Uranium fate in Hanford sediment altered by simulated acid waste solutions

Abstract

Infiltration of aqueous acidic waste to the subsurface may induce conditions that alter contaminant transport. Experiments were conducted to examine the effects of low pore water pH and associated changes to sediment properties on U(VI) behavior in sediments. Macroscopic batch experiments were combined with a variety of bulk characterization studies (Mössbauer and laser spectroscopy), micron-scale inspections (μ-XRF), and molecular scale interrogations (XANES) with the objectives to: 1) determine the extent of U(VI) partitioning to Hanford sediments exposed to acidic waste simulants and held at pH = 2, pH = 5, or under neutral conditions (pH = 8) at varying ionic strength, and in the presence of air [bench-top (BT) experiments] or in the absence of air [glove-box (GB) experiments]; and 2) determine the uranium micron-scale solid phase and associated valence state resulting from the experimental conditions. The investigation showed minimal overall changes in Fe mineralogy as a result of sediment exposure to acid solutions, but an increase in the highly reactive nano Fe fraction of the sediment. Greater uranium partitioning was observed at pH = 5 than at pH = 2 and 8. The μ-XRF inspections and XANES analyses confirmed that high concentration areas on sediment surfaces were rich in U(VI) in the BT experiments, and both U(IV) and U(VI) in the GB experiments. The laser spectroscopy data showed that uranyl phosphates {e.g., metaautunite [Ca(UO2)2(PO4)2·10–12H2O] and phosphuranylite [KCa(H3O)3(UO2)7(PO4)4O4·8H2O]} may have formed in the BT experiments. In the GB experiments, in addition to U(IV) phases, U(VI) phases may have also formed similar to those that are naturally present in the sediment, but at higher concentrations. The results provide insights about U(VI) mobility beneath acidic waste disposal sites.