Cesium Extraction from Cs_0.8Ba_0.4Ti₈O₁₆ Hollandite Nuclear Waste Form Ceramics in Nitric Acid Solutions

The leaching of Cs from well-characterized Cs-bearing hollandite powders with ideal formula Cs_0.8Ba_0.4Ti₈O₁₆ was studied at 200 °C under static oxidizing conditions achieved using dilute nitric acid solutions. A variety of techniques were used to elucidate the leaching mechanism including x-ray powder diffraction, magnetic susceptibility, x-ray absorption near edge structure, and electron microscopy. Under the conditions of the study, Cs is leached from Cs-hollandite according to a deceleratory rate law with contracting geometries. The principal reaction products are rutile and brookite. The leaching mechanism involves the formation of a quasi-continuous, and hence relatively impermeable, secondary titanium oxide phase (rutile and/or brookite) on the surfaces of the hollandite crystallites. The driving force for the leaching reaction appears to be the oxidation of Ti³⁺ in the structure by the oxidizing acid, which then promotes extraction of Cs and Ba from the hollandite tunnels followed by tunnel destabilization and transformation to rutile and brookite.