Kinetics of volatile organic compound sorption/desorption on clay minerals

摘要

Soils surrounding industrial sites or at locales where industrial chemicals are utilized, fre- quently 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 signifi- cant fractions of VOC's adsorb onto surfaces other than the external mineral surfaces. The fraction of mass sorbed on the mineral surface ranged from 30/100 to 75/100 of the equilibrium sorbed mass.