Binding of pyrene to fulvic acid fractions isolated by polarity-based sequential elution

Binding between pyrene and four fulvic acid factions sequentially eluted by 0.01 M HCl (F1), 0.01 M HCl + 20% methanol (F2), 0.01 M HCl + 40% methanol (F3), and 0.01 M HCl + 100% methanol (F4) was measured by fluorescence quenching. The influences of the functional groups of the fractions under different pH conditions on the binding behavior are discussed. The results showed that the binding coefficient (log K(DOC) ) of pyrene for F1 to F4 increased from 4.19 to 5.3. Log K(DOC) was positively correlated with the aromaticity of fulvic acid fractions but negatively correlated with the ratio of (O + N)/C and the ratio of UV absorption at 250 nm and 365 nm (E₂/E₃). As an indicator of polarity around the pyrene molecule, the ratio of intensity of pyrene at 374 nm and 384 nm (I₁/I₃) decreased with the increase of fulvic acid fraction concentrations. All fractions decreased in ability to quench as the pH increased. For a given pH value, the quenching ability followed the order of F1 < F2 < F3 < F4. The differences in the effects of pH on the quenching ability of fulvic acid fractions could be attributed mainly to the conformation of the fulvic acid fractions. Hydrophobic effect and the interactions of π-electrons from the aromatic regions of the fulvic acid and pyrene played dominant roles in the binding process.