| In recent years,the leakage of petroleum products and their derivatives,unreasonable discharge of industrial wastewater and domestic sewage,and the excessive use of organic fertilizers has caused the hydrophobic organic contamination in groundwater system,which is a worldwide problem.Surfactant-enhanced aquifer remediation(SEAR)with advantages of simple operation,low remediation cost and high remediation efficiency has been widely used in groundwater remediation for hydrophobic organic contaminants(HOCs)contamination.However,the concentration of surfactant used in current investigation and application of SEAR technology is generally much higher than its critical micelle concentration(CMC).The application of high-concentration surfactants will not only cause secondary pollution of the groundwater system in the remediation area,but also have a masking effect on the related processes involved in solubilization.As a result,the mechanism of surfactant partition,adsorption and aggregates migration involved in solubilization,especially the partition that has a significant impact on solubilization,is not very clear.Therefore,under the premise of ensuring a certain remediation efficiency and the comparability of surfactant concentration,the exploration of the solubilization process using low-concentration surfactants can not only improve the understanding of researchers on the solubilization process of SEAR technology,but also has important significance for optimizing the cost-effectiveness of SEAR technology.Therefore,based on investigating the partitioning behavior and effects of surfactants solubilization on n-alkane in porous media,the experiments(i.e.,batch solubilization experiments,static partition experiments,one-dimensional flow column solubilization experiments,aggregates migration experiments,and static redistribution experiments)combined with theoretical analysis were used in the thesis to comprehensively investigate the process and behavior mechanism of surfactant solubilization of residual n-alkane under continuous flow and flow interruption conditions,especially the mechanism of nonionic surfactant with significant partitioning effect in the residual non-aqueous phase liquids(NAPLs),and theoretical mechanism of the partition effects of nonionic surfactant involved in solubilization process.The relevant research content and results mainly include the following three sections:First,the solubilization behavior and partition effect of surfactant on residual n-alkane in high flow areas were investigated through one-dimensional flow column under continuous flow condition.Three typical surfactants(Triton X-100,SDBS and rhamnolipid)and one cosolvent n-propanol were selected to investigate the batch dynamic solubilization on ndodecane to determine the appropriate low-concentration surfactant dosage used in the following one-dimensional flow column experiments.The n-propanol,as a cosolvent,was chosen for comparative study.The results indicated that under the premise of ensuring a certain remediation efficiency and the comparability of the surfactant concentration,the application of low-concentration Triton X-100,SDBS,rhamnolipid and n-propanol in subsequent experiments were determined to be 270,270,240 mg/L and 210 g/L,respectively.The results of solubilization behavior and partition effects of surfactants on n-alkane under continuous flow condition showed that compared to the rapid equilibrium dissolution of ndodecane by n-propanol,the rate-limited solubilization of dodecane by anionic SDBS based on aggregate formation mechanism was quite different.The increase of pore-water velocity clearly inhibited the rate-limited solubilization process of low concentration SDBS,which was consistent with the solubilization results of conventional high-concentration surfactants and conformed to the law of rate-limited solubilization.Compared to n-propanol and anionic SDBS,the relatively rapid bulk partition inhibited the rate-limited solubilization of nonionic Triton X-100 to n-dodecane.Moreover,the apparent equilibrium solubilization concentration of n-dodecane in Triton X-100 solution decreased significantly with the decrease of porewater velocity,which was much different from the law of rate-limited solubilization.Compared with anionic SDBS and nonionic Triton X-100,the balance between rate-limited solubilization of dodecane by rhamnolipid and retention loss of dodecane-rhamnolipid aggregates during transport in the saturated porous media induced a similar apparent equilibrium solubilization concentration at different pore-water velocities.In addition,due to the large particle size of n-dodecane-rhamnolipid aggregates,the preferential flow was more obvious at high pore-water velocity.Second,one-dimensional flow column experiments under flow interruption condition were conducted to investigate the solubilization of residual n-dodecane by surfactants and their behavioral mechanisms in low-flow or no-flow areas.The results of the comparative study shown that after continuous injection of Triton X-100 solution,the nonionic Triton X-100 entered the residual n-dodecane through partitioning,and thus establishment the source of Triton X-100 in the one-dimensional flow column.During the flow interruption solubilization process,the rate-limited unsaturated solubilization of residual n-dodecane by Triton X-100 baesd on the aggregate formation mechanism promoted the Triton X-100 source re-release Triton X-100 into the aqueous phase,resulting in the Triton X-100 concentration in the solubilization effluent higher than that of in the injected mother liquor.The results of the flow interruption solubilization experiments of Triton X-100 under different concentrations indicated that the application of low concentration surfactant was the key for the generation of flow interruption positive peaks in Triton X-100 breakthrough curves.The application of high concentration Triton X-100 would create a masking effect.Finally,the effect of pore-water velocity on the flow interruption solubilization process illustrated that the increase of pore-water velocity could promote the migration of n-dodecane-Triton X-100 aggregates in saturated porous media and make its migration behavior appear preferential flow,thus leading to the more prominent positive peaks in n-dodecane elution curves and Triton X-100 breakthrough curves under the same duration of flow interruption in onedimensional flow column experiments.Third,static batch redistribution experiments were carried out to investigate the theoretical mechanism of the partition effects of n-dodecane solubilization by nonionic Triton X-100.The result of the static batch redistribution of Triton X-100 into the phosphate buffer solution(PBS)showed that the non-equilibrium partition between the n-dodecane phase and the background PBS drived Triton X-100,which had been partitioned into the n-dodecane,to rapidly redistribute through the oil-water interface into PBS and tended to establish a new partition equilibrium.The Triton X-100 redistributed into the PBS exhibited a certain solubilization effect on n-dodecane.The result of the static batch redistribution of Triton X-100 into the 1.5 CMC Triton X-100 solution indicated that when Triton X-100 reached the partition equilibrium between the n-dodecane phase and Triton X-100 aqueous phase,that is,there was no partition non-equilibrium driving force,the rate-limited solubilization of residual n-dodecane by Triton X-100 based on the aggregate formation mechanism could also drive the redistribution of Triton X-100 into the Triton X-100 solution.In addition,since the rate-limited solubilization of n-dodecane by Triton X-100 was relatively slower than the partition,the redistribution process driven by rate-limited solubilization was therefore significantly slower than that driven by partition non-equilibrium.The result of the static batch redistribution of Triton X-100 into the effluent of the one-dimensional flow column illustrated that the rate-limited unsaturated solubilization based on aggregate formation mechanism also drived Triton X-100 to redistribute into the aqueous phase,and the solubilization concentration of hydrophobic organic compounds in the aqueous solution had a significant effect on the occurrence of the redistribution process of Triton X-100.The result of theoretical mechanism investigation showed that the rate-limited solubilization based on the aggregate formation mechanism allowed the Triton X-100 in aqueous phase to continuously enter the n-dodecane-Triton X-100 aggregates making the Triton X-100 molecules involved in partitioning in aqueous phase decrease.The original partition equilibrium was therefore destroyed and caused the Triton X-100 molecules partitioned into the residual n-dodecane phase to re-enter the aqueous phase through redistribution to establish the partition equilibrium.Finally,the result of the flushing stage under flow interruption condition indicated that the desorption of Triton X-100 from the porous media surface could also increase the concentration of Triton X-100 in effluent,but the contribution was not as obvious as the redistribution effect of Triton X-100 from the residual n-dodecane. |
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