| Hydrophobic organic compounds (HOCs), such as polycyclic aromatic hydrocarbons (PAHs), organ chlorine pesticides and petroleum hydrocarbon, are one of the most difficult contaminants for the remediation of organic polluted soil. Due to the high toxicity, low water solubility and anti-biological availability, this kind of contaminants often show strong tendency to be sorbed onto soil, and it is difficult to remove them through the physical or biological approaches. Surfactants can increase the solubility and mobility of HOCs, and make them easy to be separated from soil by leaching. For these reasons, Surfactant-enhanced remediation (SER) has become a promising technology for the removal of HOCs.Technique of traditional SER has several merits:short implementation period and high treating efficiency. However, because of the difficulty of the separation of surfactant and solubilized contaminants, the repeated use of surfactant is limited and cost is the major constraint of application and promotion of SER. In addition, the residual washing/flushing agent caused by incomplete treatment will also lead to secondary pollution to the environment.The amphiphilic switchable surfactants contain certain groups that can in response to the external stimuli, thus the molecular structure and solubility of switchable surfactant can be controlled through changing the stimuli, and the surface activities will be converted as well. In this study, a novel principle based on the switchable surfactant named Reversible surfactant-enhanced remediation (RSER) was proposed. The goal of RSER is to solve the key scientific problem of the separation of surfactant and organic contaminants using the solubilization-release cycle of switchable surfactant.Azobenzene modified photoresponsive switchable surfactant can generate cis-/trans-isomerization under the irradiation of UV/visible light without a specific place or the addition of a third substance, therefore, this kind of switchable surfactant has the potential to be an ideal choice of RSER. In the dissertation, the photochemical behavior and relevant surface chemical properties of a photoresponsive surfactant 4-butylazobenzene-4’-(oxyethyl)trimethylammonium bromide (AZTMA) were studied. The solubilization capacity of three typical PAHs (pyrene, phenanthrene, and acenaphthene) by AZTMA and AZTMA-Tween80 mixed system were investigated in detail. For the purpose of optimizing the solubilization system and improving the remediation efficiency, the absorption behavior of AZTMA and its cationic-nonionic surfactant system on bentonite were studied as well. At last, the remediation efficiency of PAHs contaminated soil by mixed AZTMA-Tween80 solutions was tested. The objectives of our work are trying to meliorating the traditional SER technology by switchable, and verify the feasibility of RSER. The innovative research results are as follows:(1) The photoresponsive switchable surfactant AZTMA was synthesized by the by the methods of Williamson ether-forming reaction and amination reaction. The results of reversible control experiments showed that the irradiation of UV (365 nm) and visible light (≥420 nm) could cause the photo-isomerization of AZTMA. The CMC values of trans- (activity state) and cis- (inactivity state) AZTMA were 2.0 and 5.1 mmol/L, respectively; and the mean diameters trans-/cis- AZTMA were 2.4 and 1.7 nm, respectively, indicated that the surface activity of AZTMA changed dramatically before and after light irradiation. By calculating, it could be found that the ratio of interface Gibbs free energy and minimum radius of hydration (-Gad0/Amin) was lower than that of SDS, indicating that the biological toxicity of was acceptable, which could meet the requirements of SER technology and had the application potential in RSER.(2) Trans-AZTMA showed strong solubilization capacity on pyrene, phenanthrene and acenaphthene, the degrees of water solubility enhancement of the PAHs by AZTMA follow the order pyrene> phenanthrene> acenaphthene. Also, the values micelle-phase/aqueous-phase partition coefficient (Kmc) mirrored the octanol/water distribution coefficients (Kow) of the PAHs and excellent linearity was obtained. After irradiated by visble light, AZTMA would lost its solubilization ability, the molar solubilization ratio (MSR) of cis-AZTMA was reduced and more than 60% of PAHs could be released from the micelles.(3) Upon non-ideal mixing, AZTMA formed mixed micelles with a conventional nonionic surfactant Tween80, which displayed a synergistic effect on the solubilization of PAHs, and the CMC of mixed surfactant solution was lower than that of single. The synergistic solubilization at AZTMA:Tween80 = 2:8 proved to be optimal. Experimental results have also demonstrated that, after irradiation with UV light, PAHs could be released from the micelles, the greater hydrophobic of PAHs, the higher release efficiency achieved. In summary, the cationic-nonionic mixed surfactant system of AZTMA still retained the photochemical reversible characteristics.(4) Typical clay mineral bentonite showed strong sorption capacity on AZTMA. The main sorption mechanism of AZTMA on bentonite was cation exchange and it was based monolaycr sorption. Thermodynamic parameters demonstrated that the sorption was endothermic and spontaneous, and the confusion of the system increased the sorption of AZTMA to the layer of montmorillonite in bentonite. Coexistence cations in the soil could inhibit the sorption of AZTMA, the inhibitory effect fellow:Ca2+>K+>Na+, indicated that the greater hydrated radius of cation, the more obvious the inhibitory effect achieved; while the presence of organic matter in soil could improve the organic phase distribution effect of AZTMA, thus promoted the sorption of AZTMA. Mixed AZTMA-Tween80 sorption experiments proved that the space barrier effect caused by Tween80 could lower the sorption losses of AZTMA, at the same time, due to the competitive sorption, the adsorbed amount of Tween80 would be reduced as well, so that the total adsorbed amount of the mixed system was below that of single AZTMA and Tween80. XRD characterization results showed that AZTMA and its cationic-nonionic mixed system could effectively increase the the bottom layer spacing of montmorillonite in bentonite, in tne range of concentration studied, the d001 value of original soil could be increased from 1.51 to 2.15 nm.(5) Experiments on the removal of PAHs from contaminated soil showed that AZTMA-Tween80 mixed micelles gathered on soil/PAHs and water/PAHs interfaces, this could wash off the PAHs from the soil particles and solubilized into the hydrophobic core effectively. The elution ability of PAHs reached maxium while the mass ratio of AZTMA and Tween80 was 2:3, the efficiency were 50.37%,74.94% and 90.29% for pyrene, phenanthrene and acenaphthene, respectively; and the equilibrium time was about 180 min. In addition, the inorganic salts in soil could further improve the elution efficiency. Repeated experiments verified that AZTMA-Tween80 system can be recycled in the process of soil remediation. However, due to the adsorption loss of surfactants, the recycling efficiency would decline but it began to flatten since the secondary experiment. |
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