Rapid immobilisation of U(VI) by Eucalyptus bark: Adsorption without reduction

Organic matter is increasingly shown to influence the mobility of uranium (U) in the environment. The mobility of U likely depends on whether the organic matter is in dissolved or solid form, with the latter able to retard U mobility. In this work, column experiments were used to reveal that solid organic matter, in the form of well characterized tree bark from Eucalyptus globulus, dramatically reduced the mobility of aqueous U(VI) which was introduced as uranyl nitrate [UO2(NO3)2]. Eucalyptus globulus bark contains high levels of carboxylic and phenolic acid groups which are known to bind to U. Admixtures containing 20 wt. % tree bark and sand were compared to columns containing sand only. We show that soluble U is adsorbed onto the tree bark, likely via a cation exchange with calcium, with no change in U oxidation state as confirmed by X-ray Absorption Near Edge Structure (XANES) analyses. Cation concentrations in column outflow solutions indicated that U was retained in the columns containing tree bark but was released from the sand-only columns. These results demonstrate that solid organic matter such as tree bark has potential applications in trapping U, possibly within permeable reactive barriers, without necessitating further engineering to reduce U(VI) to U(IV). Building on previous work on organic sedimentary U-deposits, this study also helps understand processes of U enrichment from groundwater as observed in environments high in organic matter including wetlands and sediment-hosted ore deposits.