Preparation And Study Of ²³⁹Pu Highly-Adsorbed Pillared Clay

There is a large amount of high-level nuclear waste currently stored in China,and²³⁹Pu is one of the key high-level radionuclides.Clay minerals are potential backfill materials for high-radioactive nuclear waste reservoirs because of their strong adsorption properties and low permeability.However,the ability of clay colloids to assist the transportation of plutonium in water where clay rocks are used as part of a multiple barrier system has been established.In this thesis,modified clays with high specific surface area,strong sorptive properties and low mobility were prepared by the pillaring process.The internal structure of clays was studied by small-angle X-ray scattering and small-angle neutron scattering.In order to improve the adsorption of aqueous ²³⁹Pu solution species and also minimize the migration of ²³⁹Pu,the interaction between pillared and ²³⁹Pu were studied.Finally,the adsorption performance of Pu uptake by clays and pillared clays was compared,and the adsorption properties of various sorbents are discussed.The results of the study are as follows:?1?The ability of bentonite and montmorillonite pillared by Al-oxide and mixed?Ln-Al?-oxides?Ln=La,Ce?to remove ²³⁹Pu solution species from water were comparatively investigated at pH 7 and pH 4.Not all of the oxide pillars improved Pu uptake compared with sodium montmorillonite.At neutral and acidic pH only?Ce-Al?-oxide pillared clays showed the ability to remove Pu over the concentration range studied(1.35?10^-8–8?10^-8 mol dm^-3)with distribution coefficient?K_D?values>10⁴.?2?Small-angle scattering and neutron contrast variation with H₂O/D₂O mixtures was used to verify the ingress of water in the calcined products after hydrophilicity was introduced by an NH₃-H₂O vapor treatment.The size and shape of the?La/Ce?-Al oxo-hydroxy pillaring cations?2 nm spheres?was determined by small-angle x-ray scattering from the pillaring solutions.XPS analysis of the?Ce-Al?-oxide pillared clays indicates the presence of Ce⁴⁺as cerium dioxide.The progressive improvement in sorption performance in the order of pillar type was found to be:Al₂O₃<La₂O₃-Al₂O₃<<CeO₂-Al₂O_3.The reflects the increasing access of Pu solution species to the clay mineral layers by changes to the basal spacing and specific surface area,and also to the increasing stability of the pillars to the test conditions.?3?TEM,SANS and WAXS were used to determine the internal structure and particle size of laponite delaminated by Al-oxide and mix?Ln-Al?-oxides.Neutron scattering from the modified clays revealed nanostructural features corresponding to platelets of length 10-25 nm and 1-2 nm in thickness.?4?With the exception of the product that was freeze-dried prior to calcination,laponite pillared by Al-oxide and?Ce-Al?-oxides showed respectively,a five times increase in ²³⁹Pu uptake compared with the starting clay in the slightly alkaline condition.Although the modified bentonite exhibited superior sorption distribution coefficients in both near-neutral and acidic environments,the advantage of delaminated pillared laponite as a high affinity sorbent?K_D>10⁴?for filtration and sorbent bed applications is that at the near-neutral pH condition,Pu sorption equilibrium was achieved twice as fast as that recorded for the lamellar pillared bentonite.