Former manufactured gas plants (FMGPs) operated throughout the U.S. and Eu- rope between the early-1800s and mid-1900s and produced a flammable gas for heating and lighting for commercial and residential purposes. The U.S. Environ- mental Protection Agency (EPA) indicated that there may be as many as 50,000 FMGPs and related sites in the U.S, with the majority of these sites expected to have some level of contamination [217]. The primary contaminant of concern at FMGPs is tar, which was produced as a by-product of the gasification of coal and petroleum, and was frequently released into the environment either intentionally or through leaks in plant infrastructure [128].
FMGP tars possess a number of characteristics that make them among the most challenging of nonaqueous phase liquids (NAPLs) to remediate. They are generally denser than water (i.e., dense NAPLs or DNAPLs) and are usually significantly more viscous than water or other NAPLS, with reported dynamic viscosities ranging from 10 to 500,000 cP [18]. FMGP tars have exhibited the ability to alter the wetting characteristics of porous media, further decreasing 1Reprinted (adapted) with permission from Hauswirth, S. C., et al. (2012)Environmental Science
recovery efficiencies [88, 172]. Additionally, the tars are chemically complex and can compositionally vary greatly from site to site, and within a given site, due to differences in the source materials and processes used in the plant operation, as well as due to environmental alteration after being released into the subsurface [27, 54]. These variations can have significant implications for the physiochemical properties and subsurface behavior of tars, and subsequently to risk assessment and remedial design at contaminated FMGP sites.
A major source of the variation in tar characteristics is a result of differences in the gas production process used at the FMGP. Coal tars were a by-product of coal carbonization. Carburetted water gas (CWG) tars were a by-product of a process that first used coal to produce water-gas and then sprayed oil in the gas stream. Oil-gas tars were a by-product of gas produced from cracking oils. Birak and Miller [18] provide a summary of historic tar data highlighting differences between tars produced by different processes. Briefly, coal tars were considered distinct in that they contained phenolic acids and nitrogenous bases, and lacked paraffinic compounds. Being petroleum based, oil-gas and CWG tars were almost entirely absent of the acids and bases, but contained a small amount of aliphatic compounds (<5%). Early CWG plants used mostly anthracite coal, and the resultant tars were also generally absent of the acids and bases. In later years, bituminous coal was sometimes used and the resulting CWG tars could more closely resemble coal tars. Though much of the recent literature does not distinguish between these tar types, referring to tars at FMGPs as “coal tars,” CWG tars are more likely to be sources of contamination at FMGP sites because they had less commercial value than coal tars and were more likely to be disposed
of on-site [18, 70].
Interfacial tension (IFT) is a property of major importance in determining the behavior of NAPLs in porous media. Unlike most DNAPLs (e.g., chlorinated sol- vents), FMGP tars exhibit IFTs that are strongly pH dependent. This behavior was first reported by Barranco and Dawson [17], who observed a drop in IFT from an average of 23.5 mN/m for pH 3.4–9.1 to 0.6 mN/m at pH 12.4. Zheng and Powers [242] reported a similar finding and suggested that asphaltenes, the fraction of tar that is insoluble in a given short alkane (usually n-pentane or n- heptane), are responsible for the observed IFT behavior. They observed that the IFT of asphaltenes dissolved in toluene decreased with increasing asphaltene concentration. Asphaltenes are composed of a large number of individual species, but generally consist of relatively high-molecular weight polar compounds con- taining aromatic, aliphatic, and heteroatomic groups [8, 64, 107, 132, 199]. The reduction of IFT at high pH is believed to be due to the deprotonation of acidic groups within the asphaltenes, resulting in the formation of surface active ionic compounds [17, 242]. These two studies combined, however, measured IFT as a function of pH for only two FMGP tars and a creosote sample, and therefore provide an incomplete picture of the potential range of behavior for FMGP tars. Hauswirth et al. [75] recently found that the pH dependence of IFT can be exploited for the remediation of tar-contaminated porous media. The use of basic aqueous solutions containing xanthan gum to increase viscosity were capable of removing over 90% of residual CWG tar (sample B1-07 in this study) in column studies [75]. To assess the widespread applicability of such a method to FMGP tars, a better understanding of tar IFT behavior is necessary.
The overall goal of this work is to advance our understanding of the factors that influence the IFT of FMGP tars. The specific objectives of this study are the following: (1) to characterize tars from eight wells at two FMGPs, as well as a commercially-available coal tar using a variety of analytical techniques, (2) to measure the IFT as a function of pH for the tars, as well as synthetic DNAPLs containing asphaltenes and resins extracted from the tars, and (3) to correlate the results of the tar characterization with the pH-dependent IFT behavior of the tars.