The effect of sub-boiling temperatures on mass transfer from former manufactured gas plant residuals

Abstract

The dissolution of polycyclic aromatic hydrocarbons (PAHs) from coal tar at former manufactured gas plant (FMGP) sites is a long-term threat to groundwater quality. The dissolution rate is often limited by an increase in the viscosity of the non-aqueous phase liquid (NAPL) as the lower molecular weight compounds are depleted over time, and this slow mass transfer prevents the effective application of remediation technologies that rely on NAPL-to-water mass transfer to remove or degrade mass. Increasing subsurface temperatures has the potential to increase mass transfer at FMGP sites by increasing PAH solubility and reducing NAPL viscosity. This study investigated the mass transfer of PAH compounds from a synthetic NAPL mixture and FMGP residual at 25, 50 and 80 °C using well-mixed batch experiments. Effective solubilities increased by up to an order of magnitude and mass transfer rate coefficients increased by up to a factor of 45. Enhancements were greater for higher molecular weight compounds, and for the more complex FMGP NAPL compared to the synthetic mixture due to a more substantial decrease in NAPL viscosity. Simulations using a screening-level model demonstrated the potential for sub-boiling temperature to increase NAPL mass removal at FMGP sites, with increases in concentration up to a factor of seven, and 6 to 87 % of mass remaining after heating to 80 °C for 120 days compared to 25 °C.