Complex formation of Np(V) with humic acid and polyacrylic acid

By using ²³⁹Np and/or ²³⁷Np, the complexation of Np(V) with Aldrich humic acid and polyacrylic acid has been studied at the concentration of Np(V) from ∼ 10^-13 to ∼ 10^-4 M by the solvent extraction method using TTA and 1,10-phenanthroline in isoamyl alcohol. By defining the apparent formation constant as β_α = [ML]/([M]C_Rα), where C_R is the total concentration of proton exchanging sites and α is the degree of dissociation, [M] and [ML] respectively denote the concentrations of free and bound NpO₂⁺, log β_α have been obtained at pH 6-7 in 0.1 and 1.0 M NaClO₄. For both humate and polyacrylate, log β_α slightly increased with the degree of dissociation, and slightly decreased with ionic strength. While the variation of Np(V) concentration had no appreciable influence on the log β_α of polyacrylate (ranging from 1.8 to 3.0 depending on pH and ionic strength), log β_α of humate decreased from 4.7-5.3 (depending on pH and ionic strength) at [M] < 10^-9M to 2.8-3.1 at [M] = 10^-5 M, indicating the co-existence of weak and strong binding sites in the humic acid.