Mechanistic Characterization of Adsorption and Slow Desorption of Phenanthrene Aged in Soils

Abstract

Long-term adsorption of phenanthrene to soils was characterized in a silt-loam (LHS), a sandy soil (SBS), and a podzolized soil (CNS) by use of the Polanyi-Manes model, a Langmuir-type model, and a black carbon-water distribution coefficient (K(BC)) at a relative aqueous concentration (C(e)/S(w)) of 0.002-0.32. Aqueous desorption kinetic tests and temperature-programmed desorption (TPD) were also used to evaluate phenanthrene diffusivities and desorption activation energies. Adsorption contribution in soils was 48-70% after 30 days and 64-95% after 270 days. Significant increases in adsorption capacity with aging suggest that accessibility of phenanthrene to fractions of SBS soil matrix was controlled by sorptive diffusion at narrow meso- and micropore constrictions. Similar trends were not significant for LHS silt-loam or CNS podzol. Analysis of TPD profiles reveal desorption activation energies of 35-53 kJ/mol and diffusivities of 1.6 x 10(-7-)9.7 x 10(-8) cm2/s. TPD tests also indicate that the fraction of phenanthrene mass not diffusing from soils was located within micropores and narrow width mesopores with a corresponding volume of 1.83 x 10(-5-)6.37 x 10(-5) cm3/g. These values were consistent with the modeled adsorption contributions, thus demonstrating the need for such complimentary analytical approach in the risk assessment of organic contaminants.