Fougèrite, a Fe(II)/Fe(III) layered double hydroxide that was identified twenty years ago in hydromorphic soils, is a naturally-occurring form of green rust (GR). GR layers are positively charged because of the occurrence of mixed Fe2+/3+ valence states within the layers. The layer charge is compensated by anions in the interlayer space. These interlayer anions are exchangeable with anions present in the pore water. Where present, GR participate to the biogeochemical cycling of anions, including nutrients or pollutants, in the environment. In this study, we carried out a detailed analysis of green rust ion exchange properties, with the aim of contributing to a better understanding of the biogeochemical cycle of elements in soils. In particular, we investigated the Cl-/SO42- exchange mechanism on two samples having similar layer structure, but particle size distributions either in the nanometer or in the micrometer size range. Samples were characterized by a combination of chemical and physical methods, including synchrotron high-energy X-Ray scattering and synchrotron in situ and time-resolved X-ray diffraction. The replacement of Cl- by SO42- followed a 2:1 stoichiometry, and led to an increase of interlayer distances, in agreement with an anion exchange mechanism without dissolution or recrystallization. Total anion exchange capacity and Cl-/SO42- selectivity coefficients (Kex) were inferred from modelling of aqueous chemistry data. Exchange data were adequately described by an anion exchange thermodynamic model using the Rothmund-Kornfeld convention with a Kex value of 0.58 and a β value of 3 for nanometric samples and 0.14 and a β value of 5 for micrometric samples.