An experimental investigation into the rate-limited solubilization of liquid organic compounds in micellar surfactant solutions.

Abstract

The effectiveness and efficiency of surfactant-enhanced aquifer remediation (SEAR) technologies will be largely controlled by the extent and rates of solubilization mass transfer. This study examines the influence of surfactant and organic liquid properties on the solubilization rates of octane, decane, and dodecane in micellar solutions of purified dodecyl alcohol ethoxylates (with average ethoxylate chain lengths of 8, 12, 17, and 30) and the commercial surfactants Witconol SN-120 $\rm(C\sb{10-12}H\sb{21-25}(OCH\sb2CH\sb2)\sb9OH)$ and Witconol 2722 $\rm(C\sb{18}H\sb{34}O\sb2C\sb6H\sb{10}O\sb4(OCH\sb2CH \sb2)\sb{20}).$ Two types of experimental systems are used to quantify solubilization rates: completely mixed batch reactor and segregated-phase flow reactor. Concentration-time data from the batch reactor experiments reveal that the time to reach equilibrium increases with an increase in ethoxylate chain length on the purified surfactants and with a decrease in solute chain length. Concentration-time data are fit with a linear driving force model to yield effective mass transfer coefficients. Correlations of mass transfer coefficients are developed that suggest that surfactant structure, solute size, and micelle capacities are key factors in predicting solubilization rates. The flow rate-concentration data from the flow reactor were modeled using a two-dimensional simulator, assuming a linear driving force mass transfer term at the interface. Fitted mass transfer coefficients (using a volume concentration driving force) are found to be independent of solute chain length and surfactant ethoxylate chain length (except for Witconol 2722). The mass transfer coefficients from the flow reactor are smaller than those from the batch reactor. The experimental results are used to identify potential rate-limiting mechanisms in each experimental system. The mechanisms of diffusion, micellar dissociation, monomer sorption, desorption of the organic-laden micelle (budding), and collision transfer are considered. Experimental observations suggest that collision transfer and budding are the dominant rate-limiting mechanisms in the batch and flow reactor systems, respectively. It was also shown that flow reactor trends with alkane chain length are similar to rate-limited mass transfer trends exhibited during flow interruption in soil columns, while batch reactor trends with alkane chain length are similar to trends observed during flow variation in soil columns. It is anticipated that these processes will influence the efficiency of SEAR at the field scale.