Remediation of Former Manufactured Gas Plant Tars Using Alkaline Flushing

The remediation of former manufactured gas plant (FMGP) tars in the subsurface is particularly difficult due to the wetting behavior and high viscosities of these dense non-aqueous liquids (DNAPLs). Alkaline flooding is a technique which has proven effective in improving the recovery of crude oils, which share some characteristics with FMGP tars. For this study, we measured the effect of NaOH solutions on interfacial tension and conducted column experiments to investigate the feasibility of applying this technique to FMGP tars. The pendant drop technique was used to measure interfacial tensions for solutions ranging from 0-1% NaOH. Column experiments were conducted by contaminating sands with tars recovered from a FMGP then flushing the columns with NaOH solutions. A final, 70% v/v ethanol cosolvent flush was conducted to investigate the effectiveness of a two-stage remediation approach. The mass removal of tar, as well as 26 individual PAHs, was measured, along with the aqueous phase mass flux of PAHs after each flushing stage. The interfacial tension was reduced from about 20 mN/m with pure water to a minimum of 0.05 mN/m at a concentration of 0.1% NaOH. In the column experiments, alkaline flushing resulted in a 50% reduction of the residual saturation. Aqueous phase PAH concentrations, however, were similar before and after the alkaline flushing stage. The combination of alkaline and cosolvent flushing resulted in an overall reduction of 95% of the total mass of the 16 EPA PAHs. Final aqueous phase concentrations were reduced significantly for lower molecular weight PAHs, but increased slightly for the higher molecular weight compounds, likely due to their increased mole fraction within the remaining tar. Additional work is being conducted to improve the effectiveness of the alkaline flushing through the use of surfactants and polymers.