8. Sorption Values for Selenium, Molybdenum, and Technetium

Selenium, molybdenum, and technetium belong to groups VIB, VIA, and VIIA, of the periodic table, respectively, and form highly soluble and mobile oxo-anions in aqueous solutions under oxidising and moderately reducing conditions (molybdenum only exists as the hexavalent MoO₄ ²⁻ oxo-anion under all conditions). As uranium fission products, isotopes of these elements are common in radioactive wastes. Based on good evidence for selenate, Se(VI), and on supplemental information and chemical analogies for molybdate, Mo(VI), and pertechnetate, Tc(VII), these elements are preferentially taken up by the sulphate–aluminate minerals (ettringite, monosulphate) in their highest oxidation states, where the oxo-anions can substitute for sulphate ions. Thus, sorption of these ions would be favoured by a high content of sulphate-aluminate minerals and a low aqueous concentration of competing sulphate ions. This is also relevant for selenite oxo-anions, Se(IV), but selenite sorbs additionally on all other mineral phases of importance in HCP. Under strongly reducing conditions, Se(−II) and Tc(IV) become relevant. No reliable information is available for Se(−II), and zero sorption must be assumed. On the other hand, technetium (IV) sorbs strongly onto hydrated cement paste and CSH phases, which is consistent with its hydrolysis behaviour.