Sorption and diffusion of benzene, toluene, and the xylenes in unsaturated soil

Widespread use and transport of volatile organic compounds (VOCS) has resulted in contamination of soils and groundwater worldwide. Government regulation and public demand have mandated the cleanup of existing contaminated sites. Contamination of soils and groundwater by benzene, toluene, and the xylenes (BTX) is common due to their frequent occurrence in petroleum products and use as solvents. This research focused on the sorption and diffusive transport of BTX in unsaturated soils. The individual soil processes directly affecting VOC transport were examined as separately as possible to eliminate confounding effects from the different soil processes.; Batch solid-liquid isotherms were carried out to examine the sorption behavior, sorption mechanisms, and interactions between binary solute systems of BTX sorbing to soil. Results indicated that BTX sorbed predominately through two mechanisms: methyl group interaction and hydrogen bonding between the aromatic electrons and soil-bound hydrogen bonding functional groups. Formation of BTX dimers enhanced the sorption in binary solute isotherms of the weaker sorbing BTX species.; Diffusion through the soil liquid phase as a function of moisture content was examined in four different soils using chloride ion. Existing tortuosity correlations did not accurately predict the liquid-phase diffusion resistance as a function of moisture content. A resistance model analogous to electrical resistances was developed and verified that separately considered the intraparticle and interparticle regions of the soil. This resistance model utilized readily-obtained input parameters and was independent of the average particle size.; An apparatus was constructed to evaluate diffusion of VOCs through unsaturated soils. Breakthrough diffusion fluxes of BTX through the soil at relative saturations (RS) up to 0.60 were dominated by interparticle vapor phase diffusion. A vapor-phase resistance model was developed to describe vapor phase diffusion resistance.; A conceptual and mathematical model describing diffusive transport of VOCs through unsaturated soils was developed. Finite difference methods were used for equation solution. This model reasonably predicted the diffusive breakthrough flux profiles for BTX. The modeling results were sensitive to the interparticle vapor phase tortuosity and the contaminant distribution isotherm used.