Efficient Solubilization Performance And Mechanism For Chlorinated Hydrocarbons In Aquifers Of Anionic-nonionic Gemini Surfactants

Chlorinated hydrocarbons are a type of contaminants widely found in aquifers,characterized by high toxicity and low water solubility.The adsorbed and residual phase chlorinated hydrocarbons in the pores of aquifer media are highly immobile and are continuously leached into the groundwater through reverse diffusion.This process results in a long-term contamination source and is difficult to be removed.Surfactant enhanced aquifer remediation(SEAR)technology can effectively remove residual chlorinated hydrocarbons in aquifers through the solubilization of surfactants.Actual site remediation was often screened surfactants directly from commercially available surfactants,but the screened nonionic/anionic surfactants were not produced with SEAR technology in mind,which result in the molecular structure and properties of the surfactant are not fully applicable to solubilization chlorinated hydrocarbon contaminants in aquifers.Such as nonionic surfactants polysorbate oleate(Tween80),which form micelles that are basically uncharged,resulting in weak electrostatic repulsion between them and the aquifer medium.They were easy to be adsorbed by the medium and lost.Most anionic surfactants,such as sodium dodecyl sulfate(SDS),have a high Krafft point.They were easy to be precipitated and lost in the low-temperature and high-salt environments of the aquifer.Therefore,the development of novel surfactants simultaneously possess high solubilization capacity,low-temperature resistance,salt tolerance,and media adsorption resistance is critical to improving the remediation efficiency of chlorinated hydrocarbons in aquifers.It has been shown that the solubilization properties of surfactant could be effectively improved by linking two single-chain surfactant molecules with spacer groups to form the Gemini surfactant,the ethylene oxide group(EO)had lowtemperature resistance,the anionic sulfonate group(-SO3-)had better salt tolerance and resistance to media adsorption,the ester group and straight-chain alkyl group had better biodegradability.Accordingly,in this thesis,two surfactant molecules with nonionic hydrophilic EO groups and hydrophobic straight-chain alkyl tails were linked to a straight-chain alkyl group by esterification to form a double-chain Gemini structure;and-SO3-were symmetrically incorporated into the resulting molecules.Four Gemini surfactants,named GEOnS-m(GEO3S-12,GEO5S-12,GEO7S-12,and GEO7S-10)were synthesized by regulating the polymerization of the EO groups and the number of carbon atoms in the hydrophobic straight-chain alkyl tails.Then the solubilization performance and the structure-activity relationship of GEOnS-m were investigated.The applicability of GEOnS-m in aquifers were evaluated.The process and mechanism of solubilization of chlorinated hydrocarbons by GEOnS-m were investigated to accurately predict the remediation rate and effectiveness of contaminated aquifers.The influence of environmental factors and engineering control parameters on the removal effect of chlorinated hydrocarbons was also determined.The process and effect of solubilization and remediation of chlorinated hydrocarbons contaminated aquifer by GEOnS-m were investigated,and the technical system for solubilization and remediation of chlorinated hydrocarbons contaminated aquifer by GEOnS-m was constructed.The environmental safety and application cost of GEOnS-m were also analyzed.The main conclusions are as follows:(1)Properties and applicability evaluation in aquifer remediation of GEOnS-mThe critical micelle concentration of GEOnS-m can be as low as 2.14×10-3 mM,which is lower than that of SDS and Tween80.The viscosity of GEOnS-m solution is 1.26～1.53 cP,which is similar to that of water.The zeta potential of the GEOnS-m micelles is-88.6～-8.76 mV.GEOnS-m have excellent solubilization capacity for chlorinated hydrocarbons such as tetrachloroethylene(PCE),carbon tetrachloride,trichloroethylene and chlorobenzene.The solubilization capacity of GEOnS-m for PCE could be 2.8～5.0 and 2.2～3.9 times of that of SDS and Tween80,respectively.GEOnSm are suitable for remediation of chlorinated hydrocarbon contaminated aquifers.GEOnS-m solutions had no precipitation under the aquifer temperature(7～22℃)and the high salt environment(ion concentration of 50 mM).While the temperature was lowered from 25℃ to 5℃,the decrease in solubilization capacity of GEOnS-m was 24.8%-31.7%,which was lower than that of SDS(86.6%)and Tween80(40.6%).The solubilization capacity of GEOnS-m for chlorinated hydrocarbons increased as the ion concentration increased.The adsorption loss of GEOnS-m on fine sand media was 25.3%～86.3%and 22.0%～85.7%lower than that of SDS and Tween80,respectively.GEOnS-m had a primary biodegradability higher than 90%and were environmentfriendly surfactants.Each of the four GEOnS-m has distinct advantages.GEO3S-12 has the best resistance to media adsorption while GEO5S-12 has the best salt tolerance.GEO7S-12 has the best low-temperature resistance while GEO7S-10 has the highest primary biodegradability.