Nonideal Binding between Dissolved Humic Acids and Polyaromatic Hydrocarbons

Interactions between hydrophobic organic chemicals (HOCs) and dissolved organic matter (DOM) are of environmental significance due to their influence on mobility and bioavailability of HOCs. The linear dissolution concept has been widely used to describe the interactions between HOCs and DOM, but it may not be correct. To date there is no systematic evaluation of nonideal interactions between HOCs and DOM. Therefore, this study employed a dialysis method to investigate sorption, desorption, and competition of two polyaromatic hydrocarbons (PAHs), phenanthrene (PHE) and pyrene (PYR), by two DOMs at pHs of 4, 7, and 11. Nonlinear interactions between PAHs and DOM and desorption hysteresis were consistently observed. The isotherm nonlinearity factor, n value, increased significantly after the addition of cosolutes, indicating the occupation of specific binding sites by the cosolute molecules. Significant influence of pH on PAHs-DOM interaction was also observed (higher binding coefficients, stronger desorption hysteresis, and increased nonlinearity at lower pH). This study for the first time systematically showed the nonideal binding behavior of PAHs by DOM. A more complete model rather than linear distribution is required to describe the interactions between HOCs and DOM. Conformation changes of DOM molecules were proposed to explain the interactions between HOCs and DOM.