Compositional and pH effects on the interfacial tension between complex tar mixtures and aqueous solutions.

Tars at former manufactured gas plants (FMGPs) are a major environmental concern and present a number of challenges to remediators. This experimental study investigates the relationship between composition and tar-water interfacial tension (IFT), a property of primary importance in determining the transport of tar in porous media. Nine field-collected FMGP tars and a commercially available coal tar were characterized by means of fractionation, gas chromatography, Fourier transform infrared (FTIR) spectrometry, and vapor pressure osmometry. The tar-aqueous IFT of the tars, as well as resins and asphaltenes extracted therefrom, were measured over a range of pH. The IFTs were found to be strongly dependent on pH, with the lowest values obtained at high pH. The reduction of IFT at high pH was found to correlate well with the I(C═O) values from the FTIR analysis, which provide an indication of the relative amount of carbonyl groups present. Reductions of IFT at low pH were also observed and found to correlate well with the extractable base concentration. The aromaticity and asphaltene average molar mass are also correlated with IFT reductions at both low and high pH, suggestive of compositional patterns related to the tar source material.