Adsorption characteristics of Cs and cation selectivity of todorokite

Todorokite has a high ion exchange capacity and can be a good adsorbent to fix Cs. Here, the adsorption behavior of Cs and the cation selectivity of two todorokites synthesized using two different methods (Oxi-TOD and Red-TOD) were investigated by kinetic and isotherm experiments. Todorokite exhibited a range of physicochemical properties depending on the synthesis method, while the Cs adsorption capacity also varied accordingly. Todorokite, possessing a high specific surface area and pore volume combined with the low average oxidation state of the manganese ion, exhibited a high adsorption capacity. The todorokite attained adsorption equilibrium within 48 h and followed the pseudo-second model. For the adsorption isotherm models, the correlation coefficient was high in the Langmuir model by linear regression and in the Redlich–Peterson model by nonlinear regression. For the adsorption experiment, todorokite had at least two adsorption sites for Cs: a high adsorption preference site with small capacity and a low adsorption preference site with large capacity. The result of the competition experiment between Cs and other cations showed that the ion selectivity was generally similar between the two todorokites except for Cs, K, and Rb, which exhibited similar ionic radii. The selectivity for Oxi-TOD was Li < Na < K < Rb, Cs < Sr while that for Red-TOD was Li < Na < K, Rb, Cs < Sr.