(6d) 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.