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.