The effect of a nonionic surfactant on the desorption of naphthalene from soil in a packed column

The effect of a nonionic surfactant (Witconol SN-70) on the desorption of a PAH (naphthalene) from a model soil (1.29% organic content) was investigated to determine the surfactant's effectiveness in for soil remediation. The surfactant sorbs onto the soil in a Langmuir type fashion, with a maximum sorption of around 3 mg/g. The critical micelle concentration occurs near 0.05% surfactant, and the molar solubilization ratio for naphthalene is 0.02. The equilibrium partition coefficient for naphthalene desorbing from the artificially contaminated model soil is 4.2. The addition of surfactant above 0.05% aqueous concentration produces a marked decrease in the apparent partition coefficient, from around 4.0 at 0.05% to around 0.5 at 1.0% surfactant. Experiments were performed in which the contaminated soil (30 to 80 ppm naphthalene) was placed in a packed column and a continuous feed of varying surfactant concentrations flowed through the soil matrix at an interstitial velocity of 1.0 cm/hr. The addition of 1.0% surfactant to the feed reduced the total throughput volume required to wash the soil from over 5 L to less than 1 L. Further experiments indicated that the naphthalene on the solid is in local equilibrium with that in the fluid phase with 0.3% surfactant in the feed for interstitial velocities of 1 cm/hr up to 15 cm/hr. No discernible mass transfer resistances were shown in the absence of surfactant. The breakthrough profile for soil allowed to contact the naphthalene for 66 days was not appreciably different than soil with overnight contact in the presence of 0.5% surfactant feed. Modeling of the system as an equilibrium CSTR assuming no surfactant sorption produced a relatively good fit with the data, which indicates that column behaves as a well mixed reactor (i.e. no concentration gradients), and also supports the hypothesis of resistance free mass transfer from the solid to the fluid. These experiments clearly indicate that Witconol SN-70 is effective in the removal of sorbed PAH from laboratory-contaminated soil of moderate organic content.