Polycyclic aromatic hydrocarbon desorption mechanisms from manufactured gas plant site samples
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The objective of this research was to elucidate mechanisms controlling the desorption of Polycyclic Aromatic Hydrocarbon (PAH) from manufactured gas plant (MGP) site samples. Twelve MGP site samples were characterized and found to vary in chemical and physical characteristics. Four-month desorption studies were conducted on each sample. PAH desorption from these samples was found to be biphasic with an initial fast release of the PAHs from the soil to the aqueous phase followed by a slower release of the PAHs to the aqueous phase. The extent and rates of PAH release were variable. The extent of PAH release tended to correlate to the apparent degree of weathering and aromaticity of the organic carbon. However, there were outliers for each trend. For coal gas samples, the rate of PAH release trended with apparent extent of weathering. This suggested that for coal gas samples, time-dependent, diffusion-based mechanisms may be responsible for slow desorbing PAHs. Deuterated PAHs were spiked onto a subset of the MGP samples. Similar four-month desorption patterns of the spiked deuterated PAHs, initially in contact with the sample material for hours, and native PAHs, having been in contact with the sample material for decades, illustrated that a time-independent matrix sorption mechanism limited the release of PAHs from an oil gas sample. This suggested that soot adsorption is possible mechanism that limits the release of PAHs from oil gas samples. Desorption enthalpies were also determined on a similar subset of samples. Desorption enthalpies for the fast releasing fractions tended to be less than 40 KJ/mol, indicating that the fast releasing PAHs were bound by weak van der Waals interactions. The desorption enthalpy for the slow releasing fraction of naphthalene from an oil gas site was consistent with literature values for the surface adsorption of naphthalene to graphite. Overall, this research indicates that both diffusionbased and soot adsorption mechanisms can control the release of PAHs from MGP soils.