Experimental evidence of incomplete fluorescence quenching of pyrene bound to humic substances: implications for Koc measurements

Abstract

Fluorescence quenching (FQ) is extensively used for quantitative assessment of partition coefficients (K_OC) of polycyclic aromatic hydrocarbons (PAHs) to natural organic materials – humic substances (HS). The presence of bound PAHs with incompletely quenched fluorescence would lead to underestimation of the K_OC values measured by this technique. The goal of this work was to prove the validity of this assumption using an original experimental setup, which implied FQ measurements upon excitation into two distinct vibronically coupled electronic states. Pyrene was used as a fluorescent probe, and aquatic fulvic acid (SRFA) and leonardite humic acid (CHP) were used as the humic materials with low and high binding affinity for pyrene, respectively. Excitation of pyrene into the forbidden (S₀–S₁) and allowed (S₀–S₂) electronic states yielded two pairs of nonidentical FQ curves. This was indicative of incomplete quenching of the bound pyrene, and the divergence of the two FQ curves was much more pronounced for CHP as compared to SRFA. The two component model of fluorescence response formation was proposed to estimate the K_OC values from the data obtained. The resulting pyrene K_OC value for CHP (220 ± 20) g L⁻¹ was a factor 3 higher compared to the K_OC value determined with the use of the Stern–Volmer formalism (68 ± 2) g L⁻¹. At the same time for aquatic FA the difference in FQ curves was almost negligible, which enables the use of the Stern–Volmer formalism for weakly interacting HS and PAHs.