The Effect of Na⁺, Mg²⁺, and Ca²⁺ Ions on the Formation of Eu (III) -Humate Complex

Abstract

Formation of Eu (III) -humate at various concentrations of Na⁺, Mg²⁺, and Ca²⁺ in a wide pH range (4<pH<8) was studied by the solvent extraction method using ¹⁵²Eu tracer. Concentrations of Na⁺, Mg²⁺, and Ca²⁺ ranged from 0.020 to 1.0 M, 10^-6 to 10^-3 M, and 10^-6 to 10^-3 M, respectively. The dependence of the stability on pH and concentrations of Na⁺, Mg²⁺, or Ca²⁺ was interpreted by considering the polyelectrolyte nature of humic acid. Concentration ratios of Eu (III) -humate complex to free Eu (III) ion in fresh water and seawater was estimated based on the relations between the stability of Eu (III) -humate complex and concentrations of Na⁺, Mg²⁺, or Ca²⁺. Speciation calculations of Eu (III) species dissolved in fresh water and seawater were conducted based on the relations. As a result, it was suggested that humate complex could be more important species of Eu (III) dissolved in fresh water compared with inorganic complexes such as carbonate and hydroxide. In seawater, however, the carbonate complex can be more important species, since Mg²⁺ and Ca²⁺ in seawater at a 10^-2 M level greatly inhibit the formation of the humate complex of Eu (III).