Mechanisms of Se(IV) Co-precipitation with Ferrihydrite at Acidic and Alkaline Conditions and Its Behavior during Aging

Abstract

Understanding the form of Se(IV) co-precipitated with ferrihydrite and its subsequent behavior during phase transformation is critical to predicting its long-term fate in a range of natural and engineered settings. In this work, Se(IV)-ferrihydrite co-precipitates formed at different pH were characterized with chemical extraction, transmission electron microscopy (TEM), and X-ray absorption spectroscopy (XAS) to determine how Se(IV) is associated with ferrihydrite. Results show that despite efficient removal, the mode and stability of Se(IV) retention in the co-precipitates varied with pH. At pH 5, Se(IV) was removed dominantly as a ferric selenite-like phase intimately associated with ferrihydrite, while at pH 10, it was mostly present as a surface species on ferrihydrite. Similarly, the behavior of Se(IV) and the extent of its retention during phase transformation varied with pH. At pH 5, Se(IV) remained completely associated with the solid phase despite the phase change, whereas it was partially released back into solution at pH 10. Regardless of this difference in behavior, TEM and XAS results show that Se(IV) was retained within the crystalline post-aging products and possibly occluded in nanopore and defect structures. These results demonstrate a potential long-term immobilization pathway for Se(IV) even after phase transformation. This work presents one of the first direct insights on Se(IV) co-precipitation and its behavior in response to iron phase transformations.

Evolution of the reactive surface area of ferrihydrite: time, pH, and temperature dependency of growth by Ostwald ripening

“…For this reason, it is used and widely studied as model material to elucidate the adsorption of cations, anions, and organic matter. [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19] The surface area of ferrihydrite is an essential characteristic particularly from the perspective of surface complexation modelling. One reason is that ferrihydrite particles are usually charged.…”

Comparing Schwertmannite and Hydrobasaluminite Dissolution in Ammonium Oxalate (pH 3.0): Implications for Metal Speciation Studies by Sequential Extraction

“…The implications of our study are relevant because they could reveal specific metal-mineral associations and dissipate doubts about metal transport and storage in AMD-affected soils and sediments. The relevance of our results can also expand to other environments where amorphous Al-Fe phases play an important role in metal mobility, such as naturally contaminated rocks, aquifers and soils [23][24][25][26][27][28].…”

Retention and multiphase transformation of selenium oxyanions during the formation of magnetite via iron(ii) hydroxide and green rust

“…It is known that mineral formation processes can positively affect the immobilization of dissolved species. In case of selenium oxyanions, this was already demonstrated for the formation of hematite or goethite via ferrihydrite 34,35 .…”