Particle-scale investigation of PAH desorption kinetics and thermodynamicsfrom sediment

Dredged sediment from Milwaukee Harbor showed two primary classes of partic les in the <2 mm size range: a lighter density coal- and wood-derived fract ion with 62% of total PAHs and a heavier-density sand, silt, and clay fract ion containing the remaining 38% of the PAHs. Room-temperature PAH desorpti on kinetic studies on separated sediment fractions revealed slow desorption rates for the coal-derived particles and fast desorption rates for the cla y/silt particles. The effect of temperature on PAH release was measured by thermal program desorption mass spectrometry to investigate the desorption activation energies for PAHs on the different sediment particles, Three act ivated diffusion-based models and an activated first order rate model were used to describe the thermal desorption of PAHs for four molecular weight c lasses. PAH binding with the coal-derived particles was associated with hig h activation energies, typically in the range of 115 -139 kJ/mol. PAHs boun d to the clay/silt material had much lower activation energy, i.e., in the range of 37-41 kJ/mol for molecular weight 202. Among the desorption models tested, a spherical diffusion model with PAHs located like a rind on the o uter 1-3 mum region best described the PAH thermal desorption response for coal-derived particles. This internal PAH distribution pattern on coal-deri ved particles is based on prior direct measurement of PAH locations at the subparticle scale. These studies reveal that heterogeneous particle types i n sediment exhibit much different amounts and binding of PAHs. PAHs associa ted with coal-derived particles aged over several decades in the field appe ar to be far from reaching an equilibrium sorption state due to the extreme ly slow diffusivities through the polymer-like coal matrix. These results p rovide an improved mechanistic perspective for the understanding of PAH mob ility and bioavailability in sediments.