Multi-cation exchanges involved in cesium and potassium sorption mechanisms on vermiculite and micaceous structures

International audienceVermiculite and micaceous minerals are relevant Cs$^+$ sorbent in soils and sediments. To understand Cs$^+$ bioavailability in soils resulting from multi-cation exchanges, sorption of cs$^+$ onto clay minerals have been carried out in batch experiments with solutions containing Ca$^{2+}$+, Mg$^{2+}$ and K$^+$. A sequence between a vermiculite and various micaceous structures were achieved by conditioning a vermiculite at various amounts of K. Competing cation exchanges were investigated according to the concentration of Cs$^+$. The contribution of K$^+$ on trace Cs$^+$ desorption is probed by applying different concentrations of K$^+$ on Cs-doped vermiculite and micaceous structures. Cs sorption isotherms at chemical equilibrium were combined with elemental mass balances in solution and structural analyses. Cs$^+$ replace easily Mg$^{2+}$ > Ca$^{2+}$ and competes scarcely with K$^+$. Cs$^+$ is strongly adsorbed on the various matrix and a K/Cs ratio about a thousand is required to remobilize Cs$^+$. Cs$^+$ is exchangeable as long as the clay interlayer space remains open to Ca$^{2+}$. However, excess of K$^+$, as well as Cs$^+$, in solution leads to the collapse of the interlayer spaces that locks the Cs into the structure. Once K$^+$ and/or Cs$^+$ collapse the interlayer space, the external sorption sites are then particularly involved in Cs sorption. Subsequently, Cs$^+$ exchanges preferentially with Ca$^{2+}$ rather than Mg$^{2+}$. Mg$^{2+}$ is extruded from the interlayer space by Cs$^+$ and K$^+$ adsorption, excluded from short interlayer space and replaced by Ca$^{2+}$ as Cs$^+$ desorbs