Effects of ionic strength on the binding of phenanthrene and pyrene to humic substances: three-stage variation model

This study compared the effects of ionic strength on the binding constants (Kdoc) of selected polycyclic aromatic hydrocarbons (PAHs) (phenanthrene and pyrene) and a terrestrial humic acid (Leonardite Humic Acid) in different electrolyte solutions (KCl, KBr, MgCl2 and MgSO4). Distinct trends were found in Kdoc variation depending upon the range of ionic strength resulting from added electrolytes. These trends demonstrated similar shapes for all the systems studied, while degree of variation increased with hydrophobicity of the PAHs. Furthermore, different types of electrolytes had different effects on the interactions between humic acid (HA) and the PAHs. These differences were primarily caused by types of cation, not anion. To describe the complicated effects of ionic strength on Kdoc, we developed a three-stage variation model that encompasses increasing and decreasing trends and plateaus in Kdoc associated with ionic strength, as well as the mechanisms behind these trends, including the variation of HA structure configuration, HA aggregation and the salting-out effect. This model illustrated the importance of sufficient experimental data when interpreting the influence of ionic strength on the trends in Kdoc variation.