Anisotropic Morphological Changes in Goethite during Fe²⁺-Catalyzed Recrystallization

When goethite is exposed to aqueous Fe²⁺, rapid and extensive Fe atom exchange can occur between solid-phase Fe³⁺ and aqueous Fe²⁺ in a process referred to as Fe²⁺-catalyzed recrystallization. This process can lead to the structural incorporation or release of trace elements, which has important implications for contaminant remediation and nutrient biogeochemical cycling. Prior work found that the process did not cause major changes to the goethite structure or morphology. Here, we further investigated if and how goethite morphology and aggregation behavior changed temporally during Fe²⁺-catalyzed recrystallization. On the basis of existing literature, we hypothesized that Fe²⁺-catalyzed recrystallization of goethite would not result in changes to individual particle morphology or interparticle interactions. To test this, we reacted nanoparticulate goethite with aqueous Fe²⁺ at pH 7.5 over 30 days and used transmission electron microscopy (TEM), cryogenic TEM, and ⁵⁵Fe as an isotope tracer to observe changes in particle dimensions, aggregation, and isotopic composition over time. Over the course of 30 days, the goethite particles substantially recrystallized, and the particle dimensions changed anisotropically, resulting in a preferential increase in the mean particle width. The temporal changes in goethite morphology could not be completely explained by a single mineral-transformation mechanism but rather indicated that multiple transformation mechanisms occurred concurrently. Collectively, these results demonstrate that the morphology of goethite nanoparticles does change during recrystallization, which is an important step toward identifying the driving force(s) of recrystallization.