Stabilization of U(V) and U(VI) in Goethite Formed by Recrystallization of Fe-Oxyhydroxysulfates

Schwertmannite and jarosite are naturally occurring iron (Fe) oxyhydroxysulfates with strong sorption capacities for hexavalent uranium [U(VI)] in various acidic sulfate-rich environments. These metastable minerals commonly undergo recrystallization, particularly in the presence of dissolved Fe²⁺ [Fe(II)_aq], which may influence the fate of associated U(VI). Here, we quantified molecular-level changes in U repartitioning and speciation when U(VI)-sorbed schwertmannite and jarosite reacted with Fe(II)_aq under near-neutral and anaerobic conditions over 2 weeks. The results show that Fe(II)_aq additions promoted rapid mineral transformation to goethite via a dissolution-reprecipitation pathway, proceeding (near-completely) for schwertmannite but slowly and incompletely for jarosite. Importantly, even at early transformation stages when goethite likely only started forming on the surface of the transforming minerals, the recrystallization process led to near-complete retention of U, predominantly as U(VI), within the structure of the neo-formed goethite. Subsequent U reduction to U(V) increased with time but remained incomplete, even after extensive mineral transformation in the presence of 1-50 mM Fe(II)_aq for 2 weeks. The results demonstrate that Fe(II)-promoted recrystallization of Fe-oxyhydroxysulfates can rapidly and persistently lock both U(VI) and U(V) into chemically stable goethite, with important implications for predicting U behavior and designing remediation strategies in various acidic and U-contaminated environments.