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

Abstract

Molar-based Setschenow constants ( K s) 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 K s values, varying from 0.105±0.009 M −1 (for NaClO 4 and pyrene) to 1.29±0.17 M −1 (for K 2SO 4 and perylene). In general, salting out effects with these electrolytes were observed in the order Ca 2+>Na +>K + and SO 4 2−>Cl −>ClO 4 −, consistent with previous reports. However, the expected salting out trend of perylene > pyrene > naphthalene was only observed with K 2SO 4. In CaCl 2 solutions, the K s value of pyrene was significantly lower than that of naphthalene. For NaCl, KCl and NaClO 4, pyrene K s 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 K s values were in reasonable agreement with observed K s 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.