Aqueous Solubility of Binary and Multicomponent, Tarlike PAH Mixtures

Polycyclic aromatic hydrocarbons (PAHs) are common components of many materials, such as petroleum and various types of tars. They are generally present as mixtures, occurring both naturally and as byproducts of fuel processing operations. It is well known that manufactured gas production resulted in subsurface contamination by PAH-containing tars and non-aqueous phase liquids (NAPLs). In an effort to better characterize the phase and partitioning behavior of PAH mixtures, one must consider that PAH mixtures sometimes exist in places where contact with water is to be expected, e.g., the subsurface, and that this might affect the fate and transport of these compounds and the stability of the source material. Hence, the ability to understand PAH mixture dissolution is important for many environmental and commercial applications.

Since tars and NAPLs may evolve to a limited number of components, this experimental study is aimed at understanding the aqueous phase solubility of transitional multicomponent mixtures, not containing the wide range of compounds that characterize authentic tars. We have been interested in determining whether these mixtures partition into water as individual, phase-separated species or following some solution behavior. Solution theory is quite different for liquids than for solids, and this raises the question as to which predictive model might be best.

The measured aqueous solubilities of binary and multicomponent PAH mixtures have shown fair, unexplained variability in this study, falling both above and below predicted values. Nevertheless, recent results show that binary and ternary PAH mixtures trend towards phase-separated dissolution behavior, that is, the components of the mixtures dissolve independently of one another and approach predicted pure species solubility concentrations in the aqueous phase. As more PAHs are added to create tarlike mixtures, the aqueous solubilities of these higher-component mixtures seem to better fit the values predicted by solid-solution theory, though there is still significant failure to predict actual values. These results serve as a preliminary indication of behavior, suggesting that as PAH mixtures become enriched in components to a point at which only a discrete number of compounds exist in the mixture, solubility might be roughly predicted by ideal solid-solution theory or by pure component values. Though there was no reason to initially hypothesize that the multicomponent, tarlike mixtures in this study would follow the liquid-solution theory commonly reported for NAPLs, the behavior observed here is quite different from what one would expect if that model were used. Of course, this is significant for those dealing with PAH mixtures in natural settings.