(257o) Solubility of Polycyclic Aromatic Hydrocarbons in Sub-Critical Water: A Predictive Approach Using EoS/GE Models

In this study, we compare the capabilities of
predictive Equation of State/Gibbs free energy (EoS/G^E) models to
provide an accurate description of the solid-liquid phase equilibrium (SLE) of
solid polycyclic aromatic hydrocarbons (PAHs) in sub-critical water (SBCW).
Polycyclic aromatic hydrocarbons (PAH) are contaminants, which are well
distributed globally, and are capable of causing a variety of toxic effects on
the human body.  Because the solubility of PAH in water is very low at room
temperature, and that it increases dramatically above 100^oC,
subcritical water has been employed as an alternative to liquid organic
solvents for extracting PAHs. Since solubility data for these systems are
limited, reliable thermodynamic models are needed, to estimate PAH solubility
over wide temperature and pressure ranges for development and design of
subcritical water technologies

We have used EoS/G^E models, specifically
the Linear Combination of Vidal and Michelsen (LCVM) mixing rule and the
Modified Huron-Vidal second order mixing rule (MHV2), in conjunction with the
Peng-Robinson Equation of State (PR-EoS), to predict the solubility of PAHs in
SBCW, as a function of temperature. We compared the solubility predictions
obtained from these EoS/G^E models, with the solubility estimated
using UNIFAC activity coefficient models previously reported in the
literature.  Our results indicate that all the EoS/G^E models provide
reasonably good prediction of PAH solubilities, with the PR-LCVM model yielding
the most accurate predictions.  Further, an analysis of the temperature
dependence of the solubility of PAHs in SBCW, showed a significant rise in
solubility over and above a specific temperature range. We analyse the possible
cause for the increased solubility at high temperatures, through an examination
of the thermodynamic driving forces responsible for the solubility of PAHs in
SBCW, by estimating the temperature dependence of the partial molar excess
entropy, enthalpy and Gibbs energy of selected PAHs.      

Fig.1. Deviations between experimental and calculated
solubility for all data points, obtained using the PR-LCVM model.

Fig.2 Partial molar excess Gibbs free energy ( , blue),
enthalpy (, red), and entropy  (, green) of (a) Naphthalene
(b) Anthracene (c) Chrysene, predicted using PR-LCVM (solid), M-UNIFAC
(Dortmund) (dashed) and experimental data (symbols).