The effect of bentonite on the interaction of I with PbO

In the long-term disposal of nuclear fuel waste, the radioactive fission product ¹²⁹I requires special attention. This is because of its long half-life (1.7·10⁷ yr), and the fact that it exists in solution as an anion (I⁻ or IO₃⁻) and does not intereact strongly with most geological materials such as clays and rock. Mixtures of bentonite and sand are being evaluated as candidate buffer materials to surround nuclear fuel waste containers in an underground disposal vault in Canada. Since bentonite and sand sorb only minor amounts of I⁻, research is being conducted to identify materials that could be mixed with the buffer material to selectively ‘sorb’ ¹²⁹I⁻, and consequently retard its movement through the buffer after the waste containers are breached. PbO has been proposed as a potential buffer additive, since it has been found to be very effective in removing I⁻ from solution. However, when bentonite is present (≥75 wt% of the total solids) in the PbO/I⁻ system, the amount of I⁻ removed from a solution with an initial I⁻ concentration of 10⁻⁵ mol/l is significantly decreased. On the other hand, kaolinite has little effect. The same phenomenon occurs when bentonite is physically separated from the PbO by a semipermeable membrane. In the PbO/I⁻ system, X-ray diffraction analysis indicates that the Pb phases present are PbO, Pb₃(CO₃)₂(OH)₂, and 7PbO·PbI₂·2H₂O; however, when bentonite is present, 7PbO·PbI₂·2H₂O is not detectable. When bentonite is treated with NaOAc to remove carbonates and then added to the Pb/I⁻ system, the amount of I⁻ released into solution is much less than in the system containing untreated bentonite. Furthermore, the addition of CaCO₃ or a solution of NaHCO₃ to the Pb/I⁻ system causes a release of I⁻ into solution. The data indicate that bentonite affects the stability of 7PbO·PbI₂·2H₂O, shifting the equilibrium between it and Pb₃(CO₃)₂(OH)₂ by increasing the HCO₃⁻ activity in the system (the bentonite contains 0.29% carbonate-C). The results indicate that PbO would not be an effective additive to a buffer material in a nuclear fuel waste disposal vault for the selective removal of ¹²⁹I⁻ from solution.