PHYSICOCHEMICAL TRANSPORT PROCESSES AFFECTING THE REMOVAL OF RESIDUALDNAPL BY NONIONIC SURFACTANT SOLUTIONS

Aquifers contaminated with dense nonaqueous phase liquids (DNAPLs) are extremely difficult to remediate with standard pump-and-treat methods . Enhanced remediation methods, such as flushing with cosolvent or sur factant solutions, promise to reduce remediation times but result in c omplex physicochemical systems for which we still lack both fundamenta l understanding and reliable process-based models. The overall objecti ve of this work was to observe and quantify various physicochemical tr ansport processes acting during the removal of a typical residual DNAP L (tetrachloroethylene, PCE) by solutions containing a nonionic surfac tant (Triton X-100). To achieve this goal, we measured the phase behav ior of the water/PCE/Triton X-100 system in batch systems and performe d a set of glass bead column experiments to investigate residual PCE r emoval mechanisms, nonreactive tracer transport, and Triton X-100 tran sport. We observed and quantified removal of residual PCE by a number of processes (dissolution, micellar emulsions/microemulsion transport, macroemulsion transport, and DNAPL mobilization) as a function of the surfactant concentration used to elute the PCE from the porous medium . We concluded that macroemulsion transport was an important process, accounting for up to 30% of total PCE removal-a process which, to date , has not been accounted for in mathematical models of surfactant-enha nced remediation. We also observed that viscous fingering developed du ring elution of surfactant in the idealized one-dimensional column sys tem, suggesting that this phenomenon will also affect the efficiency o f surfactant recovery in field-scale applications.