Particle-scale characterization and mass transfer of polycyclic aromatic hydrocarbons from contaminated soil to organic sorbent

This dissertation focuses on the fate and the movement of polycyclic aromatic hydrocarbons (PAHs) from contaminated soil. The main objectives of this study are to understand the relationship between PAH association with contaminated soil and its availability, and to provide fundamental understandings of mass transfer phenomena of PAHs between the soil or sediment and its surroundings in the aspect of in-situ stabilization of the contaminant by adding sorbent. As a case study, a PAH-contaminated industrial site soil was studied. Various carbonaceous materials including coal, coke, pitch, and tar decanter sludge were identified and most of the PAHs were found to be associated with the polymeric matrix of tar sludge or hard pitch as discrete particles, or as coatings on soil mineral particles, or as complex aggregates. The PAH availability from these particles was very low due to the hindered diffusive release with very small apparent diffusivities. Significant concentrations of PAHs were observed in the interior of solid tar aggregates. The release of PAHs from the interior of such particles would require diffusion over a substantial distance. These findings explain the results from three years of phytoremediation of the site soil, for which no significant changes in the total PAH concentrations were observed in the test plot samples. Sorption isotherms and kinetics were studied for phenantherene and pyrene with organic model sorbents: polyoxymethylene (POM), coke, and activated carbon (AC). These findings were combined with the direct observation of the diffusion of phenanthrene and pyrene using microprobe laser-desorption laser-ionization mass spectroscopy (muL2MS). POM showed reasonable agreement between the independent muL2MS-measurements and the predicted intraparticle concentration profiles from kinetic batch experiments and a polymer diffusion model. For coke and AC, the muL 2MS-measurements showed faster radial diffusion into the particle interior than predicted from diffusion models. A numerical model based on the intraparticle diffusion was employed to simulate long-term effects of sorbent amendment on changes in aqueous concentration and mass transfer between different soil domains. The model could reproduce the laboratory scale experiments qualitatively. The model was applied to different contamination scenarios and provided sound predictions of the likely long-term changes in the system.