Phase Transfer Catalyst Enhanced Remediation Of Trichloroethene Contaminated Groundwater By Potassium Permanganate Oxidation: The Efficiency And Mechanism

Trichloroethene（TCE）is a chlorinated organic solvent,which is widely used in consumption goods and industrial field.Because of its low solubility,low adsorption,relatively long durability,low biodegradability,high density and low viscosity,TCE migrates underground and persists in the aquifer.As a result,TCE is one of the most ubiquitous dense non-aqueous phase liquids（DNAPL）in contaminated soil and groundwater.Therefore,the research of technologies for the remediation of TCE polluted groundwater is of great practical significance.In-situ chemical oxidation is an important technology for the groundwater remediation.In recent years,it has been widely used in the remediation of polluted groundwater due to the advantages of cost-effectiveness and the efficient degradation of dissolved pollutants.However,the remediation goals and complete restoration are difficult to achieve in the remediation of DNAPL polluted groundwater.On the one hand,the solubility of DNAPL in the groundwater is low,and there is the difference in the polarity between water-soluble amendment and DNAPL.As a result,the amendment cannot enter DNAPL effectively,and reaction rate is limited.On the other hand,the heterogeneity of the aquifer is resulted in uneven migration and distribution of the amendment in the aquifer;and thus,the contact between the amendment and DNAPL is insufficient and ineffective,resulting in low remediation efficiency.Moreover,when permanganate is chosen as an oxidant,MnO₂,the by-product of permanganate,tends to form at the DNAPL-oxidant interface and deposits on the aquifer media.As a result,the contact between DNAPL and the amendment is hindered,and the local aquifer is blocked.Thus,the mass transfer of the reaction and the migration of the amendment are limited.Focused on above challenges,this dissertation proposes the utilization of phase transfer catalyst（PTC）for the improvement of the mass transfer efficiency of the reaction between DNAPL and the amendment;simultaneously,it proposes the utilization of xanthan gum（XG）for the viscosity regulation of the amendment to promote the transport efficiency of the amendment in the heterogeneous aquifer.Based on phase transfer catalysis,the coupling technology of phase transfer catalysis and viscosity regulation is established to further improve the remediation efficiency of DNAPL polluted heterogeneous aquifer,which will provide a theoretical basis for practical engineering remediation.In this research,the kinetics of PTC enhanced TCE oxidation by KMnO₄ were studied,and the effects of main influencing factors on the reaction rate of PTC enhanced TCE oxidation were analyzed;the mechanisms of PTC enhanced degradation were clarified.Moreover,the effectiveness of PTC enhanced TCE degradation in the homogeneous aquifer was determined,and the influences of PTC enhanced remediation on p H of the groundwater and the permeability of the aquifer were studied.Afterwards,the migration characteristics of PTC-XG enhanced amendment and the mutual effects between amendment components were investigated.Finally,the oxidative efficiencies of PTC or XG singly enhanced and PTC-XG jointly enhanced remediation of TCE polluted heterogeneous aquifer by KMnO₄ were studied;the influences of enhanced remediation on the amendment migration,p H and ORP of the groundwater,the permeability of the aquifer and reduction products of KMnO₄ were clarified.According to the above,the effectiveness of PTC-XG jointly enhanced remediation of TCE contaminated heterogeneous aquifer by KMnO₄ was comprehensively analyzed.The main conclusions of this dissertation are as follows.