Interfacial sorption/desorption studies of Pu(IV) on Silica and alumina surfaces

Abstract

AbstractCombination of sorption, desorption and kinetic experiments for Pu(IV) in silica and alumina suspensions over the pH range (2–9) have been employed to study the factors controlling Pu mobility from the near field of the repositories. These studies have been carried out using shorter lived isotope of Pu (238Pu) thereby achieving Pu concentration (∼10-9M) near the solubility limit. Kinetic experiments were carried out to investigate the mechanism of the sorption process. In order to conclude about the surface speciation, desorption and leaching studies have been carried out on short as well as long time equilibrated sorption samples. Oxidation state transformation in the kinetic experiment was determined using parallelly carried out solvent extraction procedures. Silica was found governing the sorption at pH values≲ 5.5while both the surfaces quantitatively sorb the metal ion at higher pH values. The leaching and desorption experiments, in light of kinetic experiments, concluded the formation of small polynuclear Pu(IV) oxide/hydroxide species on both the surfaces. These experiments did not show any evidence for PuO2(am.) as the surface speciation. Oxidation state determination in silica supernatants revealed V as the oxidation state of Pu, while alumina suspensions favour higher oxidations state,i.e., (VI) over (V) in the supernantant. Disproportionation of Pu(V) in the interfacial region, caused by the higher sorption affinity of alumina, produces Pu(VI) in the alumina supernatant. These findings provide new insight for defining the source term of Pu migration in geosphere.