Influence of Aqueous pH on the Interfacial Properties of Coal Tar

Coal tar is a dense nonaqueous phase liquid (DNAPL) of significant environmental concern due to its toxicity and persistence in the subsurface. The mobility and recoverability of subsurface coal tar is significantly influenced by its interfacial properties. This paper demonstrates the dependence of coal tar interfacial properties, such as interfacial tension and contact angle, on aqueous pH. Coal tar−water interfacial tension, quartz wettability, capillary pressure-fluid saturation relationships, and interfacial film presence or absence were experimentally determined in aqueous solutions of varying pH (3.4−12.4) at constant ionic strength (0.1 M). Interfacial tension varied as much as 25 dynes/cm, dramatically decreasing as pH was increased above 9. Contact angle results indicate that, as for most NAPLs, water wets quartz media under coal tar-advancing conditions over the entire pH range tested. However, unlike most NAPLs, coal tar wets quartz media under coal tar-receding conditions at acidic to neutral ...