THE SORPTION OF PB-210, BI-210 AND 210PO ON MONTMORILLONITE - A STUDY WITH EMPHASIS ON REVERSIBILITY ASPECTS AND ON THE EFFECT OF THE RADIOACTIVE DECAY OF ADSORBED NUCLIDES

The influence of the ionic strength and of pH on the adsorption/desorption processes of Pb, Bi and Po on montmorillonite has been investigated. For Pb, a strong dependence of the adsorption and desorption processes on the ionic strength was observed at pH < 7, whereas, at higher pH values, this dependence totally disappears. The ionic medium had no influence on the sorption/desorption of both Bi and Po. For all of these nuclides, large distribution ratios (R(d)) are measured. They range from 10(3) to 10(5) ml.g-1 for Pb and from 10(4) to 10(7) ml.g-1 for Bi-210 and Po-210. When adsorption and desorption coefficients are compared, an agreement is found for both adsorption/desorption R(d)'s of Pb, whereas, for Bi and Po adsorption R(d)'s were several orders of magnitude lower than those obtained for desorption. The chemical activities of free Bi and Po in the liquid phase are limited by the formation of Bi and Po-colloids prior to the sorption step. This fact could explain the differences in the R(d) values. While the adsorption of Pb was reversible, only very small amounts of Bi and Po could be desorbed from the montmorillonite (quasi-irreversible adsorption). The radioactive decay of adsorbed Pb-210 to Bi-210, which in turn decays to Po-210, can lead to significant changes in the desorption behaviour of the daughter nuclides. Whereas the sorption is nearly irreversible if Bi and Po adsorb on montmorillonite from the aqueous phase, they desorb more easily if they are generated by the radioactive decay of adsorbed Pb-210. The difference in the distribution coefficients Rd is approximately one order of magnitude in the case of Po, and more than 2 orders of magnitude in the case of Bi.