Structural Incorporation of As⁵⁺ into Hematite

Abstract

Hematite (α-Fe2O3) is one of the most common iron oxides and a sink for the toxic metalloid arsenic. Arsenic can be immobilized by adsorption to the hematite surface; however, the incorporation of As in hematite was never seriously considered. In our study we present evidence that, besides adsorption, the incorporation of As into the hematite crystals can be of great relevance for As immobilization. With the coupling of nanoresolution techniques and X-ray absorption spectroscopy the presence of As (up to 1.9 wt %) within the hematite crystals could be demonstrated. The incorporated As(5+) displays a short-range order similar to angelellite-like clusters, epitaxially intergrown with hematite. Angelellite (Fe4As2O11), a triclinic iron arsenate with structural relations to hematite, can epitaxially intergrow along the (210) plane with the (0001) plane of hematite. This structural composite of hematite and angelellite-like clusters represents a new immobilization mechanism and potentially long-lasting storage facility for As(5+) by iron oxides.

Fate of adsorbed arsenate during phase transformation of ferrihydrite in the presence of gypsum and alkaline conditions

“…Again, the larger "a" and "c" values in the ferrihydrite-gypsum-arsenate system are attributed to As being structurally incorporated into the hematite lattice. A similar observation was made by Bolanz et al (2013), who reported a positive correlation between the "c" parameter of hematite and increasing concentrations of As in the hematite structure. However, they were unable to identify a correlation between the "a" parameter and As concentrations.…”

“…In spite of this transformation, aqueous As concentrations decrease (Das et al, 2014). This observation is supported by other recent studies and attributed to the structural incorporation of As into hematite (Bolanz et al, 2013;Muramatsu et al, 2012). Although the mechanism of this incorporation and bonding environment is different (due to different experimental conditions: pH, co-precipitation vs. adsorption, Fe/As molar ratios), all of these studies indicate that As should not be released to the aqueous phase if ferrihydrite transforms to hematite.…”

Fire Promotes Arsenic Mobilization and Rapid Arsenic(III) Formation in Soil via Thermal Alteration of Arsenic-Bearing Iron Oxides

“…For the ferrihydrite:soil mixture, As(III) aq and total As aq mobilization was greatest for higher temperature/longer duration heating treatments. This may partly be a consequence of increasing abundance of solid-phase As(III), for despite the fact that both As(III) and As(V) species can efficiently adsorb to a wide range of Fe-(hydr)oxides (Dixit and Hering, 2003;Morin et al, 2008;Burton et al, 2009;Bolanz et al, 2013), As(III) species are typically desorbed more readily (Herbel and Fendorf, 2006;Fendorf et al, 2010b). In addition, while ferrihydrite can be a very efficient host for both As(V) and As(III) (Dixit and Hering, 2003), maghemite is generally less efficient and its efficacy is highly contingent upon particle size (greater sorption efficiency for smaller particles) (Auffan et al, 2008).…”

Rare elements — setting markers of the formation of the manganese and iron ore deposits of Kalahari and Postmasburg areas (South Africa). Communication 1. Kalahari manganese field

“…Особо следует отметить данные о присутствии As 5+ в кристаллах гематита (до 1.9 масс. %) в виде ангеллитовых кластеров, эпитаксически срастающихся с гематитом [Bolanz et al, 2013]. Это свидетельствует о том, что наряду с сорбцией As 5+ происходит его инкорпорация в гематит.…”