Investigating interactions of phenanthrene with dissolved organic matter: Limitations of Stern–Volmer plot

Although linear binding isotherms of hydrophobic organic chemicals (HOCs) with dissolved organic matter (DOM) are widely reported, several studies showed nonlinear HOC–DOM interactions. This study pointed out that fluorescence static quenching modeling (FSQM), which often uses a Stern–Volmer type plot to process the data from fluorescence quenching experiments, is conceptually different from the classic Stern–Volmer equation. We also emphasized that although linear Stern–Volmer plots are generally observed in literature, it does not necessarily indicate a linear HOC–DOM interaction. According to both mathematical simulation and laboratory sorption experiments in this study, nonlinear interactions could be concealed by the use of Stern–Volmer plot. Moreover, this study tested the two assumptions for applying FSQM to process binding data. Our results showed that binding coefficient (KDOC) for phenanthrene is neither independent of free solute concentration, nor DOM concentration, which is a critical limitation for using FSQM in a form of Stern–Volmer equation to examine HOC–DOM interactions. Therefore, the true characteristics of HOC–DOM interactions need to be examined using different ways of experimental design and data processing.