Transformation of nanosized ferrihydrite (Fh) to crystalline iron (hydr)oxides causes desorption of pre-adsorbed metal and metalloid oxyanions to solution. This process impacts environmental water quality by affecting contaminant transport and fate, as well as how synthetic ferrihydrite can be used to remove potential pollutants from municipal and industrial waste streams. The effects of different types of surface complexes (outer-sphere, inner-sphere, monodentate, bidentate, etc.) on the rate and pathway of ferrihydrite transformation to goethite (Gt) and hematite (Ht) remain only partly understood. We investigate the impacts of adsorbed nitrate, sulfate, and arsenate on the kinetics and pathways of Fh transformation. The oxyanion complexes range from weak electrostatic outer-sphere for NO3-, to a mix of outer-sphere and inner-sphere for SO42-, to predominantly strong bidentate inner-sphere for H2AsO4-. Pure and adsorbed ferrihydrite samples were buffered at pH 5.5, aged for up to 50 days at 70 °C, and then characterized by powder x-ray diffraction (pXRD) and synchrotron pair distribution function (PDF) analysis. Results showed a significant decrease in the rate of Fh transformation with increasing strength of the oxyanions bonding on the surface such that pure Fh was entirely transformed to Gt and Ht in less than two weeks, while As-adsorbed samples were still stable (untransformed) after 50 days. Solution chemical analysis results showed that with more Fh transformation, nitrate and sulfate were progressively desorbed from the surface, whereas As remained fully adsorbed to the Fh. This study shows that the rate of adsorbed Fh transformation is controlled by the type and strength of complexation on the surface.

Oxyanion surface complex type controls the rate and pathway of ferrihydrite transformation

Macromolecular Chemistry: The Second Century

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ACS Spring 2020 National Meeting & Expo

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