Goethite and Hematite Nucleation and Growth from Ferrihydrite: Effects of Oxyanion Surface Complexes

The presence of oxyanions, such as nitrate (NO₃^-) and phosphate (PO₄^3-), regulates the nucleation and growth of goethite (Gt) and hematite (Hm) during the transformation of ferrihydrite (Fh). Our previous studies showed that oxyanion surface complexes control the rate and pathway of Fh transformation to Gt and Hm. However, how oxyanion surface complexes control the mechanism of Gt and Hm nucleation and growth during the Fh transformation is still unclear. We used synchrotron scattering methods and cryogenic transmission electron microscopy to investigate the effects of NO₃^- outer-sphere complexes and PO₄^3- inner-sphere complexes on the mechanism of Gt and Hm formation from Fh. Our TEM results indicated that Gt particles form through a two-step model in which Fh particles first transform to Gt nanoparticles and then crystallographically align and grow to larger particles by oriented attachment (OA). In contrast, for the formation of Hm, imaging shows that Fh particles first aggregate and then transform to Hm through interface nucleation. This is consistent with our X-ray scattering results, which demonstrate that NO₃^- outer-sphere and PO₄^3- inner-sphere complexes promote the formation of Gt and Hm, respectively. These results have implications for understanding the coupled interactions of oxyanions and iron oxy-hydroxides in Earth-surface environments.