Iron (Fe) oxy-hydroxides such as ferrihydrite (Fh) are ubiquitous in surface environments. Because of their high surface area and high reactive surface, they can immobilize environmental contaminants and nutrients (e.g., oxyanions) through adsorption. Ferrihydrite is metastable and eventually transforms to hematite (Hm), goethite (Gt), and lepidocrocite (Lp). Although the Fh formation and transformation and oxyanion adsorption on its surface have been separately studied, the coupled interaction of these processes is only partly understood. The impact of oxyanion surface complexes on the rate and pathway of Fh transformation was studied. Results show that AsO43- and SO42- inner-sphere complexes decrease the rate of Fh transformation and induce the formation of Hm. In contrast, NO3- outer-sphere complexes promote the formation of Gt. We then investigated the impact of oxyanion (AsO43- and PO43-) surface loading on the rate and pathway of Fh transformation. The results show that the rate of Fh transformation decreases, and more Hm forms with increasing the oxyanion surface loading. Cryogenic transmission electron microscopy (Cryo-TEM) was also used to study the effect of oxyanion surface complexes (NO3- and PO43-) on the nucleation and growth of Gt and Hm during Fh transformation. Our results show that Gt first was formed from Fh dissolution and then grew by oriented attachment. In contrast, Hm formed after the aggregation of Fh particles. We propose that NO3- outer-sphere complexes hydrate the surface and promote the Gt formation through a dissolution/crystallization pathway, while PO43- inner-sphere complexes dehydrate the surface and induce more Hm through an aggregation pathway. In the final project, we investigated the formation of Fh from Fe oxy-hydroxide clusters. The results showed that increasing pH increased the size and structural order of particles that resemble 2-line Fh. Also, the particle size of aged samples at pHs 1.5 and 2.5 increased with time, and they transformed to Gt and Lp. In this work, we develop new ways to study the formation and transformation of Fh. These methods and information can be used to develop further studies towards a comprehensive understanding of Fh formation and transformation in other environmental conditions, such as redox systems. / Doctor of Philosophy / Iron (Fe) oxy-hydroxides nanoparticles are composed of Fe, oxygen (O), and water (H2O). One of the most famous Fe nanoparticles is ferrihydrite (Fh), which is commonly found in soils, sediments, and water. Ferrihydrite surface is positively charged and adsorbs negatively charged ions such as oxyanions, which are important contaminants (e.g., arsenate; AsO43- and sulfate; SO42) and nutrients (e.g., phosphate; PO43- and nitrate; NO3-) in drinking water in the US and around the world. The structure of Fh is not stable, and it transforms to other Fe oxy-hydroxides such as goethite (Gt), hematite (Hm), and lepidocrocite (Lp). Pre-adsorbed oxyanions may release during Fh transformation and impact water and soil quality. Additionally, oxyanion adsorption may affect the formation and transformation of Fh, which is not fully understood. In this work, we investigate how oxyanions change the rate and pathway of Fh transformation. The results show that the strong binding of AsO43- and SO42- slows down the rate of Fh transformation and favors the formation of Hm as opposed to Gt. While weak adsorption of NO3- promotes the formation of more Gt. We also study the transformation of Fh in the presence of different oxyanions (AsO43- and PO43-) concentrations. The results show that the rate at which Fh transforms to Gt and Hm decreases, and more Hm forms with increasing the concentration of oxyanions on the Fh surface. Interestingly, results also show that weekly bounded NO3- and SO42- could be released to the solution phase, while strongly adsorbed AsO43- and PO43- could remain on the surface during the ferrihydrite transformation. We also used an imaging technique (Cryogenic transmission electron microscopy; Cryo-TEM) to study the effect of oxyanion surface complexes (NO3- and PO43-) on the formation and growth of Gt and Hm during the Fh transformation. The results show that Gt first was formed from Fh and then grew to larger particles. In contrast, for the formation of Hm, Fh particles first aggregate, form larger particles, and then transform to Hm. In the final project, we investigate the formation of Fh from Fe oxy-hydroxide cluster precursors. Results show that Fe oxy-hydroxide clusters can be the potential precursors for forming Fh during the rapid hydrolysis of Fe(III) solutions. However, when these precursors are aged, they do not form Fh and transform to Gt and Lp, which have stable structures. In this work, we develop new ways to study the formation and transformation of Fh, which will have implications for understanding how and when contaminant remobilization will occur during Fh formation and transformation.