The Sorption of ²¹⁰Pb, ²¹⁰Bi and ²¹⁰Po on Montmorillonite: A Study with Emphasis on Reversibility Aspects and on the Effect of the Radioactive Decay of Adsorbed Nuclides

Abstract

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 &lt;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 (<em>R</em>_d) are measured. They range from 10³ to 10⁵ ml. g^-1 for Pb and from 10⁴ to 10⁷ ml • g^-1 for ²¹⁰Bi and ²¹⁰Po. When adsorption and desorption coefficients are compared, an agreement is found for both adsorption/desorption <em>R</em>_d's of Pb, whereas, for Bi and Po adsorption <em>R</em>_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 <em>R</em>_d values. While the adsorption of Pb was reversible, only very small amounts of Bi and Po could be desorbed from the montmorillonite (quasi-irreuersible adsorption).<br /> The radioactive decay of adsorbed ²¹⁰Pb to ²¹⁰Bi, which in turn decays to ²¹⁰Po, 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 montmoril-lonite from the aqueous phase, they desorb more easily if they are generated by the radioactive decay of adsorbed ²¹⁰Pb. The difference in the distribution coefficients <em>R</em>_d is approximately one order of magnitude in the case of Po, and more than 2 orders of magnitude in the case of Bi.