Effect of humic acid on the pH-dependent adsorption of terbium (III) onto geological materials

Mobilization of actinides by interaction with humic colloids in aquifers is essentially determined by the geochemical conditions. In this study, the pH dependence of the influence of humic acid on metal adsorption on a variety of geological solids (kaolinite, phyllite, diabase, granite, sand) was investigated for Tb(III) as an analogue of trivalent actinides, using ¹⁶⁰Tb as a radiotracer. Humic material was radiolabelled with ¹³¹I to allow experiments at low DOC concentrations, as encountered in subsurface systems in the far-field of a nuclear waste repository. For all solids, a changeover from mobilization to demobilization is observed on acidification. Except for phyllite, the reversal occurs at slightly acidic pH values, and is thus relevant in respect of risk assessments. A composite distribution model was employed to reproduce the changeover on the basis of the underlying constituent processes. For this purpose, humate complexation of Tb(III) and adsorption of humic acid as a function of pH were investigated as well. Although the ternary systems cannot be constructed quantitatively by combining the binary subsystems, the relevant interdependences are adequately described by the composite approach. For a more general discussion in view of the diversity of natural organic colloids, adsorption isotherms of various humic and fulvic acids on sand were compared.