Use of the ion exchange method for the determination of stability constants of trivalent metal complexes with humic and fulvic acids—Part I: Eu 3+ and Am 3+ complexes in weakly acidic conditions

Abstract

The conditional stability constants for tracer concentrations of Eu(III) and Am(III) with a red earth humic acid (REHA), a red earth fulvic acid (REFA) and a fulvic acid from weathered coal (WFA) were determined at pH 5.2–6.4 (such values are similar to those in non-calcareous soils) in the presence of HAc/NaAc or NaNO 3 by using the cation exchange method. It was found that 1 : 1 complexes were predominately formed in weakly acidic conditions. The total exchangeable proton capacities and the degrees of dissociation of these humic substances were determined by using a potentiometric titration method. The key parameters necessary for the experimental determination of the conditional stability constants of metal ions with humic substances in weakly acidic conditions by using the cation exchange method were discussed. The conditional stability constants of 1 : 1 complexes obtained in this paper were compared with the literature data of Am(III) determined by using the ion exchange method and the solvent extraction method and with the stability constants of 1 : 1 complexes of UO 2 2+ and Th 4+ with the same soil humic substances. These results indicate the great stability of bivalent UO 2 2+, trivalent Eu 3+, Am 3+ and tetravalent Th 4+ complexes with humic and fulvic acids in weakly acidic conditions.