Remediation of Uranium in the Hanford Vadose Zone Using Gas-Transported Reactants: Laboratory Scale Experiments in Support of the Deep Vadose Zone Treatability Test Plan for the Hanford Central Plateau

Abstract

This laboratory-scale investigation focused on decreasing uranium mobility in subsurface contaminated sediments in the vadose zone by in situ geochemical manipulation at low water content. This geochemical manipulation of the sediment surface phases included reduction, pH change (acidic and alkaline), and additions of chemicals (phosphate and ferric iron) to form specific precipitates. Reactants were advected into one-dimensional columns packed with uranium-contaminated sediment from the 200 Area of the Hanford Site as a reactive gas (for CO 2 , NH 3 , H 2 S, SO 2 ), with a 0.1% water content mist [for NaOH, Fe(III), HCl, PO 4 ] and with a 1% water content foam (for PO 4 ).Uranium is present in the sediment in multiple phases that include (in decreasing mobility) the following: aqueous U(VI) complexes, adsorbed uranium, reduced U(IV) precipitates, rind-carbonates, total carbonates, oxides, silicates, and phosphates. Geochemical changes were evaluated in the ability to change the mixture of surface uranium phases to less mobile forms, as defined by a series of liquid extractions that dissolve progressively less soluble phases. Although liquid extractions provide some useful information as to the generalized uranium surface phases (and are considered operational definitions of extracted phases), positive identification of surface phase changes by electron microprobe analysis is in progress. Some of the changes in uranium mobility directly involve uranium phases, whereas other changes result in precipitate coatings on uranium surface phases. The long-term implication of the uranium surface phase changes to alter uranium mass mobility in the vadose zone was then investigated using simulations of one-dimensional infiltration and downward migration of six uranium phases to the water table.In terms of the short-term decrease in uranium mobility (in decreasing order), NH 3 , NaOH mist, CO 2 , HCl mist, and Fe(III) mist showed 20% to 35% change in uranium surface phases. Difference in treatment effectiveness between sediments likely reflects mineralogy. Phosphate addition (mist or foam advected) showed inconsistent change in aqueous and adsorbed uranium, but significant coating (likely phosphates) on uranium carbonates. The two reductive gas treatments (H 2 S and SO 2 ) showed little change. For long-term decrease in uranium reduction, mineral phases created that had low solubility (phosphates and silicates) were desired, so NH 3 , phosphates (mist and foam delivered), and NaOH mist showed the greatest formation of these minerals. In addition, simulations of uranium movement in the vadose zone showed that these treatments greatly decreased uranium transport to groundwater. Advection of reactive gasses was the easiest to implement into low water content sediments at the laboratory-scale (and presumably field-scale) experiments. Both mist and foam advection show potential and need further development, but current implementation techniques move reactants shorter distances relative to reactive gasses. Overall, the am...

Remediation of Uranium in the Hanford Vadose Zone Using Ammonia Gas: FY 2010 Laboratory-Scale Experiments

“…Both of these treatments show very little aqueous or adsorbed uranium after 12 months. Phosphate treatment of BX-102 sediment (sediment 2, not shown but discussed and shown in Szecsody et al 2010) did show greatly increased aqueous and adsorbed uranium after phosphate treatment for 1 to 3 months (in contrast with results for this TX-104 sediment). The TX-104 sediment contains primarily uranium co-precipitated with carbonates, whereas the BX-102 sediment contains primarily uranium in sodium-boltwoodite.…”

Evaluation of Soil Flushing for Application to the Deep Vadose Zone in the Hanford Central Plateau

“…Recently, Szecsody et al (2010aSzecsody et al ( , 2010b applied the series of six sequential sediment extractions listed below as an operational means to quantify how uranium is distributed within a sediment sample. These extractions progress through increasingly harsh reagents.…”

“…These data suggest that some of the uranium in contaminated Hanford Site sediments is currently resistant to extraction by mild extraction solutions. The extraction solutions from Szecsody et al (2010aSzecsody et al ( , 2010b include the following elements:…”

“…One potentially significant difference is the oxidation state of uranium in the targeted solid phase. The solid phase and adsorbed uranium in the 200 Area contaminated soil is predominantly in the oxidized form uranium(VI) (Szecsody et al 2010a(Szecsody et al , 2010b, whereas uranium in natural deposits that are mined using ISR is typically present as uranium(IV), which is the reduced form. In addition, most ISR-targeted uranium deposits are located below the water table, in contrast to the vadose zone location of the uranium contamination in the 200 Area soils.…”

“…Many of the leaching processes are mild extraction solutions that are primarily designed to address uranium that is already in relatively mobile forms. As shown in Szecsody et al (2010aSzecsody et al ( , 2010b, much of the uranium in the Hanford Site vadose zone is present in forms that require a harsh extraction solution to render them mobile. Thus, use of mild extraction solutions would need to be evaluated in terms of whether these solutions can remove a sufficient fraction of the total uranium to meet remediation goals.…”