Field Application of ²³⁸U/²³⁵U Measurements To Detect Reoxidation and Mobilization of U(IV)

Abstract

Biostimulation to induce reduction of soluble U(VI) to relatively immobile U(IV) is an effective strategy for decreasing aqueous U(VI) concentrations in contaminated groundwater systems. If oxidation of U(IV) occurs following the biostimulation phase, U(VI) concentrations increase, challenging the long-term effectiveness of this technique. However, detecting U(IV) oxidation through dissolved U concentrations alone can prove difficult in locations with few groundwater wells to track the addition of U to a mass of groundwater. We propose the U/U ratio of aqueous U as an independent, reliable tracer of U(IV) remobilization via oxidation or mobilization of colloids. Reduction of U(VI) produces U-enriched U(IV), whereas remobilization of solid U(IV) should not induce isotopic fractionation. The incorporation of remobilized U(IV) with a highU/U ratio into the aqueous U(VI) pool produces an increase in U/U of aqueous U(VI). During several injections of nitrate to induce U(IV) oxidation, U/U consistently increased, suggesting U/U is broadly applicable for detecting mobilization of U(IV).

Persistence of Uranium in Old and Cold Subpermafrost Groundwater Indicated by Linking 234U-235U-238U, Groundwater Ages, and Hydrogeochemistry

“…A number of recent investigations have used variations in 238 U/ 235 U ratio to study U mobility in groundwater. ,− Natural variations of 238 U/ 235 U between geological samples at any given point in time are caused by U isotope fractionation during physicochemical reactions . This fractionation is mostly driven by the nuclear field shift (NFS) effect in which 238 U is preferentially incorporated into the reactant or product phase having lower electron density around its nucleus.…”

“…238 U/ 235 U is therefore an effective tracer of U mobility in groundwater at U-ore deposits and sites contaminated by anthropogenic activity. ,− , However, few high-precision 238 U/ 235 U measurements of groundwater are available at sites unaffected by anthropogenic activity despite the frequent occurrence of this element as a geogenic contaminant . No studies of 238 U/ 235 U exist in subarctic groundwater, despite the commonality of elevated geogenic U [e.g., Canada, , Scandinavia, ,− ].…”

The kinetics and mechanism of H2O2 decomposition at the U3O8 surface in bicarbonate solution

“…1 The solubility of U in groundwater is governed by the form of U (U (IV) , U (V) and U (VI) ), with the hexavalent U (VI) form being more soluble than U (IV) and U (V) . [2][3][4] Therefore, the presence of U (VI) facilitates U dissolution into the groundwater upon canister failure.…”

On the change in UO2 redox reactivity as a function of H2O2 exposure

“…This can generate oxidizing conditions at the fuel surface under which UO 2 is considerably more soluble 6 thereby facilitating the migration of U and other radionuclides in the environment. 9 Among the radiolysis products, H 2 O 2 has been demonstrated to be the main oxidant responsible for uranium dissolution in systems exposed to alpha-radiation. 10 H 2 O 2 can react in two different ways on the surface of UO 2 , via catalytic decomposition producing water and molecular oxygen ((1)-(3)) 11 and by oxidizing U(IV) ((4) and (5)) 12,13 respectively.…”