Determination of diffusion and adsorption coefficients for volatile organics in an organophilic clay - sand - bentonite liner

Abstract

In the design of barriers for containment of petroleum products it is essential to know the conditions for contaminant transport. In this work, a batch test method was used to determine the adsorption coefficients (Kd) of benzene, toluene, and 2-fluorotoluene (a tracer for toluene) on three soils. For Ottawa sand using soil to water ratios of 0.10–0.30 g/mL, Kd values were 2.5–1.2 mL/g (benzene), 11.3–3.6 mL/g (toluene), and 10.9–3.7 mL/g (2-fluorotoluene), respectively. Using organophilic clay at similar soil to water ratios, the Kd values were 40.9–50.0 mL/g (benzene), 154–129 mL/g (toluene), and 157–114 mL/g (2-fluorotoluene), respectively. Kd values for bentonite were 37.6–0.14 mL/g (benzene), 60.3–16.5 mL/g (toluene), and 51.2–33.6 mL/g (2-fluorotoluene) using soil to water ratios in the range 0.01–0.05. In general, for a given mixture, toluene was two to five times more adsorptive than benzene, indicating that hydrophobicity was an important factor in their adsorption. The diffusion coefficients in material comprised of 3% organophilic clay, 12% bentonite, and 85% Ottawa sand ranged from 0.48 × 10–6 to 2.5 × 10–6 cm2/s at 20°C. These values are lower than those measured for natural clay with low organic carbon content.Key words: diffusion, adsorption, volatile organics, organophilic clay, liner materials.