The Effect of the Moisture Regime on the Interaction of Fluorene with Porous Media

Movement and persistence of organic molecules in porous media is strongly influenced by their interactions with the solid phase. Understanding these interactions is important for the execution of reliable risk assessments and for proper handling and disposal of toxic organic chemicals. Transport and attenuation models often assume rapid adsorption-desorption equilibration and neglect the role of the ever-changing moisture regime at the top of the vadose zone. Adsorption of the polyaromatic hydrocarbon fluorene (C13H10), both from hexane and from water, on a cattle manure compost and on two soils-Dor (montmorillonitic, 1.9% organic matter [OM]) and Maagan-Michael (kaolinitic, 5.2% OM)-was studied. Adsorption from hexane mimics interactions with surfaces exposed to a gas phase or to an apolar liquid. Desorption was measured after loading the sorbents with fluorene dissolved in hexane, evaporating the solvent, and incubation in the wet state (above saturation of the porous medium), air-dried, or while undergoing wetting-drying cycles. Although good correlation was observed between the adsorption coefficient of fluorene from water and OM content, adsorption from hexane highly correlated with the surface charge density of the sorbent (its cation exchange capacity or its polarity). When added to the sorbents from hexane and then desorbed into water, less fluorene desorbed than predicted by its aqueous adsorption isotherms. Desorption from all sorbents decreased as the duration of incubation at air dryness increased. Thus, fluorene exhibited adsorption-desorption hysteresis, and a particularly strong adsorption-desorption hysteresis was exhibited by fluorene-loaded Maagan-Michael soil after undergoing wetting-drying cycles.