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, quam 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 pH. At basic pH, quartz media remained water wet. These results were corroborated by capillary pressure versus saturation experiments, in which spontaneous imbibition of water-indicative of a water-wet system-occurred only at basic pH. In addition, interfacial films formed at acidic to neutral pH, but not at basic pH, and whenever films were present, coal tar was observed-to strongly adhere to quartz. The effects observed in this study are postulated to occur due to the presence in coal tar of asphaltenes, compounds that are considered responsible for the pH-dependence of interfacial properties and formation of semisolid interfacial films in crude oil-water-rock systems.