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

Abstract

Polycyclic aromatic hydrocarbons (PAHs) are among those contaminants referred to as persistent organic pollutants. PAHs are compounds of great environmental concern because of their persistency in the environment, carcinogenicity, and toxicity to humans and biota. This dissertation focuses on the fate and the movement of PAHs from contaminated soil. The two key objectives of this study are to understand the relationship between PAH association with contaminated site soil and its availability, and to provide more 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 organic sorbent. The study includes the characterization of the contamination source, the availability of PAHs, and the mass transfer of the compounds from the source to the surrounding environment and to a sorbent added for remediation purpose. As a case study, a soil from an automobile manufacturing plant site contaminated with PAHs was studied. Various carbonaceous materials including coal, coke, pitch, and tar decanter sludge were identified by petrography and individual particle analyses. 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 from solid tar or pitch with apparent diffusivities of 6x10-15 for phenanthrene, 3x10-15 for pyrene, and 1x10-15 cm2/s for benzo[a]pyrene. 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. Long-term desorption tests and semi-permeable membrane device tests confirmed a very limited availability of PAHs. 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 uptake kinetics were studied for phenantherene and pyrene with three organic model sorbents: polyoxymethylene (POM), coke, and activated carbon (AC). These findings were combined with the direct observation of the long-term diffusion of phenanthrene and pyrene using microprobe laser-desorption laser-ionization mass spectroscopy (µL2MS). POM pellets showed reasonable agreement between the independent µL2MS-measurements and the predicted intraparticle concentration profiles from kinetic batch experiments and a polymer diffusion model. For coke and AC, the 1.tL2MS-measurements showed faster radial diffusion of phenanthrene and pyrene into the particle interior than predicted from diffusion models. For coke, this was accounted by a sorption retarded pore diffusion model with a particle size-dependent partitioning coefficient, and for AC by a branched pore kinetic diffusion model. 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. The model provided mechanistic understanding of the mass-transfer related phenomena that may occur in the application of an in-situ sequestration technique for mixed systems using carbonaceous sorbents.