Adsorption and Reduction of Arsenate during the Fe²⁺-Induced Transformation of Ferrihydrite

Abstract

Iron (oxyhydr)­oxides play an important role in controlling the mobility and toxicity of arsenic (As) in contaminated soils and groundwaters. Dynamic changes in subsurface geochemical conditions can impact As sequestration and remobilization since the fate of As is highly dependent on the dominant iron mineral phases present and, specifically, the pathways through which these form or transform. To assess the fate of arsenate [As­(V)] in subsurface settings, we have investigated the Fe2+-induced transformation of As­(V)-bearing ferrihydrite (As­(V)-FH) to more crystalline phases under environmentally relevant anoxic subsurface conditions. Specifically, we examined the influence of varying Fe2+ (aq)/Fe­(III)solid ratios (0.5, 1, 2) on the behavior and speciation of mineral-bound As species during the transformation of As­(V)-FH to crystalline iron-bearing phases at circumneutral pH conditions. At all Fe2+ (aq)/Fe­(III)solid ratios, goethite (GT), green rust sulfate (GRSO4), and lepidocrocite (LP) formed within the first 2 h of reaction. At low ratios (0.5 to 1), initially formed GRSO4 and/or LP dissolved as the reaction progressed, and only GT and some unreacted FH remained after 24 h. At Fe2+ (aq)/Fe­(III)solid ratio of 2, GRSO4 remained stable throughout the 24 h of reaction, alongside GT and unreacted As­(V)-FH. Despite the fact that majority of the starting As­(V)-FH transformed to other phases, the initially adsorbed As was not released into solution during the transformation reactions, and ∼99.9% of it remained mineral-bound. Nevertheless, the initial As­(V) became partially reduced to As­(III), most likely because of the surface-associated Fe2+-GT redox couple. The extent of As­(V) reduction increased from ∼34% to ∼40%, as the Fe2+ (aq)/Fe­(III)solid ratio increased from 0.5 to 2. Overall, our results provide important insights into transformation pathways of iron (oxyhydr)­oxide minerals in As contaminated, anoxic soils and sediments and demonstrate the impact that such transformations can have on As mobility and also importantly oxidation state and, hence, toxicity in these environments.

Effect of Sb on precipitation of biogenic minerals during the reduction of Sb-bearing ferrihydrites

“…They therefore contribute significantly to the speciation and the fate of elements (e.g. As, Sb) that play a central role in various biogeochemical cycles in soils and sediments (Dixit and Hering, 2003;Borch and Fendorf, 2007;Mitsunobu et al, 2008a;Frierdich and Catalano, 2012a;Ilgen and Trainor, 2012;Okkenhaug et al, 2013;Mallet et al, 2013;Perez et al, 2019). Due to their large specific surface area, iron oxides are considered to be an effective sink for Sb in shooting range soils (Okkenhaug et al, 2016;Okkenhaug et al, 2018).…”

Further insights into the Fe(ii) reduction of 2-line ferrihydrite: a semi in situ and in situ TEM study

“…It's oen used in the removal and stabilization (under ideal selective conditions) of a range of toxic elements in natural and man-made environments through adsorption and co-precipitation. 1,2,[9][10][11][12] The biotic reduction of iron(III) oxy-hydroxides such as 2-line FH by Fe-reducing bacteria plays an important role as an electron shuttle in various redox processes that control the biogeochemical cycling of transition and metalloid elements (e.g. Fe, As, Se, U, Mn) as well as organic matter/nutrients (e.g.…”

A Closer Look at Fe(II) Passivation of Goethite

“…For example, contaminant reduction by Fe oxides in the presence of Fe­(II) has been suggested to involve more than sorbed Fe­(II) simply being a better reductant than aqueous Fe­(II), but rather that electron transfer between sorbed Fe­(II) and the mineral creates a more thermodynamically stable iron oxide that makes the oxidation of Fe­(II) more thermodynamically favorable. Secondary mineral transformation of metastable Fe oxides such as ferrihydrite to goethite or magnetite, and lepidocrocite to magnetite − are also now thought to involve an initial electron transfer step, , and electron transfer current density has been suggested to control mineral transformation . Another significant and perhaps more controversial conceptual shift arising from experimental validation of Fe­(II)–Fe­(III) oxide electron transfer is the idea that Fe­(II) catalyzes Fe oxide recrystallization of stable Fe oxide phases (often referred to as Fe atom exchange).…”