Continuous Reactive Crystallization of Nanoscale Six-Line Ferrihydrite

Abstract

The first example of synthetic six-line ferrihydrite continuous reactive crystallization (5Fe2O3·9H2O) is described. The recently documented industrial crystallization of six-line ferrihydrite, an important nanomaterial to the environment and human physiology and an industrial byproduct, provided the model parameters for this study. Continuously crystallized six-line ferrihydrite was characterized across several orders of magnitude in size (nanometer to micrometer): primary crystals were ∼4 nm and aggregated into considerably larger particles. The rate of growth unit supply appeared to control the nucleation rates of the slower growth rate (high aspect ratio) and more thermodynamically stable goethite (α-FeOOH) phase and the kinetically favored six-line ferrihydrite. This suggested a pseudo-polymorphic relationship, as interplay between the relative proportions of the metastable six-line ferrihydrite and goethite (stable) crystals were observed leading up to steady state. This was thought to be a function of the kinetics of reactive crystallization (i.e., goethite forms at lower rates), controlled by the lower barrier to nucleation (interfacial energy) of six-line ferrihydrite. Both these phases were nanoscale, and although goethite primary crystals were larger in size, six-line ferrihydrite formed larger and tightly packed aggregates. This study has provided new possibilities for directing the crystallization of iron(III) oxyhydroxides where phase properties and crystallinity are significant, such as industrial crystallization and the biological occurrence inside the cavity of the protein ferritin.