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

Vermiculite and micaceous minerals are relevant Cs sorbents in soils and sediments. To understand the bioavailability of Cs in soils resulting from multi-cation exchanges, sorption of Cs onto clay minerals was performed in batch experiments with solutions containing Ca, Mg, and K. A sequence between a vermiculite and various micaceous structures has been carried out by conditioning a vermiculite at various amounts of K. Competing cation exchanges were investigated as function of Cs concentration. 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 replaces easily Mg  > Ca and competes scarcely with K. Cs is strongly adsorbed on the various matrix, and a K/Cs ratio of about a thousand is required to remobilize Cs. Cs is exchangeable as long as the clay interlayer space remains open to Ca. However, an 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 preferentially exchanges with Ca rather than Mg. Mg is extruded from the interlayer space by Cs and K adsorption, excluded from short interlayer space and replaced by Ca as Cs desorbs.