Impact of Sub-Boiling Temperatures on Mass Transfer from Former Manufactured Gas Plant Residuals

Abstract

The dissolution of polycyclic aromatic hydrocarbons (PAHs) from coal tar residuals at former manufactured gas plant (FMGP) sites constitutes a long-term groundwater contamination challenge. The mass transfer of PAHs from the non-aqueous phase liquid (NAPL) to the aqueous phase is often rate-limited, particularly as weathering increases NAPL viscosity and reduces the availability of contaminants for remediation technologies that rely on aqueous-phase treatment, such as in situ chemical oxidation (ISCO). This research investigated the potential of sub-boiling temperatures (from 25 to 80 °C) to enhance the mass transfer of PAHs from FMGP residuals. The investigation commenced with a series of batch experiments conducted in well-mixed aqueous-phase reactors, where the NAPL was held stationary at the base of the vessel to evaluate the impact of sub-boiling temperatures on the physicochemical properties governing PAH mass transfer. Increasing temperature from 25 to 80 °C elevated the effective solubility of target PAHs by up to an order of magnitude, with the most pronounced enhancement observed for higher molecular weight compounds. The overall mass transfer coefficient increased by factors of up to 45. This dual enhancement of both the driving force (solubility) and the rate coefficient yielded calculated maximum mass fluxes that, based on these measured parameters and assuming zero aqueous-phase concentration, could increase by up to 475 times under the experimental condition. The observed enhancement was greater for an actual FMGP residual relative to a synthetic PAH mixture. This disparity was primarily attributed to a substantial, temperature-driven reduction in the FMGP NAPL viscosity by a factor of ~17, which facilitated a transition from intra-NAPL diffusion-limited to water-phase controlled mass transfer. Screening-level simulations, constrained by the experimental data, illustrated that such enhancements could dramatically increase contaminant mass removal. For instance, after 120 days of heating to 80 °C, the remaining mass of naphthalene (NPH) was projected to be only 6% of that after 120 days of heating at 25 °C, with even greater relative removal achievable at higher groundwater velocities. These results provide clear evidence that heating FMGP residuals to sub-boiling temperatures improves NAPL-water mass transfer. The batch-scale findings were extended through physical flow-through chamber experiments, in which an FMGP residual was emplaced in a fixed reservoir and continuously flushed with temperature-controlled water, with periodic introduction of permanganate pulses to further quantify the impact of temperature on mass transfer. In the control experiments without the periodic addition of permanganate in the injection solution, increasing the temperature from 25 to 80 °C elevated the mass transfer coefficient for NPH by approximately one order of magnitude. Elevated temperatures (80 °C) significantly accelerated the dissolution of soluble PAHs from the NAPL, resulting in an early sharp increase and subsequent rapid decline in effluent concentrations, which was assumed to be a result of the increase in the mass transfer driving force (effective solubility) and the rate coefficient. In the 80 °C experiments, preferential depletion of soluble components reduced the NPH mole fraction from 0.317 to 0.034 within 24 pore volumes, leaving a highly viscous and physically altered residual. This physical transformation coincided with a sharp decline in mass transfer rates, providing direct evidence that the rate-limiting step shifted from interfacial mass transfer to intra-NAPL diffusion. In contrast, the role of the intermittent permanganate addition to the injection solution showed no significant effect on the overall PAH depletion at 25 °C. However, at 80 °C, the addition of permanganate yielded a further enhancement to mass transfer beyond that attributable to temperature alone. This additional effect was consistent with physical disturbance of the NAPL-water interface, potentially from gas generation during oxidation, which likely increased the effective interfacial area available for dissolution. To explore the laboratory mechanistic insights under field conditions, a demonstration project was conducted at an FMGP site in Hefei, China, implementing and comparing thermally enhanced ISCO (TECO), persulfate ISCO alone, and in situ thermal treatment (ISTT) within a sheet-piled 1723 m2 area. The thermal system established a stable sub-boiling temperature field (60 ~ 70 °C) within the TECO demonstration parcels. A dedicated investigation was conducted to quantify temperature-dependent mass transfer rates. Five monitoring wells were installed at increasing distances (0.5 to 2.5 m) from an active heating well, creating a thermal gradient with average temperatures ranging from 60 °C at the closest well to 25 °C at the farthest. After purging each well and replacing native groundwater with deionized water, PAH concentration rebound was monitored daily for 14 days under diffusion-controlled conditions, enabling mass transfer rates to be inferred from the concentration recovery data. Application of a radial diffusion model, incorporating NAPL dissolution as a source term, to these data yielded lumped mass transfer coefficients (K), which increased by an average factor of 5.7 between 30 and 60 °C. A paired t-test comparing the increase in K to the concurrent increase in effective diffusion coefficient (D) between 30 and 60 °C, confirmed that the enhancement in mass transfer was statistically greater (p < 0.05) than could be explained by accelerated aqueous-phase diffusion alone, indicating that the primary benefit of heating extended beyond simple transport enhancement in the water phase. Within the TECO demonstration area where the thermally enhanced treatment was implemented, the TECO parcels achieved >99.9% removal of PAHs from soil, with concentrations reduced to below method detection limits and no observed rebound in groundwater. In the standalone ISCO parcel, removal of higher molecular weight PAHs from soil was limited and groundwater concentration rebounded following treatment, despite a higher oxidant dosage. Consistent with the temperature-dependent mass transfer rates quantified along the thermal gradient, the greater depletion of higher molecular weight PAHs in the TECO parcels reflects the enhanced mass transfer achievable under sub-boiling conditions. This research provides evidence that sub-boiling temperatures can substantially increase mass transfer from FMGP residuals, with the greatest benefit realized for higher molecular weight compounds that are otherwise rate-limited at ambient temperatures. The enhancement observed during systematic laboratory investigations and corroborated by the results of the field study provides a mechanistic basis for improving in situ treatment strategies that rely on aqueous-phase delivery. TECO as evaluated in the field demonstration, represents one example of how sub-boiling temperatures can be leveraged to increase contaminant availability for subsequent treatment.