary energy minimum it experienced.For XG-nZVI/rGO with diameter range of V1 and V2,couple of DLVO interaction,Brownian diffusion,hydrodynamics and interception controlled the depostion mechanisms,and it occurred deposition where adhesive torques（TA）was in excess of hydrodynamic drag torgues（TH）particles subject to.Therefore,for polydisperse colloidal particles,particle size distributions of colloidal particles needed to be comprehensively considered to accurately understand their transport and deposition mechanism in saturated porous media.（5）After the injection of XG-nZVI/rGO,a stable reaction zone covering area was formed near the injection well and downstream,so it could effectively remove Cr（Ⅵ）contaminated groundwater.Moreover,DO and ORP in the injector well and downstream decreased rapidly,creating a favorable reduction environment for Cr（Ⅵ）removal.The pH increased to between 8.25 and 8.75,indicating that the slurry injection would not significantly increase the pH of the system,and it meaned eco-friendly.NO3-,as a competing electron acceptor,losed activity of XG-nZVI/rGO to some degree. |
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