Effects of ionic strength and fulvic acid on the adsorption of Tb³⁺ and Eu³⁺ onto clay

High salinity and natural organic matter are both known to facilitate migration of toxic or radioactive metals in geochemical systems, but little is known on their combined effect.
We investigated complexation of Tb³⁺ and Eu³⁺ (as analogues for trivalent actinides) with fulvic acid and their adsorption onto clay in the presence of NaCl, CaCl₂ and MgCl₂ at ionic strengths up to saturation. ¹⁶⁰Tb, ¹⁵²Eu and ¹⁴C were employed as radiotracers, allowing invesitigations at very low concentrations according to probable conditions in far-field scenarios of nuclear waste repositories.
A composite approach (linear additive model) was tested for suitability in the prediction of Kd values for solid-liquid distribution of Tb(III) and Eu(III) in the ternary system of metal ion, clay and fulvic acid based on the constituent binary subsystems. The model was found suitable in reproducing the influence of fulvic acid as a function of ionic strength for bivalent electrolyte cations. In this analysis, it could be shown that high ionic strength does not further enhance the mobilizing potential of humic matter.