Stabilization of Ferrihydrite and Lepidocrocite by Silicate during Fe(II)-Catalyzed Mineral Transformation: Impact on Particle Morphology and Silicate Distribution

Interactions between aqueous ferrous iron (Fe(II)) and secondary Fe oxyhydroxides catalyze mineral recrystallization and/or trans-formation processes in anoxic soils and sediments, where oxyanions, such assilicate, are abundant. However, the effect and the fate of silicate during Femineral recrystallization and transformation are not entirely understood andespecially remain unclear for lepidocrocite. In this study, we reacted(Si-)ferrihydrite (Si/Fe = 0, 0.05, and 0.18) and (Si-)lepidocrocite (Si/Fe =0 and 0.08) with isotopically labeled57Fe(II) (Fe(II)/Fe(III) = 0.02 and 0.2) atpH 7 for up to 4 weeks. We followed Fe mineral transformations with X-raydiffraction and tracked Fe atom exchange by measuring aqueous and solid phaseFe isotope fractions. Our results show that the extent of ferrihydritetransformation in the presence of Fe(II) was strongly influenced by the solid phase Si/Fe ratio, while increasing the Fe(II)/Fe(III) ratio (from 0.02 to 0.2) had only a minor effect. The presence of silicate increased the thickness of newly formedlepidocrocite crystallites, and elemental distribution maps of Fe(II)-reacted Si-ferrihydrites revealed that much more Si wasassociated with the remaining ferrihydrite than with the newly formed lepidocrocite. Pure lepidocrocite underwent recrystallizationin the low Fe(II) treatment and transformed to magnetite at the high Fe(II)/Fe(III) ratio. Adsorbed silicate inactivated thelepidocrocite surfaces, which strongly reduced Fe atom exchange and inhibited mineral transformation. Collectively, the results ofthis study demonstrate that Fe(II)-catalyzed Si-ferrihydrite transformation leads to the redistribution of silicate in the solid phaseand the formation of thicker lepidocrocite platelets, while lepidocrocite transformation can be completely inhibited by adsorbedsilicate. Therefore, silicate is an important factor to include when considering Fe mineral dynamics in soils under reducing conditions

Automated summary

This study investigates the effect and fate of silicate during Fe mineral recrystallization and transformation. The results show that the solid phase Si/Fe ratio strongly influences the extent of ferrihydrite transformation, while the Fe(II)/Fe(III) ratio has a minor effect. Silicate increases the thickness of lepidocrocite crystallites and inhibits the transformation of lepidocrocite when adsorbed. Silicate is an important factor to consider in understanding Fe mineral dynamics in reducing soil conditions.