Bimodal normal distribution model for binding of trivalent europium by soil fulvic acid

Complexation of trivalent europium by a well characterized soil fulvic acid was investigated by equilibrium dialysis titration method ([FA] = 24 mg · dm^-3, [Eu](t) = 9.0 · 10^-7-3.5 · 10^-6 mol · dm^-3, pH 6.0). The concentrations of the fulvate bound and free eruption were measured radiometrically using ¹⁵²Eu as a tracer. The experimental results were evaluated using a bimodal normal distribution model and a plot with two overlapping normal distributions was obtained which gave two mean log K (μ(I) = 7.0, σ(I) = 1.1; μ(II) = 4.9, σ(II) = 0.5) values for binding of europium by two classes of sites in fulvic acid. A multiligand model was assumed for fulvic acid, based upon the Eu(III) luminescence studies (⁷F₀ → ⁵D₀ excitation) on many organic acids as model ligands.