Ferrihydrite (Fh) is a highly reactive metastable iron oxyhydroxide that readily binds to oxyanions such as nitrate and phosphate through surface complexation reactions. Its transformation to larger crystalline products such as goethite (Gt) and hematite (Hm) can release these surface species to solution thereby mobilizing them in the environment. Oxyanions are known to impact the pathway and kinetics of Fh transformation, although the effects of complex type and surface coverage are only partly understood. The type and strength of oxyanion complexes are different, ranging from weak electrostatic outer-sphere for nitrate (NO3-), to a mix of outer-sphere and inner-sphere for sulfate (SO42-), to strong inner-sphere for arsenate (AsO43-) and phosphate (PO43-). We have developed a kinetic model that describes the impact of oxyanion surface complexes on the rate of Gt and Hm formation during the transformation of Fh. Oxyanion-adsorbed Fh samples were synthesized, buffered at pH 5.5 ± 0.2, and aged for up to 1200 hours at 70 ± 1.5°C. The fractions of Fh, Gt, and Hm during aging were quantified using linear combination fitting of synchrotron pair distribution function data. The results showed that the Fh transformation pathway is influenced by the type of surface complex such that with increasing strength of bonding (e.g., AsO43- and PO43-), less fraction of Gt forms compared with Hm. We propose that surface complexes and surface coverage govern the degree of hydration on the Fh surface, which in turn controls the rate and pathway of Fh transformation and release of oxyanions to solution. These findings are important for understanding and predicting the role of Fh in controlling the transport and fate of metal and metalloid oxyanions in natural and applied systems.

Oxyanion surface complex type and coverage control the pathway and kinetics of ferrihydrite transformation

ACS Spring 2022

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