(2)Solubilization process and mechanism of GEOnS-m for chlorinated hydrocarbonsThe removal mechanism of chlorinated hydrocarbons in aquifers by GEOnS-m was solubilization.The solubilization mass transfer process followed the novel model named "oil diffusion&micelle reformation model".The solubilization transfer coefficient was negatively correlated with the hydrophile-lipophile balance value and critical micelle concentration of GEOnS-m,and positively correlated with the solubility of chlorinated hydrocarbons in water.Nonpolar and polar chlorinated hydrocarbons were mainly solubilized in the hydrophobic cores and "palisade layer" near the outer shell of the GEOnS-m micelles,respectively.The solubilization process of GEOnS-m for chlorinated hydrocarbons could be divided into two stages:the rapid solubilization of micelles and the slow swelling of micelles.And the micelle changes of the four GEOnS-m were not the same.In the rapid solubilization stage of micelles,GEO3S-12 showed vesicles before and after solubilization,GEO7S-10 and GEO7S-12 showed spherical micelles before and after solubilization.The micelle diameter of GEO3S-12,GEO7S-10 and GEO7S-12 gradually increased in the solubilization process.GEO5S-12 showed rod-shaped micelles before solubilization.And the rod-shaped micelles were converted to vesicles and spherical micelles with solubilization.In the slow swelling stage of micelles,the rate of solubilization decreased.The shapes of GEO3S-12,GEO5S-12,GEO7S-10,and GEO7S-12 micelles remained unchanged,and the diameters further increased.(3)The main influencing factors and effectiveness of the solubilization removal of chlorinated hydrocarbons in aquifers by GEOnS-mThe main factors influencing the solubilization removal effect of chlorinated hydrocarbons in aquifers by GEOnS-m include environmental factors(such as twophase contact time,porous media size,residual saturation of contaminant)and engineering parameters(such as injection concentration,injection velocity,and injection mode of GEOnS-m solutions).The smaller the porous media size and the higher the residual saturation of contaminant,the more favorable to the removal of chlorinated hydrocarbons from aquifers.There was an interaction between the three environmental factors,and the relationship between the three factors and the removal effect of chlorinated hydrocarbons was consistent with the quadratic function model.The smaller the injection concentration,the higher the amount of chlorinated hydrocarbons removed by per unit of GEOnS-m.The smaller the injection velocity the better the remediation effect was,but the injection velocity could not be lower than the groundwater velocity.The remediation effect of the intermittent injection mode was better than that of the continuous injection mode.During the process of remediation for the chlorinated hydrocarbons contaminated aquifer by GEOnS-m,he concentration of chlorinated hydrocarbons in the solubilized aqueous phase could increase up to 1,000fold and was ultimately removed as groundwater was pumped out of the aquifer.The solubilization removal efficiency of chlorinated hydrocarbons by GEO3S-12,GEO5S12,GEO7S-12 and GEO7S-10 were 97.11%,96.52%,96.35%and 95.95%,respectively,which were higher than Tween80(90.66%).GEOnS-m can efficiently remediate chlorinated hydrocarbons contaminated aquifers.(4)Environmental safety and application cost of GEOnS-mThe synthetic raw materials and synthetic products of GEOnS-m belong to nontoxic/low-toxic compounds.And the preparation process is green and the reaction conditions are mild.The maximum residual concentration of GEOnS-m in the aquifer was 15.96 mg/L after flushing with 12 PV water,which was below the critical concentration(25 mg/L)for toxicity to soil organisms and aquatic organisms.The above results indicate the environmental safety of GEOnS-m.The application costs of GEO3S-12,GEO5S-12,GEO7S-12 and GEO7S-10 were 380.51,368.53,and 423.79 RMB￥/kgPCE,respectively,which were lower than that of Tween80(685.31 RMB￥/kgPCE)commonly used in SEAR.Therefore,GEOnS-m are suitable for the solubilization remediation of chlorinated hydrocarbons contaminated aquifers.