Engineering Dual-Phase Zeolite Composites for Cs-137 and Sr-90 Immobilization

The control over zeolite selectivity is crucial for numerous industrial applications, including catalysis, gas separation and ion-exchange. In the nuclear industry, natural zeolites are employed for the removal of caesium-137 and strontium-90 from aqueous nuclear decommissioning waste, via an ion-exchange process. However, sourcing a material capable of simultaneous dual-cesium and -strontium removal, such as Mud Hills clinoptilolite, which is currently employed at Sellafield, UK, is challenging. One documented method to improve the strontium affinity of natural mordenites (MOR) and clinoptilolites (HEU) is a structural transformation to a more aluminous Na-P (GIS) structure, achieved by means of a hydrothermal NaOH treatment. However, studies have generally used concentrated NaOH solutions (&gt;2M), ensuring complete transformation into the GIS-type structure, with composite materials seen as undesirable. This work reports the generation of composite zeolites through the partial transformation of polycrystalline mordenite and clinoptilolite aggregates into Na-P, with high levels of chemical control. Subsequently, the ion-exchange performance for cesium and strontium uptake are assessed, granular materials developed and trialed in a rapid, flow system using active nucleotides. The mechanism of transformation and nature of granular composites are also investigated through use of state-of-the-art quasi-simultaneous imaging and diffraction techniques performed at the recently commissioned DIAD beamline, Diamond Light Source.<br/><br/>Four series of composite zeolites were generated through partial transformation of four naturally sourced zeolites, three clinoptilolites and a mordenite. This was achieved through a partial structure transformation into Na-P, where a high level of control is exhibited and a desired blend of zeolite phases can be obtained. Batch ion-exchange experiments highlighted an increase in strontium affinity as the transformation proceeds, at the expense of cesium selectivity. Composites with around an equal blend in phases showed a significant improvement in dual-uptake behavior. Granular composites, suitable for use in ion-exchange beds, were also developed by pre-sieving of the natural zeolite prior to treatment with NaOH. SEM images suggest particle size is retained in this process through crystallization of the new phase on the surface of existing particles. X-ray computed tomography and image-guided micro-diffraction show an ‘outer shell’ comprised of Na-P, with an intact mordenite interior. This is further confirmed by diffraction tomography data. Similar analysis suggests that fully converted Na-P granules consist of large, spherical sub-particles on the exterior and similar, smaller sub-particles on the interior. These findings suggest the macroscale mechanism to be one of surface dissolution-recrystallisation, resulting in an ‘outer shell’ encapsulating a partially dissolved interior. The base then penetrates the outer shell, dissolving the interior, which then recrystallizes into Na-P. Containment by the outer shell possibly aids this process by restricting diffusion of aluminosilicate nano-parts into the bulk solution and retaining the original granule morphology.<br/><br/>Rapid ion-exchange data of composites of around a 50:50 mixture of phases shows a remarkable improvement in Sr-90 uptake compared to the parent materials. The enhancement was to such an extent that composite materials exhibited superior Sr-90 affinity when compared to industry standard, Mud Hills clinoptilolite. Critically, this was not at the expense of Cs-137 uptake, which remained high. This work creates scope for a novel route to a host of potential resins from low-cost natural mineral sources, with the added opportunity to tune material compositions for a particular waste stream. Additionally, the potential of the newly commissioned DIAD beamline for advanced materials analysis is showcased.