Enhanced Remediation Of Polycyclic Aromatic Hydrocarbons Contaminated Soils With Electrochemical Reversible Surfactant

Due to the characteristic structures and properties, the reversible surfactants have a good prospect in the remediation of organic contaminated soils. Studies showed that it may form a new method of Reversible Surfactant Enhanced Remediation(RSER) for organic contaminated soils with switchable surfactants instead of conventional surfactants. So that it becomes possible for the separation between surfactants and contaminants and recycling of surfactants during the repair process. On the basis of reducing the loss of surfactants on soils and increasing remediation efficiency, a cationic electrochemical switchable surfactant (11-Ferrocenylundecyl) trimethylammonium Bromide(FTMA) was selected, that was more extensively research and application.The sorption behavior of FTMA and FTMA-Tween80mixed surfactants on a typical clay mineral-bentonite were investigated, and the mechanism of surfactants to enhance the remediation of PAHs contaminated soils and release of PAHs were also studied. It will provide a theoretical basis of achieving an economic and efficient RSER technology. The main original conclusions are drawn as following parts:(1) Cation exchange is the main sorption mechanism of FTMA on bentonite. The isotherms can be described with Langmuir model, and it is based monolayer sorption. The sorption fits lagergren quasi-second-order kinetics equation, chemical sorption process is the main step of controlling rate. The sorption process is spontaneous and endothermic through thermodynamic parameters calculated. The confusion of the system is increasing during the sorption of FTMA to the surface or layer of bentonite. In addition, some key factors of soil would have different impacts on sorption of FTMA, the adsorbed amount decreases with increasing ionic strength; the sorption is inhibited by coexisting cations, and following order:Ca2+>K+>Na+; the impact of pH is slight and the addition of humic acid (HA) promotes the sorption of FTMA.(2) The electrochemical behavior of different ratio of FTMA-Tween80were investigated by cyclic voltammetry method. Results indicate that the△EP is50.2mV, and the ratio of Ipa/Ipc is1.28, which reveals the reversible reduction-oxidation characteristics of mixed surfactants solution. The sorption of FTMA on bentonite is inhibited with the addition of nonionic surfactant Tween80, while the sorption capacity of Tween80decreases linearly with increasing concentration of FTMA. Since the competitive sorption between two surfactants, the sorption amount of FTMA-Tween80mixed surfactants is far smaller than a single surfactant. It shows combination of cationic and nonionic surfactants can reduce the sorption loss on the soil.(3) XRD diffraction pattern analysis shows that, the diffraction peak d001of bentonite shifts to small angle significantly, and the interlayer spacing of the bottom increase with the loading amounts of FTMA increasing. Sorption state of FTMA within bentonite interlayers exhibit three arrangement models of lateral-monolayer, lateral-bilayer and paraffin-monolayer, respectively. While cationic and nonionic mixed surfactants FTMA-Tween80sorbing onto the bentonite, the interlayer spacing of bentonite increases significantly than single surfactant, and it shows a gradual increasing trend of the layer spacing with the concentration of Tween80increasing, that is consistent with the changes of sorption amounts of mixed surfactants.(4) The distribution coefficient Kd of polycyclic aromatic hydrocarbons in soil is correlated with octanol/water partition coefficient Kow values positively. In water-soil system, cationic and nonionic mixed surfactants can significantly reduce the apparent partition coefficient Kd*of PAHs, and Kd*value decreases with the content of Tween80increasing in the mixed system. Under the same concentration of surfactant, Kd*of polycyclic aromatic hydrocarbons could be ranked as follows:pyrene> pherantherene> acenaphthene.(5) Simulated experiments on the removal of PAHs from contaminated soil show that, the elution for pherantherene and acenaphthene can reach saturated state about150min and4h for pyrene. Through the first-order equation fitting, it shows that the cleaning rate of PAHs with surfactants follows order:acenaphthene> pherantherene> pyrene. The elution ability of PAHs with different ratio of FTMA-Tween80mixed surfactant are higher than single FTMA, and it is greatest when w (FTMA):w (Tween80)=1:3, the elution efficiency are81%,70%and42.5%for acenaphthene, phenanthrene and pyrene, respectively. Utilizing the redox-responsiveness to the mixed surfactant FTMA-Tween80, the results show that the release efficiency of PAHs is about70%, this could be explained that while the micelles oxidized to monomers, the molecular still also has certain solubilization abilities. Meanwhile, the unreleased PAHs should stably interact with the thick shells of the FTMA via hydrogen bonding and other interactions.The release abilities comply with the order acenaphthene> phenanthrene> pyrene, which is associated with the water solubility of PAHs.