Mineral Dynamics Revealed by Fe ²⁺ -Catalyzed Recrystallization of U-Incorporated Goethite

Iron (oxyhydr)oxides are capable of structurally incorporating and releasing trace elements through Fe²⁺-induced recrystallization. However, estimation of the extent of recrystallization varies among studies using isotopic tracers and those examining the release of occluded trace elements. To provide further insight into the dynamics of Fe²⁺-catalyzed iron (oxyhydr)oxide recrystallization, and to accurately interpret the isotopic and elemental compositions of iron (oxyhydr)oxides in the rock record, we probed the coupled dynamics of goethite isotopic exchange and trace element cycling during Fe²⁺-induced recrystallization. Herein, ⁵⁷Fe labeled Fe²⁺ was reacted with U(V)-incorporated goethite, and the resulting isotopic and elemental interactions were systematically characterized and modeled. Partial release of incorporated U (3.9%) was observed, which suggested limited recrystallization while Fe isotope measurements suggested nearly complete isotopic mixing after 30 days. A box model incorporating partial and gradual back-reaction estimated an overall goethite recrystallization extent of ∼32.8%, narrowing the gap to the observed U release (3.9%) as compared to the traditional fast back-reaction-only model (88%). This study highlights the role of Fe²⁺-driven recrystallization in controlling trace metal cycling and provides insights into the stability and reactivity of U-incorporated goethite in reducing environments. The findings refine our understanding of iron (oxyhydr)oxide recrystallization processes and their implications for trace element mobility in natural systems.