Cesium sorption reversibility and kinetics on illite, montmorillonite, and kaolinite

Abstract

Understanding sorption and desorption processes is essential to predicting the mobility of radionuclides in the environment. We investigate adsorption/desorption of cesium in both binary (Cs+one mineral) and ternary (Cs+two minerals) experiments to study component additivity and sorption reversibility over long time periods (500days). Binary Cs sorption experiments were performed with illite, montmorillonite, and kaolinite in a 5mM NaCl/0.7mM NaHCO3 solution (pH8) and Cs concentration range of 10 to 10M. The binary sorption experiments were followed by batch desorption experiments. The sorption behavior was modeled with the FIT4FD code and the results used to predict desorption behavior. Sorption to montmorillonite and kaolinite was linear over the entire concentration range but sorption to illite was non-linear, indicating the presence of multiple sorption sites. Based on the 14day batch desorption data, cesium sorption appeared irreversible at high surface loadings in the case of illite but reversible at all concentrations for montmorillonite and kaolinite. A novel experimental approach, using a dialysis membrane, was adopted in the ternary experiments, allowing investigation of the effect of a second mineral on Cs desorption from the original mineral. Cs was first sorbed to illite, montmorillonite or kaolinite, then a 3.5-5kDalton Float-A-Lyzer® dialysis bag with 0.3g of illite was introduced to each experiment inducing desorption. Nearly complete Cs desorption from kaolinite and montmorillonite was observed over the experiment, consistent with our equilibrium model, indicating complete Cs desorption from these minerals. Results from the long-term ternary experiments show significantly greater Cs desorption compared to the binary desorption experiments. Approximately ~45% of Cs desorbed from illite. However, our equilibrium model predicted ~65% desorption. Importantly, the data imply that in some cases, slow desorption kinetics rather than permanent fixation may play an important role in apparent irreversible Cs sorption.

Cesium Sorption and Desorption on Glauconite, Bentonite, Zeolite, and Diatomite

“…The nature of these centers is similar in the case of illites and smectites [54,55], but the impact of FES on cesium adsorption on montmorillonite is probably less important. Literature data for this vary: 0.55 µmol/g [50], 0.5 µmol/g [47], 0.462 µmol/g [49], 0.592 µmol/g [48]. Sorption values decrease as follows: zeolite ≥ glauconite > bentonite > diatomite ( Table 5).…”

“…The nature of these centers is similar in the case of illites and smectites [54,55], but the impact of FES on cesium adsorption on montmorillonite is probably less important. Literature data for this vary: 0.55 µmol/g [50], 0.5 µmol/g [47], 0.462 µmol/g [49], 0.592 µmol/g [48]. Considering that in our experiments the content of glauconite was 1 g/L, the FES concentration according to published data is in the range of 5 × 10 −7 M, which is comparable with the used Cs concentration of 10 −6 M. It is worth noting that in acidic conditions glauconite shows a slight dependency of sorption on pH (Figure 4), probably because of the presence of smectitic interlayers which is quite usual for illites and glauconites [35] and the mechanism of this dependency is the same as described above for bentonite.…”

“…Currently, there is no consensus in the literature regarding the mechanism of interaction of Cs with clay minerals. A number of authors provide evidence for the occurrence of exclusively ionic reactions [47][48][49][50][51], some studies indicate the formation of inter-and outer-sphere conplexes of cesium with edge centers and interlayer space [39,52,53]. The spectral methods [52,53] make it possible to detect complexes at high concentrations of cesium, but cannot be used at concentrations of Cs = 10 −6 M. The pH dependence observed in this work can be explained both by the formation of inner-and outer-sphere complexes, and by competition with leached cations, or it could be a consequence of both processes.…”

Evaluation of Ion-Exchange Characteristics of Cesium in Natural Japanese Rocks

“…However, efficient and practical remediation of contamination caused by radioactive substances is difficult. Several reports have examined cesium adsorption/desorption onto zeolite, bentonite, and mica [4][5][6][7][8][9][10][11][12]. The adsorption processes of exchangeable cations on the adsorption site of their minerals are different.…”

“…The adsorption processes of exchangeable cations on the adsorption site of their minerals are different. For example, cesium ions (Cs + ) are adsorbed in the interlayer of lamellar minerals and in the pores of zeolite [8,11,12]. However, the mechanism by which Cs + adsorb onto and desorb from all minerals on the earth has not been fully elucidated.…”