Iron oxy-hydroxide minerals such as ferrihydrite (Fh) and goethite (Gt) are pervasive in Earth-surface environments and important in biogeochemical element cycling. Recent research has suggested that their formation is preceded by the precipitation of ultrasmall molecular clusters (&lt;1 nm). These clusters are expected to be highly reactive, with the ability to immobilize environmental contaminants through adsorption. However, relatively little is understood about the structure of iron oxy-hydroxide clusters and their transformation to more stable phases. We use a new method that involves mixed flow reactors (MFR) to synthesize the clusters and study their properties and transformation behavior. Acidified Fe(NO3)3 (0.2 M) and NaOH (2 M) solutions were pumped continuously into a stirred 5-mL MFR at different rates to reach target pH conditions (1.0, 1.5, and 2.5). The resulting suspension was then passed through an in-situ x-ray scattering cell at &lt;5 min reaction time for particle size and structural analysis. The results showed that the initial hydrolysis products are structurally disordered with an average size of ~1 nm. At pH 2.5 the as-formed clusters have a Gt-like local structure that evolves to nanocrystalline Gt after aging at room temperature for up to 21 days. In comparison, the samples synthesized and aged at pH 1.0 and 1.5 are relatively stable during aging and have local structures that are distinct from Gt and Fh. Comparison of the results with computational simulations of iron oxyhydroxide clusters calculated by Das 2018 suggests that they are not Fe13 Keggin-like clusters, as has previously been suggested. This new information about the structure and transformation of iron oxy-hydroxide clusters are important for understanding the formation and behavior of highly reactive nanoparticles in natural and applied systems.

Formation and transformation of iron oxy-hydroxide clusters

Fall 2022 abstracts, posters, and presentations will launch in mid-September.

ACS Fall 2022

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.