The lanthanum precipitation method. Part 2: Quantification of the conditional interaction constant between technetium(IV) and humic substances

The interaction between colloidal Tc(IV) species and colloidal Gorleben humic substances (HS) was quantified after application of the La-precipitation method on supernatant solutions obtained under various experimental conditions but at constant ionic strength of the Gorleben groundwater (0.04 M). The determined interaction constant Log KHS (2.3 ± 0.3) remained unchanged over a large range of Tc(IV) and HS concentrations and was independent of the pH of the original supernatant solution (pH range 6–10), Tc(IV)–HS loading (10−3–10−6 mol Tc g−1 HS) and the nature of the reducing surface (Magnetite, Pyrite and Gorleben sand) used for the pertechnetate reduction.The Log KHS value determined by the La-precipitation method is lower than the Log K value obtained from a previous study where the interaction between colloidal Tc(IV) species and Gorleben humic substances was quantified using a modified Schubert approach (2.6 ± 0.3). The La-precipitation method allows to accurately determine the amount of Tc(IV) associated with HS but leads to a (small) overestimation of the free inorganic Tc(IV) species.