Modeling the two stages of surfactant-aided soil washing

This paper provides new insights into modelling the distribution of hydrophobic compounds between soil and water phases in the presence of nonionic surfactant micelles. Experimental measurements were made of various systems comprising a non-ionic surfactant, five soils of different fractional organic carbon contents, and a hydrophobic (disperse) dye. Soil-washing performance was quantified using reciprocal surfactant-soil solubilization coefficients (1/Kd). Two stages of partitioning were identified. In stage 1, the dye concentration increased slightly with increasing surfactant dose until surfactant monomers saturated the bulk solution at the critical micelle concentration (cmc). The washing performance was 1:1 proportional to the surfactant monomer concentration. Most of the surfactant in this stage is sorbed. In stage 2, above the cmc, soil-washing performance increased linearly with increasing available surfactant micelles in the bulk solution. Reciprocal surfactant-soil solubilization coefficients (1/Kd), octanol-water partition coefficients (Kow), fractional organic carbon content of the soil (foc), and surfactant concentration were correlated for each stage in the soil-washing process using two simple equations.