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

Abstract 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  K HS (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  K HS 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.