Browsing by Author "Namayandeh, Alireza"

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    Effects of Oxyanion Surface Loading on the Rate and Pathway of Ferrihydrite Transformation
    Namayandeh, Alireza; Borkiewicz, Olaf J.; Bompoti, Nefeli M.; Watson, Steven K.; Kubicki, James D.; Chrysochoou, Maria; Michel, F. Marc (American Chemical Society, 2023-10-19)
    In natural environments, ferrihydrite (Fh) reacts readily with the contaminant and nutrient oxyanions through surface complexation. While previous experiments showed that the transformation of Fh to Gt and Hm under oxic conditions at 70 °C is controlled by the type and strength of oxyanion surface complexes, the impact of surface loading on this process is only partly understood. Synchrotron scattering methods and chemical analysis were used to develop a kinetic model that describes the impact of oxyanion surface loading on the rate and pathway of Fh transformation by using arsenate (AsO43-) and phosphate (PO43-). Kinetic modeling showed that AsO43- and PO43- adsorption decreased the rate of transformation and favored Hm formation over Gt. Higher surface loadings increasingly inhibited Fh transformation with a greater effect for PO43- compared with AsO43-. This information has implications for understanding the impacts of oxyanions on the transformation of natural Fe to Gt and Hm in environmental systems.
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    Formation and transformation kinetics of iron oxy-hydroxides and effects of adsorbed oxyanions
    Namayandeh, Alireza (Virginia Tech, 2022-09-20)
    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.