Laboratory evaluation of the use of surfactants for ground water remediation and the potential for recycling them

The primary goal of this study was to develop a suitable experimental and laboratory methodology for investigating the efficiency of surfactants in removing hydrophobic contaminants from sand and the efficient recovery of used surfactant solutions for reuse. One of the goals of this study was to test the use of a hydrophile-lipophile balance (HLB) number as a suitable selection criterion. We used toluene as a model contaminant, Ottawa sand as a model porous medium, and six nontoxic, water-soluble anionic or nonionic surfactants with HLB numbers ranging from 12 to greater than 25. Four of the six surfactants were judged to be less suitable on the basis of their properties (less suitable HLB and surface tension), and on batch experiments using separatory funnel, shaker table, and centrifugatioin methods. The two most suitable surfactants were trideceth-19-carboxylic acid (TDCA), an anionic surfactant, and polyoxypropylene-polyoxyethylene block copolymer (POP-POE), a nonionic surfactant. These two surfactants were then used for sand column leach experiments, and two of the less suitable surfactants were also investigated to determine if HLB number and batch experiments satisfactorily predicted leaching effectiveness. In sand column experiments, an aqueous surfactant solution containing a mixture of 1% anionic and 1% nonionic surfactant in a 1.6% NaOH solution was the most effective in removing the toluene. Removal efficiency of alkylaryl ethoxylate carboxylic acid (AECA), which has an HLB number of 19, was only slightly less than for the chemically similar TDCA, but that of dioctyl sulfosuccinate (DOSS) with an HLB greater than 25 was much less effective. These results indicate that HLB number alone is not a perfect indicator of surfactant effectiveness for leaching hydrophobic contaminants because the chemical structure of the surfactant is also important. In the counter-current solvent extraction experiments designed to allow reuse of surfactants, the anionic surfactant solution (2% TDCA) was the most effectively recycled.