Effects of nonaqueous phase liquids on the washing of soil in the presence of nonionic surfactants.

The removal of malathion from soil by surfactant washing was investigated under various physical-chemical states of the malathion. Three distinctive phases (without nonaqueous phase liquids (NAPL), with NAPL, and the transitional zone of NAPL) were found to be important for a better understanding of the washing process. When there is no NAPL in the system, the washing process is less dependent on the surfactant dose if the applied surfactant concentration is above the critical micelle concentration. The existence of a sorption site boundary, which for the determination of different washing mechanisms, was identified. In the presence of NAPL, the washing performance is generally independent of the organic content (f(oc)) of the soils but is dominated by the concentration of the surfactant used, due to the lesser resistance for mass transfer in NAPL. If the formation of NAPL is marginal, a two-stage washing pattern is observed, which has been quantified by the term 'unit extraction'. For this two-stage system, a mathematical model was derived based on the observed initial unit extraction and final extraction capacity, which eventually resulted in a practical design equation with the use of primary parameters such as f(oc) and surfactant dose.