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

Abstract

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 Ca²⁺, Mg²⁺, 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 Mg²⁺  > Ca²⁺ 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 Ca²⁺. 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 Ca²⁺ rather than Mg²⁺. Mg²⁺ is extruded from the interlayer space by Cs⁺ and K⁺ adsorption, excluded from short interlayer space and replaced by Ca²⁺ as Cs⁺ desorbs.