The sorption of polycyclic aromatic hydrocarbons by soils in low-methanol/water mixtures

The solubility and sorption of three polycyclic aromatic hydrocarbons (2-methylnaphthalene, phenanthrene, pyrene) by two soils were measured in water and low-volume-fraction (<30%) methanol-water mixtures. A large increase in the solution hase concentration of our analytes was observed as the methanol concentration was increased from 0% to 30% by volume, and was much more pronounced for the more nonpolar compounds. The solubility enhancement of our analytes exhibited a similar semi-logarithmic relationship that appeared to be linear in low co-solvent concentrations, but deviated from linearity as the methanol concentration increased beyond 20% volume fraction. An inverse relationship was observed for hydrophobic organic compound soil partition coefficients as a function of increasing co-solvent. A semi-empirical equation was successfully applied to explain our observations if the requisite parameters needed to calibrate the model were elucidated from low-co-solvent solubility enhancement experiments.