Application Of KMnO₄ For Remediation Of TCE-contaminated Heterogeneous Aquifer Under Viscosity Control:Mechanisms And Effectiveness

The injection of remedial amendments into aquifer was one of the popular remediation alternatives for groundwater pollution.The contaminants in groundwater can be transformed into compounds having less mobility or toxicity than their original form through reactions with amendments.However,heterogeneity was often encountered in subsurface,the amendments migration was mainly in the highly permeable and hydraulically accessible zones,which results in the invalid contaminants removal in low permeability zones.Furthermore,when the chemical solution was injected into the aquifer,the density difference between the chemical solution and the groundwater causes the water flow to be stratified,which leads to the uneven distribution of the amendments in the aquifer,and the effect of the remediation.Therefore,it was a key problem as to how to improve the ability of remedial amendments migration into low permeability formations for heterogeneous aquifer remediation.Therefore,this project intends to use polymer to control the viscosity and mobility of groundwater in order to enhance the remedial amendments migration into low permeability zones,and the uniform distribution of amendments.The biological polymer xanthan gum（XG）,oxidant-potassium permanganate（KMnO₄）,and contaminant-trichloroethylene（TCE）were selected for batch experiments,one-dimensional and two-dimensional simulation tank experiments.These experiments will be carried out to study:the compatibility of polymers with remedial amendments and the rheological properties of polymer solutions;the characteristics of retention of XG fluid in the aquifer and the synchronization of polymer and remedial amendments;the polymer fluid migration and distribution characteristics in various hydrogeological conditions,especially,the effect of layer thickness and permeability contrast on polymer sweeping efficiency improvement;the migration of remedial amendments in the aquifer under the effect of density effect and viscosity modification;the effect of KMnO₄ application for remediation of TCE-contaminated heterogeneous aquifer under viscosity control.The mainly conclusions of this study are as follows:（1）Compatibility of XG and KMnO₄,rheological properties of polymer solutionsAs a kind of shear thinning fluid,XG concentration exerted profound influence on solution viscosity;The K⁺,Na⁺,Ca²⁺,and Mg²⁺can reduce the viscosity of the XG solution.KMnO₄ has a good compatibility with XG based on oxidant consumption and solution viscosity;within a certain time,water-soluble phase TCE has little effect on the viscosity of the XG solution.（2）The retention of XG fluid in the aquifer and the migration synchronization of XG and KMnO₄When XG fluid was injected into the medium,the retention of XG in porous media led to the decrease of the effective porosity,thus accelerating transport of the subsequent solution to a certain extent.The smaller the hydraulic conductivity,the greater the retention effect for XG.;The injection of XG fluid resulted in the decrease of aquifer permeability,especially for the media of fine sands and silts,hydraulic conductivities were reduced by an order of magnitude.There was a general synchronization for the relative transport of KMnO₄ and XG,although a slightly difference was detected in their migration fronts.（3）The XG fluid migration and distribution characteristics in various hydrogeological conditionsXG fluids increase the viscosity of the liquid and the in-situ pore pressure of the aquifer,therefore reduce the flow rate of fluids in high-permeability media,leading to the cross-flow between different porous media.The sweeping efficiency of the remedial amendments in the low permeability zone was accordingly improved.In the equal thickness of medium sand and fine sand,the sweeping efficiency increased by0.33 times after the 300mg/L XG was injected.The relative thickness of the sediment layer has a significant effect on the enhanced remediation effectives of XG.Compared to the layered aquifer systems of 25%medium sand-75%fine sand and 50%medium sand-50%fine sand,the utilization efficiency of XG was increased by 0.12 times and0.05 times in the 75%medium sand-25%fine sand layered system,meanwhile,the migration rate was increased by 0.005 cm/min and 0.004cm/min,respectively in the low-permeability medium.That is,the greater the thickness of the high-permeability layer,the more obvious the enhancement of remedial fluid uniform distribution.The permeability difference also influences the enhanced remediation of XG in heterogeneous aquifers.Compared to the 50%medium sand-50%fine sand,the utilization efficiency of the fluid in the 50%medium sand-50%silty sand and the migration rate in the low-permeability medium were increased by 0.33 times and0.011cm/min,respectively.The larger the difference between the permeability of two layers,the greater effect of sweeping efficiency and migration rate increasing in low permeable region by viscosity modification.（4）Effect of density effect on the migration of KMnO₄ in the aquiferThe density effect has little influence on the migration of KMnO₄ in fine sand aquifer.KMnO₄ showed different degrees of sinking in the medium and coarse sand aquifer.when the 1.0,2.5 and 5.0g/L KMnO₄ were injected into the coarse sand aquifer,βwere changed from 69°,36°and 25°at 5 hours to 34°,19°and 12°at 11 h,respectively.In all the experimental time,the order of angleβwas as follows:β_1.0g/L>β_2.5g/L>β_5.0g/L.That means the greater the KMnO₄ concentration,the more obvious the density effect is.When the KMnO₄-XG mixture solution was injected after 9h,theβvalues of the 2.5g/L KMnO₄ with XG（100 and 300mg/L）in the coarse sand changed from the initial 90°to 25°,58°and 74°.For the 5.0g/L KMn O₄ with XG（100,and 300mg/L）of coarse sand,theβwere 19°,26°and 43 at 9 hours,respectively.The experimental results show that when the concentration of KMnO₄ was the same,the higher the concentration of XG,the smaller the density effect.The migration of KMnO₄ in the tank was closer to the form of vertical flow,the vertical distribution of fluid was more uniform,and the density correction effect was more obvious.（5）The effect of KMnO₄ application for remediation of TCE-contaminated heterogeneous aquifer under viscosity controlWhen KMnO₄ was injected,KMnO₄ produced significant cross-flow and sinking phenomena in the heterogeneous formation.The concentrations of KMnO₄ in the lower sampling ports of the same column were significantly higher than those of the upper sampling ports.When the effluent KMnO₄ concentration was not reached the initial concentration,the order of the effluent KMnO₄ concentration was No.1>No.2>No.3>No.4.The sweeping efficiency of KMnO₄ was only 23.3%in the low permeability formation,the TCE concentration was reduced by 30.8%at the 4-5sampling port,the TCE concentration reductions were all less than 20%at other sampling ports.Therefore,the TCE removal efficiency was relatively poor;When KMnO₄-XG was injected,the sinking and cross-flow phenomena of KMnO₄ were significantly improved.The migration of KMnO₄ in the tank was closer to the form of vertical flow.The KMnO₄ sweeping efficiency increased to 48%in the low permeability formation,TCE concentration of 3-5 was reduced by 62.7%,and the TCE concentration reduction increased to about 30%in other sampling ports.At the end of the experiment,the total removal rate of TCE reached to 57.8%.