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 (0.04M). The determined interaction constant LogKHS (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)molTcg(-1) HS) and the nature of the reducing surface (Magnetite, Pyrite and Gorleben sand) used for the pertechnetate reduction. The LogKHS value determined by the La-precipitation method is lower than the LogK 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.