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(UO 2 ) 2 (PO 4 ) 2 ·10-12H 2 O] and phosphuranylite [KCa(H 3 O) 3 (UO 2 ) 7 (PO 4 ) 4 O 4 ·8H 2 O]} 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.

Conceptual Model of Uranium in the Vadose Zone for Acidic and Alkaline Wastes Discharged at the Hanford Site Central Plateau

“…The ferrous iron produced from dissolution reactions could reduce uranium, if adsorbed to iron oxide surfaces (Liger et al 1999) resulting in U(IV) precipitates, although adsorption of ferrous iron would only occur once the acidic conditions were pH buffered to greater than pH 6. However, there were no significant differences in uranium sorption/precipitation response for batch experiments under acidic conditions that were conducted under atmospheric conditions compared to those in a controlled atmosphere with no measureable oxygen and carbon dioxide (Gartman et al 2014). These data suggest that reductive processes induced by acid neutralization of the tested Hanford Site sediments are not a significant contributor to overall uranium partitioning to solids at low pH.…”

“…Sorption experiments demonstrated that uranium behavior in regions of the subsurface with low pH (e.g., pH 2) shows increasing sorption of uranium to the solid phase with large increases in ionic strength and sodium nitrate concentration (i.e., 1M sodium nitrate increased to 8 M sodium nitrate) (Gartman et al 2014). This behavior would be transient and occur during disposal, likely only in zones where the neutralization capacity became exhausted.…”

“…The conceptual model descriptions in this report were developed based on results of recent laboratory experiments examining the geochemical processes that occur when Hanford sediments are exposed to uranium-laden acidic or alkaline wastes. Details of these experiments are provided by Szecsody et al (2013) and Gartman et al (2014). In summary, batch sediment-water experiments were conducted to quantify time scales of mineral dissolution in sediments and proton or OH-adsorption capacity.…”

“…A general conceptual model for uranium at waste sites where acidic wastes were disposed is described in this section for use as a template in developing a site-specific conceptual model. Primary experimental support for the conceptual model is described by Szecsody et al (2013) and Gartman et al (2014). The conceptual model incorporates the impacts of acid neutralization on uranium transport as influenced by uranium and non-uranium bearing mineral dissolution/precipitation and changes in uranium aqueous species partitioning (i.e., sorption) that are imposed by this mineral dissolution/precipitation and the waste fluid constituents.…”

Factors influencing U(VI) adsorption onto soil from a candidate very low level radioactive waste disposal site in China

“…A com pa ra ble con clu sion was drawn by Mishra [33], who in ves ti gated the sorp tion be hav ior of ura nium in ag ri cul tural soils. Gartman et al [34] stud ied U(VI) fate in Hanford sed i ment and found that sed i ment sorp tion fit the Freundlich iso therm in the ab sence of air ex per i ments.…”