Iron sulfide (FeSx-1), a major constituent of acid-volatile sulfides formed under sulfate reducing conditions, can be an effective reducing agent for sequestering redox-sensitive radionuclides like U in subsurface environments. Past studies have shown that FeS can convert highly mobile U(VI) (e.g., dissolved U(VI)O22+ species) to highly immobile reduced solid forms (e.g., U(VI)O2(s)). More recent studies have demonstrated FeS can also serve as an effective redox buffer by scavenging oxidants such as oxygen, while forming various iron (hydr)oxides and sulfur species, depending on pH and pe. In such U(IV)/FeS systems, as oxygen concentrations increase relative to FeS, oxidation of reduced U(IV) may occur, leading to the remobilization of previously sequestered U during an anoxic to oxic transition. In this presentation, the rate controlling reactions and mechanisms of oxidative mobilization of reduced U(IV) species (e.g., uraninite and non-crystalline U(IV)) in U/FeS systems are summarized. The ability of FeS to serve as a redox buffer in suboxic and oxic systems (e.g., from low to high relative O2/FeS molar ratios), and the subsequent potential for intermediate oxidants (e.g., Fe(III) (hydr)oxides and reactive oxygen species formed by FeS oxidation) for facilitating U(IV) remobilization are also discussed. These findings illustrate the important role of the redox cycling of iron and sulfur in suboxic systems in controlling pe and U speciation, with direct implications for the long-term stability of reduced solid phase forms of U(IV) species against oxidative mobilization in subsurface groundwater systems.

Control of U(IV) transformation and mobilization by nano-scale FeSx-1 in suboxic and oxic systems

255th National Meeting (2018)

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.