Ion exchange in green rust: Influence of crystal size and structural defects

Green rust (GR) is a Fe(II)/Fe(III) layered double hydroxide (LDH). Its layers are positively charged because of the occurrence of mixed Fe2+/Fe3+ valence states. The charge is compensated by exchangeable anions located in the interlayer space. Green rust is one of the few inorganic materials with large anion adsorption capacity. The importance of this anion adsorption capacity of LDH in general has been highlighted in many studies, but we are still missing a comprehensive mechanistic understanding and a rigorous thermodynamic framework to predict anion exchange processes in natural and engineered environments where green rust is present.
We combined for the first time laboratory experiments and synchrotrons in situ and time resolved X-ray diffraction measurements with geochemical modeling to decipher and quantify the mechanism of anion exchange in green rust having nanometer or micrometer sizes. We studied more specifically the Cl-:SO42- exchange.
We showed that the overall exchange process can be seen as a simple anion exchange mechanism without dissolution-recrystallization or interstratification processes. The replacement of Cl- by SO42- followed a 2:1 stoichiometry, and led to an increase of interlayer distances from 8.0 ± 0.1 Å to 11.0 ± 0.1 Å. An increase of SO42- occupancy in the interlayer led to an increase of the relative affinity of the exchanger for SO42- compared to Cl-. An ion exchange thermodynamic model based on the Rothmund and Kornfeld convention made it possible to predict the interlayer composition in a large range of conditions. The model parameters were different for the nanometric GR particles and the micrometric GR particles. This difference of affinity may originate from differences in layer stacking order in the two types of samples, the nanometric particles being turbostratic while the micrometric particles being 3D-ordered. We consider that 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 hydromorphic soils.