Assessing desorption resistance of PAHs in dissolved humic substances by membrane-based passive samplers

Binding of hydrophobic organic compounds (HOCs) to humic substances (HS) has significant influence on their fate and bioavailability in an aquatic environment, yet very little is known on the desorption behavior of HOCs in dissolved HS. In this study, triolein-embedded cellulose acetate membranes (TECAMs) were used as passive samplers in dissolved HS-containing solutions to extract the free polycyclic aromatic hydrocarbons (PAHs). The partitioning coefficients (K(OC)) of PAHs in dissolved HS from different sources were determined. Then the rapidly desorbable fractions (f(rd)) of HOCs in dissolved HS were quantified by a quasi-equilibrium model. Results show that the magnitude of K(OC) of PAHs correlated strongly with the O/C atomic ratio and molecular weight of HS. The PAHs with lower K(OW) values desorbed almost reversibly from an aquatic fulvic acid, a sediment humic acid (HA), a lignite HA, and Aldrich HA. However, for strongly hydrophobic chrysene binding to the lignite HA, some extent of desorption resistance was found and attributed to sorption hysteresis as well as steric hindrance. The present study illustrated that the quality of HS and the property of solute may play key roles in determining the desorption reversibility.