Technetium! Hydrosphere/Geosphere jRedox Potential/SorptionSummaty The various pathways of Tc into the hydrosphere and the geosphere are summarized and the chemistry of Tc in natural waters is discussed. Sorption experiments with 5 selected sediments and 5 corresponding groundwaters of different salinity are described. Analytical data of the sediments and the groundwaters are presented and evaluated with respect to their influence on the chemistry of Tc. Sorption and desorption experiments are performed under aerobic and under anaerobic conditions (inert gas box).At higher redox potentials Eh (aerobic conditions) where TcOi is the stable species of Tc very low sorption ratios of about 0.1 to 0.3 ml/g are measured, the lower values being observed at higher salinity. The sorption of TcO, is reversible and explained by physical adsorption at the surface of the grains of the sediments.At low Eh (strongly anaerobic conditions) where TcO(OH)3 or T^, • 2 H,0, respectively, is the stable species of Tc high sorption ratios are measured (of the order of 10' ml/g at initial concentrations of 10"' mol/1 of TcO^) which do notdepend markedly on salinity, but at higher salinity the final sorption values are attained faster. Now, sorption is npt reversible and the irreversibleprocessistheformationofTcO(OH), orTcO, -21150, respectively, foUowed by strong sorption of this species. When Eh is increased, the sorption ratios drop rather sharply at 170±60mV.In all experiments only TcO, is found in Solution and no colloidal forms of Tc are detected.

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