Kinetics of volatile organic compound sorption/desorption on clay minerals

Abstract Soils surrounding industrial sites or at locales where industrial chemicals are utilized, frequently become contaminated through unsuitable discharge of potentially hazardous organic compounds. The fate and transport of these chemicals must be sufficiently understood to predict detrimental environmental impacts and to develop technically and economically appropriate remedial action to minimize environmental degradation. Improving our understanding of the processes involved in organic pollutant vapor transport is important because the gas phase is often the most mobile, and therefore most potentially hazardous phase. In order to gain a better understanding of the basic kinetic processes affecting soil adsorption/desorption of volatile organic compounds (VOC's) in the vapor phase, we conducted VOC adsorption/desorption experiments using oven-dry clay minerals. Transient, isothermal, gravimetric sorption experiments using volatile organic compounds (VOC's) acetone, benzene and toluene onto pure clay minerals obtained from Ward's Scientific (kaolinite, illite, and Ca-montmorillonite) suggest a biphasic sorption mechanism on these minerals. Experimental results indicate that hydrophobic sorption onto oven-dry clay minerals with negligible soil organic matter is controlled by rates of inter-particle Fickian diffusion mechanisms, intra-particle Fickian diffusion mechanisms, and sorption kinetics. Using an analytical solution to Fick's Second Law where sorption is partitioned into macroscopic and microscopic domains, each with unique diffusion time constants, enables precise prediction of experimental sorption observations. Correlation coefficients of 0.999 were found between the parameter optimized analytical solution and very large sets of experimental data. Macroscopic diffusion coefficients ranged from 10 −2 to 10 −4 cm 2 /min, while microscopic diffusion coefficients ranged from 10 −12 to 10 −17 cm 2 /min. Sorption rates suggest that significant fractions of VOC's adsorb onto surfaces other than the external mineral surfaces. The fraction of mass sorbed on the mineral surface ranged from 30% to 75% of the equilibrium sorbed mass. Equilibrium sorption mass correlates approximately to mineral specific surface area and, similarly, the fraction of `irreversible' sorption appears to be proportional to the specific surface area of the clay mineral. Finally, the experimental results suggest that sorption processes (once the sorbate has diffused to the sorbent) are not instantaneous. Due to the experimental inter-particle length scales and intra-particle diffusion processes, diffusional time constants associated with each process during sorption experiments were generally found to be within two orders of magnitude. Such close agreement would not be expected if particles larger than a few micrometers (i.e., aggregates) comprised part of the sorbing soil system. Although the experiments were conducted using oven-dry clays, it is hoped that these results offer an improved conceptual model of clay mineral sorption processes to better explain field observations of VOC persistence in the subsurface.