Desorption mechanisms of cesium from illite and vermiculite

It is known that cesium ion, Cs+, is strongly sorbed to micaceous minerals. However, the desorption of Cs+ at a trace sorption level with time in the presence of different salt ions is not well understood. In this study, we conducted long-term sorption and desorption experiments of Cs+ with illite and vermiculite at room temperature to study the effects of sorption time and co-existing cations on the desorption. A small amount of Cs+ (50 nM Cs+ spiked with 900 Bq 137Cs) was sorbed to the illite and vermiculite in the presence of 1 mM K+ or Ca2+, or 1 mM K+ and 100 mM Ca2+ over 8 weeks, which was then desorbed in the presence of Prussian blue (PB) nanoparticles over 12 weeks. The PB nanoparticles were used to inhibit the re-sorption of desorbed Cs+. More than 90% of Cs+ was sorbed to the minerals in the presence of Ca2+; meanwhile, only 50–70% of Cs+ was in the presence of K+. For all samples other than the illite with Ca2+ (Ca-illite), more than 80% of Cs+ were desorbed within a few days, and almost all Cs+ was desorbed at the end of the experiment. The large and fast desorption of Cs+ indicated a large part of Cs+ sorbed to these minerals were indeed labile in the presence of a strong sorbent like PB nanoparticles. These desorption trends were hardly influenced by a change of the sorption time. The desorption of Cs+ from the Ca-illite was slow, taking more than one month before 80% desorption for the sample with 1-day sorption, and the desorption amount only reached less than 90%. This slow desorption of Cs+ from the Ca-illite became even slower with the sorption time from one day to two weeks, and only 70% of sorbed Cs+ was desorbed at the end of the experiment for the latter. The mechanisms of Cs+ desorption from the Ca-illite was quantitatively explained by fitting to a pseudo first-order desorption model, suggesting that 30–40% of Cs+ was sorbed to the peripheral region of the interlayer of the Ca-illite and diffused into the interior part. The rest of sorbed Cs+ can be desorbed relatively fast. As this Cs+ was most likely sorbed to frayed edge sites in the Ca-illite, these results suggested that a part of the sorbed Cs+ (70 - 60%) was labile. Thus, the expansion and collapse of the peripheral regions of the interlayers induced by co-existing cations and interlayer migration of Cs+ are important processes constraining the sorption and desorption of Cs+ to/from the micaceous minerals. In addition, compared with the desorption from the pure minerals examined in this study, the desorption of Cs+ from real soils was slower likely due to weathering and/or the formation of aggregates.