Low-concentration Solubilization Behavior Of Surfactants And Its Effect On Chemical Oxidation Of Perchloroethene

In remediation of soil and groundwater contaminated by hydrophobic organic compounds（HOCs）, one of widely applied roles of surfactant is solubilization. Some studies suggested that, in contrast to conceptualized micelle-based mechanism for solubilization of HOCs starting at surfactant concentration higher than critical micelle concentration（CMC）, some surfactants demonstrated solubilization for some HOCs（e.g. hexadecane） at concentrations below CMC. However, the mechanism for this sub-CMC solubilization behavior has not been systematically investigated in prior studies. Besides, in situ chemical oxidation（ISCO） has been a popular method applied to the remediation of soil and groundwater contaminated by HOCs in recent decades, especially for the sites contaminated by chlorinated solvents. It is of importance to better understand the effect of molecular structure of chlorinated hydrocarbons on the oxidation degradation and the feasibility of applying surfactant to ISCO in order to improve the remediation efficiency. In this paper, solubilization of hexadecane by low-concentration surfactant was systematically investigated. Effect of molecular structure of chlorinated hydrocarbons on oxidation degradation by sodium persulfate and effect of biosurfactant dirhamnolipid（diRL） on the oxidation of PCE by sodium persulfate were also investigated.Firstly, solubilizaiton of hexadecane by two surfactants, SDBS and Triton X-100, at concentrations near the critical micelle concentration（CMC） and the related aggregation behavior was investigated in this study. Solubilization was observed at surfactant concentrations lower than CMC, and the apparent solubility of hexadecane increased linearly with surfactant concentration for both surfactants. The capacity of SDBS to solubilize hexadecane is stronger at concentrations below CMC than above CMC. In contrast, Triton X-100 shows no difference. The results of dynamic light scattering（DLS） and cryogenic TEM analysis show aggregate formation at surfactant concentrations lower than CMC. DLS-based size of the aggregates（d） decreases with increasing surfactant concentration. Zeta potential of the SDBS aggregates decreases with increasing SDBS concentration, whereas it increases for Triton X-100. The surface excess（Г） of SDBS calculated based on hexadecane solubility and aggregate size data increases rapidly with increasing bulk concentration, and then asymptotically approaches the maximum surface excess(Г_max). Conversely, there is only a minor increase in Г for Triton X-100. Comparison of Г and d indicates that excess of surfactant molecules at aggregate surface has great impact on surface curvature. The results of this study demonstrate formation of aggregates at surfactant concentrations below CMC for hexadecane solubilization, and indicate the potential of employing low-concentration strategy for surfactant application such as remediation of HOC contaminated sites.Secondly, oxidative dechlorination of five chlorinated hydrocarbons（i.e. PCE,1,1,2-TCA, 1,2-DCA, 1,3-DCP） by sodium persulfate at ambient temperature was investigated, focusing on the effect of molecular structure of chlorinated hydrocarbons. As shown in results, the oxidative reaction of chlorinated hydrocarbon by excess persulfate followed pseudo-first-order rate law, pseudo-first-order reaction rate constant follows kPCE>k1,2-DCP>k1,1,2-TCA>k1,3-DCP>k1,2-DCA. The unsaturated carbon bond, number of chlorine substituents and position, and carbon chain length etc. had great influence on the reaction rate, and the unsaturated carbon bond has the most significant effect. For saturated chlorinated hydrocarbons with same number of chlorine substitute, the molecular structure has influence on the reactivity of alkyl radical intermediates formed during reaction,subsequently influencing the reaction rate. 1,2-DCP or 1,3-DCP had higher reaction rate than 1,2-DCA due to more ractive primary alky radical intermediates formed. For five chlorinated hydrocarbons, chlorine substituents were finally completely transformed to chloride ion.Finally, oxidation of PCE by sodium persulfate in the diRL reaction system was investigated. PBS solution and ethanol solution reaction system were also investigated for comparison. As shown in results, solubilization of PCE by diRL only occurred at diRL concentrations higher than CMC, the apparent solubility of PCE increased linearly with diRL concentration. Ethanol had strong solubilization for PCE, the apparent solubility of PCE increased linearly with ethanol concentration at low ethanol concentration, in contrast, apparent solubility of PCE increased rapidly in non-linear pattern. In PBS, diRL and ethanol reaction system, the oxidative reaction of PCE by persulfate followed pseudo-first-order rate law, and pseudo-first-order reaction rate constant follows kPBS>kdiRL>kethanol. diRL and ethanol had inhibited persulfate oxidizing PCE and had great influence on the reaction scheme. In diRL or ethanol reaction system, PCE was not completely dechlorinated, by forming a certain chlorinated intermediates. It was indicated that diRL or ethanol was not conductive to the combined application with persulfate to the remediation of sites contaminated by HOCs, although both di RL and ethanol can improve the solubility of HOCs in aqueous solution.The paper disclosed the solubilization mechanism of hexadecane by SDBS and Triton X-100 at concentrations below CMC and the relation between molecular structure of chlorinated hydrocarbons and oxidative degradation by sodium persulfate. The feasibility of applying biosurfactant diRL to ISCO using persulfate as oxidant remediating sites contaminated by PCE was also investigated. Results in this paper provided theoretical basis for the economic application of surfactant in the remediation of sites contaminated by HOCs and ISCO degradation of chlorinated hydrocarbons.