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Ion Exchange to cope with decommissioning

Herbas, Melodie Noelle marielle

[Thesis]. Manchester, UK: The University of Manchester; 2018.

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Abstract

This PhD project studied the caesium and strontium removal capacities of six nanoporous materials. Indeed, it was linked in with the SIXEP plant present at Sellafield site, UK, which is currently using natural clinoptilolite, originally mined from the Mojave Desert, to remove radioactive caesium and strontium ions from wastewater effluents. At the moment the feeds entering the SIXEP plant correspond to the best working conditions for clinoptilolite. As the feeds are moving toward more acidic pH and complex solutions and as clinoptilolite works at neutral pH, new nanoporous materials are required. This project had been divided into sub-projects, which all had the same purpose of finding the best material for caesium and strontium removal under complex solutions. The first part of the PhD focused on the synthesis of CST, SNT, ETS-4 and ETS-10 materials following literature reviews. The two last materials studied were provided by Fortum Power and Heat Oy for CsTreat material and by Sellafield Ltd for clinoptilolite material. The second part of the PhD consisted in studying the exchange capacities for caesium and strontium ions for each material studied under batch conditions using several water compositions relevant to Sellafield Ltd. Each water composition had different competing ions or pH and were used to classify the materials between them. From that it was found that Clinoptilolite stays the best materials for caesium and strontium removal. The third part of the project was the gamma irradiation of the materials. This irradiation had an aim to check the stability of the structure of the studied materials under gamma irradiation. The materials were exposed to four different gamma doses which were 0.5; 1; 3 and 6 MGy, the last one corresponding to the maximum dose were structural changes could happen. This revealed that all the materials, except ETS-4 material, were stable under gamma irradiation and that no structural changes were observable. The fourth part of the project was an extension of the third part in which the materials were Cs exchanged before being gamma irradiated to a dose of 6 MGy. This showed that the caesium exchange process is not affected by radiation. Finally, the unexchanged materials were alpha irradiated using a 15 MeV He2+ beam at the Dalton Cumbria Facility. Two irradiations were performed on CsTreat, CST, ETS-4, ETS-10 and clinoptilolite material. It was found that all the materials were damaged by alpha irradiation and displayed amorphous framework To resume, it appeared that clinoptilolite still stays one of the best materials for the removal of caesium and strontium ions in a wide range of wastewater composition. Additionally, it was discovered that ETS-10 material could be a good alternative to clinoptilolite material as its structure is stable under gamma irradiation and it displayed good exchange capacities when competing ions were present in the feeds. CST could also be a substitute as it shows good ion exchange properties under a broad range of pH. SNT material was great for strontium removal while CsTreat was working well for caesium removal. Even though ETS- 4 revealed good ion exchange capacities they were lower than clinoptilolite, ETS- 10 or CST materials.