Testing a surface tension-based model to predict the salting out of polycyclic aromatic hydrocarbons in model environmental solutions

Molar-based Setschenow constants (Ks) for six alkali and alkaline earth metal-based inorganic salts were determined at 20°C to evaluate their influence on the solubilities, and thus the aqueous activity coefficients, of three polycyclic aromatic hydrocarbons (PAHs). The six salts tested exhibited a wide range of Ks values, varying from 0.105±0.009 M−1 (for NaClO4 and pyrene) to 1.29±0.17 M−1 (for K2SO4 and perylene). In general, salting out effects with these electrolytes were observed in the order Ca2+>Na+>K+ and SO42−>Cl−>ClO4−, consistent with previous reports. However, the expected salting out trend of perylene > pyrene > naphthalene was only observed with K2SO4. In CaCl2 solutions, the Ks value of pyrene was significantly lower than that of naphthalene. For NaCl, KCl and NaClO4, pyrene Ks values were found to be lower than, but not significantly different from, those of naphthalene. Setschenow constants for all six salts were predicted using a semi-empirical, thermodynamically-based equation that relates the standard free energy change associated with transferring solutes from water to a salt solution to the difference in surface tensions between the two solutions. With this equation, predicted Ks values were in reasonable agreement with observed Ks values (generally within±50%). Lack of better agreement between predicted and observed values likely reflects the inability of the simple surface tension model to account for all interactions among the cations, anions, PAH molecules and water molecules in the respective systems.