The sorption of Tc0 4 and Re0 4 anions on Mg/Al layered double hydroxide (LDH) has been investigated by batch experiments at 25° C in order to test its applicability for the removal of Tc(VII) from radioactive wastewaters. The oxo-anions TcOr and ReOr, which are similar in size and structure, show practically the same sorption behaviour onto LDH. The sorption of TcOr onto LDH is found to be a stepwise ion exchange. The main reaction is the exchange of one OH" of the LDH by M0 4 and thus the formation of Mg 6 Al 2 (0H) 17 (M0 4 )(s). At high concentrations ([M0 4 ]o/[LDH] 0 -1), two M0 4 ions are sorbed per LDH unit and Mg 6 Al 2 (OH) 16 (MO") 2 (s) is formed additionally. The equilibrium constants for the ion exchange reactions Mg 6 Al 2 (OH) 18 (s) + MO; -Mg 6 Al 2 (OH), 7 (M0 4 )(S) + OH'and Mg 6 Al 2 (OH) 1B (s) + 2 M0 4 --Mg 6 Al 2 (0H) 16 (M0 4 ) 2 (s) + 2 OH" are found to be K, = 1.40 ± 0.11 and K 2 = 0.47 ± 0.20, respectively.
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