Solubilization of volatile organic compounds in edible nonionic surfactants
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Volatile Organic Compounds (VOCs) are major subsurface contaminants and exist as Non-Aqueous Phase Liquids (NAPLs) in the subsurface. Pump-and-treat remediation is the most common technology used to remediate contaminated groundwater. However, experience has revealed that the traditional pump-and-treat remediation is impractical for treating NAPLs in the subsurface, especially for dense NAPLs (DNAPLs). The difficult desorption of highly hydrophobic compounds and the inability to dissolve residual saturation are the major limiting factors for traditional pump-and-treat remediation. Different chemicals have been used to enhance traditional pump-and-treat remediation. Surfactant enhanced subsurface remediation is identified as a promising technology. In situ Surfactant-Enhanced Subsurface Remediation (SESR) was developed to improve removal efficiency by surfactant solubilization (formation of micelles) and mobilization (reduction in the interfacial tension between the NAPLs and groundwater). The solubilization efficiency of VOCs in an edible surfactant system is very important for the implementation of SESR. The micelle-water partition coefficient (Km) is one of the key parameters to describe the solubilization efficiency of insoluble or sparingly soluble organic compounds in the micelles. A headspace auto-sampler and gas chromatography system was used to test the solubilization of benzene, toluene and TCE in edible nonionic surfactants (Tween 20 and Tween 80 which are mono-fatty acid esters of polyoxyethylene sorbitan) by a modification of the EPICS (Equilibrium Partitioning In Closed Systems) method. Benzene and toluene were used to represent light NAPLS (LNAPLs) and TCE was chosen to represent DNAPL. Preliminary tests were conducted to determine the equilibration time required for the partitioning of selected VOCs among the aqueous, micellar, and gaseous phases. Different surfactant concentrations (Tween 20 at concentrations of 330 mg/L, 3300 mg/L, and 13200 mg/L, and Tween 80 at concentrations of 749 mg/L and 3210 mg/L) were used to investigate the solubilization. The fraction of VOCs partitioned into micelle, fm, was proportional to the surfactant concentration. The micelle-water partition coefficient, Km, for benzene was 1823.4±8.5 in Tween 20 solutions and 2650.4±36.6 in Tween 80 solutions. Km for toluene was 5755.4±21.8 in Tween 20 solutions and 8780.4±14.1 in Tween 80 solutions. Km for TCE was 4710.5±6.7 in Tween 20 solutions and 6634.0±33.0 in Tween 80 solutions. Experimental results show that Tween 80 micelles have more solubilizing capacity than Tween 20 micelles for benzene, toluene and TCE. Results also show that Km increased as the hydrophobicity of the contaminant increased (benzene, TCE, toluene). The effect of temperature on solubilization was studied for these three typical VOCs in 13200 mg/L Tween 20 solution and 3210 mg/L Tween 80 solution. 5°C, 10°C, 15°C, and 20°C were chosen to represent the subsurface temperature range. Experimental results show that more VOCs partitioned into the headspace from the aqueous phase with an increase of temperature. It is also shown that there is little change of fm and only a slight increase of Km with temperature. It is likely this is because the selected temperature range was much lower than the cloud point (the temperature at which micellar solution appears turbid), and therefore the enlargement of micelle size caused by an increase in temperature was not apparent. The effect of ionic strength on the solubilization of benzene, toluene and TCE in 13200 mg/L Tween 20 solution and 3210 mg/L Tween 80 solution was studied in 0.1M, 0.3M, 0.5M and 0.64M of NaCl solutions. A concentration of 0.64M of NaCl was chosen to simulate seawater. The temperature for the ionic strength effect tests was controlled at 20°C. Experimental results show that more VOCs partitioned into the headspace and micellar phase from the aqueous phase with the increase of ionic strength. It is also shown that fm and Km increased with ionic strength. It is likely this is because the addition of electrolytes to nonionic surfactant solutions (Tween 20 and Tween 80) causes an increase in the aggregation number of the micelles.