Single‐Phase Microemulsification of a Complex Light‐Nonaqueous‐Phase‐Liquid: Laboratory Evaluation of Several Mixtures of Surfactant/Alcohol Solutions

Abstract

A recent advance in conventional pump‐and‐treat technology for aquifer remediation involves the use of surfactant‐alcohol mixtures that will form a clear, transparent, thermodynamically stable oil‐in‐water microemulsion on contact with a residual non‐aqueousphase‐liquid (NAPL). An initial screening of 86 commercial‐grade surfactants for aqueous solubility resulted in selection of 58 that were further tested in batch experiments to evaluate the capacity to solubilize a complex NAPL waste collected from a Superfund site (Operable Unit OU‐1) at Hill AFB, UT. The selected group of 58 surfactants represented six classes of anionic, nine classes of nonionic, and one class of amphoteric surfactants. Batch studies on NAPL solubilization identified a number of surfactants suitable for use in the field demonstration phase of the project; a further criterion in surfactant selection was that the flushing solution had a viscosity <2 cp. The best surfactants among this group had HLB (hydrophilic‐lipophilic balance) values between 12 and 13, and solubilized 10 to 20 g L −1 of the OU‐1 NAPL when the surfactant concentration was 3%. Column tests using NAPL‐coated glass beads showed that the more efficient surfactants could remove >90% of the NAPL after flushing with <10 pore volumes. Brij 97, an ethoxylated alcohol ether surfactant, showed a high capacity for solubilizing the OU‐1 NAPL. In a column test using contaminated Hill AFB aquifer material, flushing with a mixture of 3% Brij 97 and 2.5% n‐pentanol removed essentially all of the mass of nine target analytes in the NAPL after flushing with <10 pore volumes without mobilizing the NAPL or destabilizing aquifer colloids.