Electrochemically Reversible Solubilization Of Typical Polycyclic Aromatic Hydrocarbons By A Redox-Active Surfactant

Surfactant-enhanced remediation (SER) is one of the most efficient, the least time-consuming and a promising technique among the remedy approaches for the contaminated soil and groundwater. Howere, Micelles comprise stable thermodynamic system in which it is difficult to separate the organic pollutant from surfactant solutions and the scale of application is limited. If the uptake by and release from micelles of solubilizate can be reversibly regulated by external stimuli, it would be possible to control the release rate of solubilized contaminants, and which is the available way to settle this problem. In the present dissertation, much reviewing was made on the principles, the current research results and the main problems of surfactant-enhanced remediation. A redox-active surfactant was synthesized, the surface tension and electrochemical behavior of this ferrocene-containing surfactant were studied. The solubilization of polynuclear aromatic hydrocarbons (PAHs) by redox-active surfactant was investigated in detail. At last, the solubilization efficiency of the selected PAHs by mixed surfactant solutions was tested and compared with that in single ones. The objectives of the present dissertation are to prospect novel agents and approaches for SER which is to reduce the concentration of surfactant, the expenditure of technique. The main original conclusions are drawn as following parts:(1) A novel redox-active cationic surfactant (11-Ferrocenylundecyl) trimethyl ammonium Bromide (hereinafter referred to FTMA) was synthesized by the main method of Friedel-Crafts reaction and Amination reaction. The synthesized target compounds were characterized by1H NMR and13C NMR, the purity of FTMA was97%. Surface tensions of FTMA and FTMA+were measured at30℃condition used the platinum plate method. The critical micellar concentration (CMC) values were changed from0.6mmol·L-1to lmmol·L-1when FTMA was oxidized to FTMA+, resulting the surface activity occurred significant changes, the surface tension value was also increased from38.5mN·m-1to55.3mN·m-1at the concentration of the CMC. The electrochemical behavior of FTMA in0.2mol·L-1Li2SO4solution was investigated by cyclic voltammetry methods. Results indicate that the oxidation potential and reduction potential were0.457V and0.4168V respectively, the△EP is40.2mV, and the ratio of Ipa/IPc is1.26. So FTMA possessed well reversible characteristic, which realized reversibility between reduction and oxidation freely. Otherwise, we finished the changes between FTMA and FTMA+by electrochemical method, and the conversion efficiency between the reduced (FTMA) and oxidized forms (FTMA+) was found to be more than90%; Dynamic light scattering measurements were performed on solutions of the reduced, oxidized and re-reduced form of FTMA solution at different concentrations to examine how the micellar size of the surfactant is affected by its redox reaction on the electrode. The mean micellar sizes of FTMA were found to be8.6,2.5and7.8nm for the reduced, oxidized and re-reduced forms, respectively. This indicates that oxidation of the ferrocenyl group caused the micellar size to decrease and its re-reduction allows the size to reture to the initial value.(2) The apparent water solubilities of pyrene, phenanthrene and acenaphthene by redox-active micelles of FTMA were determined. The results suggested that the solubilization capacity of PAHs in FTMA solutions were higher than the other classical cationic surfactants, the water solubility enhancements of pollutants by ferroccenyl surfactant were obvious around CMC. The solubility of the selected PAHs could be ranked as follows:pyrene> pherantherene> acenaphthene. In addition, the apparent water solubilities of each PAH by oxidized FTMA (FTMA+) were also determined. The results indicated that the solubilization capacity of FTMA for the selected PAHs was much higher than those in FTMA+, the values of molar solubilization ratio(MSR) for FTMA+were reduced of85%compared with FTMA. Furthermore, these changes were reversible and electrochemically controlled.(3) By utilizing the redox-responsiveness of the surfactant with ferrocenyl moiety, controllable loading and release of various substances can be conveniently realized. The cumulative release efficiency (R) of PAHs solubilized in ferroceney surfactant micelles by oxidization of FTMA to FTMA+was studied, the values of R decreased as the surfactant concentration increased, and more than50%of the PAHs were released at each surfactant concentration. Indeed, even64%of the acenaphthene could be released after21h at low FTMA concentration. And the cumulative release efficiency of selected PAHs follows the order of acenaphthene> pherantherene> pyrene, which was contrary to the results of water solubility enhancements of PAHs by FTMA. However, more than about20%of the PAHs could not be released within21h, even at the lowest FTMA concentration. It is believed that the unreleased PAHs should stably interact with the thick shells of the FTMA via hydrogen bonding and other interactions.(4) The effects of mixed cationic-nonionic surfactants, FTMA mixed with Tween80on the solubility enhancement of PAHs in the aqueous phase were investigated. The efficiency of solubilization of PAHs in single-, and mixed-surfactant solutions were also compared. The critical micellar concentrations of mixed surfactants were sharply lower than that of sole FTMA and Tween80, and the experimental CMCs are lower than the ideal CMCs at almost all mixed surfactant solution compositions. Mixed FTMA-Tween80at the ratio of2:8exhibited lowest CMC which was0.082mmol·L-1. PAHs are solubilized synergistically in mixed FTMA-Tween80surfactant solutions, especially at low surfactant concentrations. The sequence of synergistic solubilization for different ratios of mixed-surfactant solutions is FTMA:Tween80(2:8)> FTMA:Tween80(1:9)> FTMA: Tween80(3:7)> FTMA:Tween80(5:5)> FTMA:Tween80(7:3). By utilizing the electrochemistry oxidation of the mixed surfactant FTMA-Tween80, controllable loading and release of PAHs can be also conveniently realized. And the cumulative release efficiency of PAHs in mixed surfactants was much larger than those in single FTMA solutions at the same concentration of FTMA. The work presented here demonstrates that mixed cationic-nonionic surfactants would be preferred over single surfactants for solubilization remediation of PAHs which could decrease the surfactant loss and remediation cost, increase the recovery ratio of FTMA.