Mechanistic and Thermodynamic Insights into Anion Exchange by Green Rust

Abstract

Fougerite is a naturally occurring green rust, that is, a layered double hydroxide (LDH) containing iron (Fe). Fougerite was identified in natural settings such as hydromorphic soils. It is one of the few inorganic materials with large anion adsorption capacity that stems from the presence of isomorphic substitutions of Fe 2+ by Fe 3+ in its layers. The importance of anion adsorption in the interlayer of LDH has often been highlighted, but we are still missing a mechanistic understanding and a thermodynamic framework to predict the anion uptake by green rust. We combined laboratory and in operando synchrotron X-ray diffraction and scattering experiments with geochemical modeling to contribute to filling this gap. We showed that the overall exchange process in green rusts having nanometer and micrometer sizes can be seen as a simple anion exchange mechanism without dissolution−recrystallization or interstratification processes. A thermodynamic model of ion exchange, based on the Rothmund and Kornfeld convention, made it possible to predict the interlayer composition in a large range of conditions. This multiscale characterization can serve as a starting point for the building of robust and mechanistic geochemical models that will allow predicting the role of green rust on the geochemical cycle of ions, including nutrients, in soils.

Retention of Iodide and Chloride by Formation of a Green Rust Solid Solution GR-Cl1–xIx: A Multiscale Approach

“…This weak interaction between hydrated halides and the octahedral sheet is the origin of the high mobility of these anionic species within the interlayer, 53 thereby explaining why Cl − in GR-Cl can be readily exchanged by, for example, SO 4 2− present in the contacting water. 35 4. CONCLUSIONS AND OUTLOOK GR compounds were prepared by aerial oxidation of Fe(OH) 2 in suspension in the presence of chloride and iodide considering various atomic ratios, at constant ionic strength of halogens.…”

5 Minerals and the Emergence of Life

“…The mineral fougerite, however, belongs to an entirely different structural class. These are the layered anionic clays with solvent-accessible interstitial galleries [47,48,[50][51][52][53][54] (Figure 4). They distinguish themselves from solids such as greigite by several properties particularly pertinent to emergence-of-life scenarios:…”

“…We have described bio-relevant properties of GR in more detail recently [47] and will in the following focus on a few key features. Fougerite as a natural form of GRs is obviously less well-behaved than its synthetic counterparts with occasional Fe-sites being replaced by other divalent metal cations such as Ni, Mg, Al, and Zn [54]. While the mineralogists' GR samples are usually prepared with a single type of interstitial anion (such as chloride, carbonate, sulfate, etc.…”

“…Clay-like assemblages substantially morph in response to the chemical composition of their solvent environment. This is due to their interstitial spaces being contiguous with the outside bulk solvent and the resulting possibility of counterions with strongly differing bulkiness being able to diffuse into these galleries [54]. In GR, the beststudied examples are those of the so-called GR1 and GR2-variants containing chloride and sulfate counterions, respectively.…”

“…Tetragonal anions such as sulfate (and possibly phosphate), by contrast, arrange themselves into two superimposed interlayers [65]. Individual nanocrystals can harbor mixed populations of flat and tetragonal anions resulting in undulated geometries of layer/interlayer stacks (Figure 5 [54,66]).…”