Comparison of Naphthalene Diffusion and Nonequilibrium Adsorption-Desorption Experiments

Abstract

Diffusion of hydrophobic organic compounds (HOC) is a key process controlling transport of contaminants in soils. However, the separate effects of sorption and diffusion on net (effective) HOC diffusion are not fully understood. In this study, effective diffusion of naphthalene in five unsaturated soils was evaluated by: (i) naphthalene adsorption-desorption experiments (batch method), (ii) naphthalene effective diffusion experiments (half-cell method), and (iii) trace-gas diffusivity experiments (chamber method). There was no soil type effect on gas diffusivity in repacked unsaturated soil, but a pronounced soil type effect on naphthalene sorption behavior. Varying degree of adsorption nonlinearity (Freundlich n′a) and apparent adsorption-desorption nonsingularity (ω) and ω increased with decreasing n′a were observed. In the half-cell experiments, gas diffusion was the governing naphthalene transport mechanism. Three effective diffusion coefficients were calculated from the half-cell experiments, based on concentration profile from either the whole (Deff) cell, the source (desorption) half-cell (Deff,D), or the recipient (adsorption) half-cell (Deff,A). Generally, the observed Deff decreased with naphthalene-soil contact time, because of aging effects. The Deff, Deff,A, and Deff,D values (half-cell method) could only to some extend be estimated from Freundlich isotherm parameters (batch method). A suggested index of effective diffusion nonsingularity, H = Deff,D/Deff,A, showed that H was correlated with ω and inversely correlated with n′a Thus, the sorption nonlinearity (n′a) was found to provide good indications of degree of nonsingularity in both HOC adsorption-desorption and effective diffusion. The combination of batch and half-cell experiments generally gave useful insight towards understanding and predicting the influence of sorption nonlinearity and nonequilibrium on HOC diffusion.