Ion exchange and adsorption methods are effective ways for the removal of Cs⁺ ions from radioactive effluents. In the practical application process, most of adsorbents used as powdered materials show difficulties in term of separation from the contaminated liquid, or pressure challenges through a high flow resistance in fix bed processes. A suitable grain size preparation of adsorbents can overcome these drawbacks by tailoring desired size particle and facilitating column operations with a low flow resistance. However, the particle size of the adsorbents can be critical on the breakthrough exhausted points and the efficiency of the adsorbent in a continuous treatment process.

This presentation will focus on the influence of the grain size of geopolymer based adsorbent on their Cs⁺ adsorption performances both in batch and fixed-bed process. For that purpose, synthesized geopolymer was used as prepared and as binder to support NaY zeolite particle in 20 wt% charged composite material. These adsorbents were prepared with three various grain sizes (50/100/500 µm) to remove Cs⁺ in batch and column operations. The efficiency and adsorption characteristics were investigated through kinetics, adsorption isotherms and breakthrough curves experimental data. We characterized the porosity and microstructure of the adsorbents and compared their adsorption properties in the exchange process. Comparison of batch and column adsorption experiments coupled with modelling for column study is used for a detailed explanation of various process parameters. The results of these experiments show some challenges for bed fixed column utilization by the choice of the grain size and the importance to more accurately optimize the design of column adsorption system to assess the transport of Cs in geopolymer derived system.

A chief characteristic of hollandite is a high tolerance for large cations (Ba²⁺, Cs⁺), which are traditionally problematic to immobilize. Prior work demonstrated a positive correlation between tunnel A-site Cs content, thermodynamic stability, and a corresponding decrease in elemental release. This work investigated the stability relationship among two suites of samples, which varied in their tunnel A-site occupancy of small cations, namely, potassium (K) produced by two different synthetic routes. The K-hollandite samples, K_xMg_x/2Ti_8-x/2O₁₆, (0<𝑥<2), were synthesized by solid-state as well as sol-gel methods. High-temperature oxide melt solution calorimetry was applied to measure their formation enthalpies to identify stability trends. It was found that phase stability tracks with A-site cation content (K⁺), which correlates well with our previous studies on Cs-hollandite. Thermochemistry, structural features, and electrical conductivity measurements will be discussed in light of density functional theory models and current structure-property relations for these materials systems. Tunnel-structured materials are of interest for a wide range of applications from nuclear waste immobilization to electrochemical energy storage.

Significant success has been observed in loading hollandite-structured phases with the fission product elements cesium and barium. While some understanding of the limits to the content and stability of the phases has been obtained via experimental scoping studies, a detailed understanding that would allow efficient design of these waste forms is still lacking. That issue is addressed in this effort where these systems have been extensively modeled, and where those models are being extended to the simultaneous incorporation of actinide elements. First principles calculations were thus performed on actinide-bearing aluminum-substituted hollandite phases to examine the potential use of the structures for also effectively immobilize U, Np, and Pu. The DFT-calculated formation enthalpies suggest the relative stabilities of these structures, providing likely targets for synthesis studies. These are used together with CALPHAD modeling of the phases using the compound energy formalism to determine their ultimate phase stability. The results provide an emerging picture of solid solubilities and potential ability to design highly loaded waste forms.

The Center for Hierarchical Waste Form Materials (CHWM) is composed of different international teams and aims to develop hierarchical materials for an efficient immobilization of radioactive elements. In this frame, aluminosilicated-based materials have been considered for the selective trapping and conditioning of Cs.

First, geopolymers, which are alkali-activated materials composed of tetrahedra of aluminate and silicate obtained at ambient temperature and pressure, were studied as adsorbent for Cs. Their ability to entrap Cs by ionic exchange is strongly depending on their Si/Al ratio. Indeed, an adapted Si/Al ratio is needed to create mesoporosity and allow the access of the geopolymer grain center for a high adsorption capacity with a fast kinetic. However, their selectivity for Cs is very limited because geopolymers are amorphous. Moreover, their leaching resistance is not as good as those of crystalline materials such as zeolites.

The material synthesis has been modified by curing the same precursor solutions hydrothermally. This leads to the formation of crystalline zeolitic structures instead of amorphous geopolymers. Depending on the Si/Al ratio and the curing time, different zeolite phases can be obtained (Faujasite, NaP1, Analcime…), which impact the Cs adsorption properties. Indeed, the crystallographic parameters of the zeolite have to present an adapted cage size to selectively host Cs by ionic exchange. While the formation of NaP1 does not significantly modify the Cs trapping properties, the synthesis of Analcime instead of geopolymer strongly reduces the Cs adsorption properties because the size of the hydrated Cs⁺ ion is larger than the micropore channels of the zeolitic structure. However, it has been shown that, for a specific Si/Al ratio, a mixed of NaP1-ANA is obtained with larger micropores (or specific defects) particularly adapted for the Cs adsorption. Therefore, this material presents a high capacity as well as an important selectivity for Cs toward Na.