Temperature-dependent Fe(II) thresholds for magnetite formation during Fe(II)-catalyzed ferrihydrite transformation

摘要:

Fe(II)-catalyzed ferrihydrite (Fh) transformation is a critical process controlling secondary Fe mineral formation, Fe redox reactivity, and the cycling of associated nutrients and trace elements in anoxic and suboxic environments. Although aqueous Fe(II) (Fe(II)aq) availability is known to regulate Fh transformation pathways, the critical Fe(II)aq concentration required for magnetite (Mt) formation at different temperatures remains poorly understood. Here, we investigated Fe(II)-catalyzed Fh transformation at 25, 45, and 65 °C under anoxic, circumneutral pH conditions across a range of initial Fe(II)aq concentrations. Time-resolved analyses of mineral phases, Fe(II) speciation, particle morphology, and magnetic properties were combined with thermodynamic calculations to determine how temperature regulates transformation kinetics, pathway selection, and Mt formation. Increasing temperature significantly reduced the apparent Fe(II)aq threshold for Mt formation, from > 10.0 mM at 25 °C to 0.6–0.8 mM at 45 °C and 0.3–0.4 mM at 65 °C. Mineralogical evolution followed a two-stage pathway: rapid Fh dissolution and accumulation of lepidocrocite (Lp) and goethite (Gt) during Stage I, followed by Lp consumption and competitive partitioning between Gt growth and Mt formation during Stage II. Elevated temperature shifted the dominant Stage II pathway from template-directed Gt growth toward Lp-to-Mt conversion, changing the overall pathway from Fh → Lp/Gt → Gt to Fh → Lp/Gt → Mt/Gt. Thermodynamic calculations further showed that Stage I nucleation barriers favored Lp and Gt over direct Mt formation, whereas during Stage II, higher temperature enhanced the nucleation driving force and lowered the nucleation barrier for Lp-to-Mt conversion, while the corresponding thermodynamic parameters for Lp-to-Gt conversion remained nearly unchanged. Fe(II) speciation further showed that higher temperatures promoted Fe(II) adsorption and structural Fe(II) incorporation, facilitating the formation of mixed-valence Mt. The delayed appearance of Mt after Lp accumulation, together with rapid Lp consumption and enhanced Fe(II) incorporation, indicates that temperature-promoted Mt formation proceeds mainly through dissolution-reprecipitation rather than direct Fh-to-Mt solid-state conversion or oriented-attachment-dominated growth. The resulting Mt particles displayed temperature-dependent morphological and magnetic characteristics. These findings demonstrate that moderate warming expands the Fe(II)aq window for Mt formation by altering both the thermodynamic accessibility and kinetic competition of secondary mineralization pathways, providing new constraints for predicting Fe mineral phase selection in thermally dynamic biogeochemical environments.

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