Measuring the saturation limit of low-volatility organic compounds in soils: Implications for estimates of dermal absorption

Estimating dermal absorption from contaminated soils typically requires extrapolations from measurements obtained on soils artificially contaminated at much larger concentrations. Such extrapolations should be constrained by the fact that maximum absorption will occur for the largest possible concentration of chemical on the soil without neat chemical being present; i.e., at the soil saturation limit (Ssoil). Saturation limits of two low-volatility model compounds (4-cyanophenol and methyl paraben) were determined on the 38–63 μm sieve fraction of four soils with different fractions of organic carbon (foc = 0.015–0.45) and specific surface areas (σsoil = 4–34 m2 g− 1) using two methods: equilibrium uptake into silicone rubber membranes and differential scanning calorimetry. Except for Pahokee peat, which had the largest foc, a model assuming contributions from both surface adsorption and organic carbon absorption provided excellent predictions of Ssoil. In all soils, the surface saturation concentration of both chemicals was estimated at 2.2 mg m− 2. The saturation concentration of 4-cyanophenol in the soil organic carbon was 1.7-fold higher than methyl paraben, which is consistent with the estimated solubility limits of these two chemicals in octanol.