Metaschoepite Dissolution in Sediment Column Systems—Implications for Uranium Speciation and Transport

Abstract

Metaschoepite is commonly found in U-contaminated environments and metaschoepite-bearing wastes may be managed via shallow or deep disposal. Understanding metaschoepite dissolution and tracking the fate of any liberated U is thus important. Here, discrete horizons of metaschoepite (UO 3 • nH 2 O) particles were emplaced in flowing sediment/groundwater columns representative of the UK Sellafield Ltd. site. The column systems either remained oxic or became anoxic due to electron donor additions, and the columns were sacrificed after 6-and 12-months for analysis. Solution chemistry, extractions, and bulk and micro/nano-focus X-ray spectroscopies were used to track changes in U distribution and behavior. In the oxic columns, U migration was extensive, with UO 2 2+ identified in effluents after 6-months of reaction using fluorescence spectroscopy. Unusually, in the electron-donor amended columns, during microbially mediated sulfate reduction, significant amounts of UO 2 -like colloids (>60% of the added U) were found in the effluents using TEM. XAS analysis of the U remaining associated with the reduced sediments confirmed the presence of trace U(VI), noncrystalline U(IV), and biogenic UO 2 , with UO 2 becoming more dominant with time. This study highlights the potential for U(IV) colloid production from U(VI) solids under reducing conditions and the complexity of U biogeochemistry in dynamic systems.

Stabilization Effects of Mn(II)-Salts on Metaschoepite in Soil under Different Water Regimes

“…Majority of U released from metaschoepite were sorbed to the sand in a chemical fraction with slow desorption, and the transportation of metaschoepite in the sand mainly depends on its dissolution and the interaction of dissolved UO 2 2+ with the sand particles. Bower et al investigated the dissolution of synthetic metaschoepite (UO 3 ·1.3H 2 O) and the biogeochemistry of liberated U in complex sediment/groundwater systems . In the oxic columns, metaschoepite dissolution resulted in significant U­(VI) transport, while the reaction with iron-containing sediment species retarded uranium migration; in the electron donor-amended columns (1 mmol L –1 of acetate/lactate 50:50 was added), U­(VI), noncrystalline U­(IV), and UO 2 were all observed in the sediment.…”

“…Bower et al investigated the dissolution of synthetic metaschoepite (UO 3 •1.3H 2 O) and the biogeochemistry of liberated U in complex sediment/groundwater systems. 14 In the oxic columns, metaschoepite dissolution resulted in significant U(VI) transport, while the reaction with iron-containing sediment species retarded uranium migration; in the electron donor-amended columns (1 mmol L −1 of acetate/lactate 50:50 was added), U(VI), noncrystalline U(IV), and UO 2 were all observed in the sediment. The study confirmed the transformation of liberated U(VI) in metaschoepite-amended soils to U(IV) colloid under reducing conditions.…”

“…Generally, natural or constructed wetlands were applied for the remediation of uranium-contaminated water based on the redox reduction of U(VI). 41 Further, shallow or deep disposal was usually used in the management of metaschoepite-bearing wastes, 14 and chemical leaching was applied to remediate DUcontaminated soils. 42 The present study provided a simple method using the Mn(II)-salts as the stabilization agents, which have a great potential in practical applications for in situ remediations of U-contaminated sites.…”

“…Under oxidizing conditions, U is primarily present in its hexavalent state as uranyl cation (UO 2 2+ ), forming numerous aqueous complexes. 13,14 However, under reducing conditions, U(IV), such as uraninite (UO 2 ) (log K sp = −54.6), is the common species, which is sparingly soluble, less mobile, and less bioavailable to plants compared to U(VI) species. 15,16 The solubility of uraninite at a nearneutral pH is ∼5 orders of magnitude lower than that of metaschoepite (∼10 mg L −1 ).…”

“…The mobility, solubility, and toxicity of U depend on the speciation and redox states, with reducing species being, in general, relatively more insoluble and immobile. Under oxidizing conditions, U is primarily present in its hexavalent state as uranyl cation (UO 2 2+ ), forming numerous aqueous complexes. , However, under reducing conditions, U­(IV), such as uraninite (UO 2 ) (log K sp = −54.6), is the common species, which is sparingly soluble, less mobile, and less bioavailable to plants compared to U­(VI) species. , The solubility of uraninite at a near-neutral pH is ∼5 orders of magnitude lower than that of metaschoepite (∼10 mg L –1 ) . Thus, reductive conversion of mobile U­(V) to immobile U­(IV) is a desirable strategy for the retardation of U migration at sites with U soil contamination.…”