Plutonium binding affinity to sediments increases with contact time

Abstract

Abstract Previous research has shown that Pu binds strongly to mineral surfaces and that its tendency to desorb decreases with time. However, the mechanisms behind this aging process are not well understood, have not been conducted under controlled laboratory conditions, and have not considered natural samples aged for long durations. In this work, sediments aged for 32\u202fyears in the presence of Pu were used to investigate the reversibility of sorption with time. Experiments utilized these aged sediments to examine the fate of Pu via multi-isotope adsorption and desorption experiments and selective extractions in the presence of a suite of ligands. None of the ligands (desferrioxamine B siderophore, fulvic acid, citric acid) or elevated concentrations of salts (NaF, NaCl, CaCl2, Na2PO4) promoted desorption of significant amounts of Pu from the surface after 32\u202fyears of aging or significantly reduced sorption of freshly added Pu. Together, these results highlight that Pu has a significantly greater affinity for binding to sediment surfaces than ligand complexation, regardless of aging period. A surface complexation model further corroborated these experimental data. However, selective extractions show that Pu on the solid phase may be redistributing into a more recalcitrant fraction with time due to an aging effect. Not including long-term Pu aging effects on sediments in risk models may lead to overestimating Pu mobility in the aqueous phase while underestimating Pu mobility in the colloidal phase.