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).