（1）The mechanisms of PTC enhanced TCE degradation by KMnO₄The main principles of PTC enhanced TCE degradation by KMnO₄ included phase transfer catalysis,the acceleration of TCE dissolution and the alleviation of MnO₂generation and precipitation.Firstly,the process of phase transfer catalysis of onium PTC was mainly as follows.The oxidant anion MnO₄^-exchanged with the anion of PTC,and the ion pair of PTC-MnO₄^-was formed.Subsequently,the ion pair entered DNAPL phase TCE and was involved in oxidation reactions.This process maintained in the whole reaction stage,which played the leading role in TCE destruction enhancement by PTC.Moreover,this process was generally applicable for multiple non-aqueous phase liquids.Secondly,PTC could remarkably accelerate TCE dissolution,but it could not significantly solubilize TCE.The acceleration of TCE dissolution mainly occurred at the early stage of oxidation reaction.Thirdly,PTC could effectively alleviate the generation of MnO₂ by promoting the formation of dissolved Mn（Ⅱ）and Mn（Ⅲ）,as well as the inhibition of Mn（Ⅲ）disproportionation.Simultaneously,PTC could reduce the average particle size of formed MnO₂ and improve the stability of colloidal MnO₂;and thus,the precipitation of MnO₂ on the aquifer media was retarded.（2）The feasibility of PTC enhanced remediation of TCE contaminated homogeneous aquifer by KMnO₄In the process of PTC enhanced remediation of TCE contaminated homogeneous aquifer by KMnO₄,PTC significantly improved the effective utilization of MnO₄^-and promoted the degradation of TCE in the aquifer.TCE removal efficiency of PTC enhanced remediation of TCE polluted aquifer by KMnO₄ reached 94.1%,which was increased by 27.4%compared to the system without PTC.Moreover,in the process of PTC enhanced remediation,the p H of the groundwater maintained relative stability,and the mass of MnO₂ that was adsorbed and intercepted by media was reduced.It effectively alleviated the decrease in the permeability of the aquifer and the bypassing of the amendment.（3）The migration characteristic of PTC-XG enhanced amendment in the aquiferShear thinning fluid XG was utilized for the viscosity regulation of the amendment.The results indicated that the PTC-XG enhanced amendment which was prepared by the mixture of PTC,XG and KMnO₄ possessed the characteristic of shear-thinning.This enhanced amendment had no obvious effect on the PTC enhanced TCE degradation.Compared to the initial concentration of MnO₄^-before the mixture,the concentration of MnO₄^-in PTC-XG enhanced amendment was not significantly decreased.In the migration of PTC-XG enhanced amendment in the aquifer,the migration of MnO₄^-was synchronous with the water flow,and the migration of PTC lagged far behind that of MnO₄^-in the absence of XG.However,in the presence of XG,the same migration rate of PTC and MnO₄^-could be achieved attributed to the enhancement of PTC migration by XG.（4）The feasibility of PTC-XG jointly enhanced remediation of TCE contaminated heterogeneous aquifer by KMnO₄In the process of PTC-XG jointly enhanced remediation of TCE contaminated heterogeneous aquifer by KMnO₄,PTC was utilized to improve mass transfer of TCE oxidation reaction,and the obstruction of MnO₂ to the mass transfer of the reaction and the migration of the amendment was alleviated.TCE removal efficiency of PTC-XG enhanced remediation of TCE polluted aquifer by KMnO₄ reached 72.7%,which was increased by 61.6%compared to the system in the absence of PTC and XG.Moreover,the mass of formed MnO₂ by the degradation of 1.0 g of TCE was decreased by 68.8%compared to the system in the absence of PTC and XG.Simultaneously,XG was utilized for the viscosity regulation of the amendment,and thus,the transport efficiencies of MnO₄^-and PTC in the low permeability zone were promoted.After the injection of 9.5-pore volume-amendment,the sweep efficiency of the oxidant in the low permeability zone was up to 99.2%.The PTC-XG jointly enhanced remediation significantly improved the remediation efficiency of TCE contaminated heterogeneous aquifer by KMnO₄.Therefore,PTC coupled with XG for the enhanced remediation of TCE contaminated aquifer by KMnO₄ was highly effective and had good potential for field application.In this research,the theoretical bases of PTC-XG jointly enhanced technology had been laid to some extent,and the effectiveness of the enhancement technology had been preliminarily investigated,which was of important significance for engineering guidance.