We have investigated the solubility of Tc0 2 • nH 2 0 in aqueous solution as a function of pH and p c o 2 · The experimental data collected shows that the solubility of Tc0 2 · nH 2 0 increases with increasing pH and hydrocarbonate concentration.By studying the dependence of the solubility on the pH and Pco 2 we have determinated the stoichiometry and stability of the predominant aqueous complexes: TcO(OH) 2 (aq), TcO(OH)J, Tc(0H) 2 C03 and Tc(OH) 3 CO;\ The behaviour of Tc(IV) in natural water systems is discussed in the light of these new data.

The equüibrium constant for the formation of UOjOH^^q) was studied at 25 °C in solutions of different ionic strengths (0.05 to 1.0 M NaClO,) by sohrent extraction using a technique involving competition with complexation by diglycolate anion. The constants are fitted to the following extended Debye-Huckel expression:1.02/1/2 log^H^^) = -5.891 -+0.367/ [UOJOH+llH+1

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The complexation behavior of humic acid towards Eu'"*" ions in 0.1ionic strength (NaClO,) aqueous Solution has been investigated in the pH ränge 5.0 to 6.0 by an ultraiiltration technique.Two classes of coordination sites (logATj = 7.9 ± 0.2 Imol"', log AT, = 6.0 ± 0.11 mol"') are found. The overall complexation capacity increases from 0.78 to 1.14 mmolg"' while the fraction of this capacity due to the strongly complexing sites goes from 33% at pH 5.0 to 82% at pH 6.0.The complexation strength of both classes of sites does not Vary with pH.Radioecological implications to the environmental behavior of trivalent lanthanides and actinides are briefly discussed.