Retention of Iodide and Chloride by Formation of a Green Rust Solid Solution GR-Cl_1–xI_x: A Multiscale Approach

Abstract

The uptake of iodide and chloride during the synthesis of green rust (GR), the Fe endmember of the layered double hydroxide (LDH) group, was investigated. GR compounds were prepared by aerial oxidation of Fe­(OH)2 in suspension, considering various I/Cl ratios at constant ionic strength. Only GR compounds formed in all experiments, and the associated I/Cl ratio increased with that of the starting suspension. No preferential uptake of any halide could be detected, and all compounds had comparable morphology. Furthermore, the height of the interlayer gallery increased with the I/Cl ratio from ∼7.7 Å for the chloride endmember to ∼8.3 Å for the iodide endmember, and the observed linear increase was attributed to increasing interlayer iodide content. In all compounds, Fe K-edge X-ray absorption spectroscopy evidenced the presence of sixfold coordinated iron with a Fe2+/Fe3+ ratio of 3, homogeneously distributed within flattened octahedral sites, with six Fe as next-nearest neighbors. The Fe short-range environment was not affected by the interlayer composition, and no halide from the interlayer could be detected. Furthermore, iodide and chloride anions are located in a water-like environment, being loosely bound by weak electrostatic interactions to the octahedral sheet likely above ferric iron. Results consistently hint at the formation of a solid solution between chloride and iodide GR endmembers, certainly facilitated by the crystallization of both compounds in the same space group. This study provides further insights into the ability of LDH to simultaneously accommodate several anionic species of various sizes. The formation of such LDH compounds in a deep geological repository for nuclear waste thus represents a possible retention barrier to the migration to the far field of anionic species like 36Cl– and 129I– mobilized from the waste matrix. The extent of retention in disposal sites will depend, among others, on the availability of GR and on the concentration of competing anions.

Retention of Iodide by Chloride Green Rust and Magnetite

“…According to these results, there is no significant preference for one anion over the other (chloride or iodide) for uptake. These results compare well with previously reported solid solution experiments, where the Cl – /I – ratio in the GR interlayer was the same as in the contacting aqueous phase. , Data from this study further show that the absence of selective uptake of one of these two anions is also valid at higher ionic strength up to 5 mol/kg (Figure ). This experimental finding is also well captured by the solid solution model.…”

“…X-ray diffractograms of the synthesized GR compounds (Figure S1) evidence the presence of only green rust minerals, and electron micrographs show that samples are made of thin hexagonal platelets (Figure S3). These results compare well with reported findings for GR-Cl samples prepared by precipitation from ferric and ferrous ions or by oxidation of Fe­(OH) 2 . , The synthesized magnetite is made of octahedral crystals of 100–200 nm, and the crystal structure was assessed by XRD analysis (Figures S2 and S4).…”

“…According to the results, changes of pH m or ionic strength do not affect the hydration shell made of six water molecules binding iodide via H atoms. Overall, XAS data are consistent with a weak electrostatic interaction with the Fe octahedral sheet, comparable to that reported in earlier studies of iodide uptake by formation of a GR-Cl 1– x I x solid solution …”

“…GR and magnetite samples have been prepared by titrating a stirred solution containing Fe­(II) and Fe­(III) with a 1 M NaOH solution until pH m 7.5 (GR, Fe­(II)/Fe­(III) = 4:1) or 9.5 (magnetite, Fe­(II)/Fe­(III) = 1:2). Although the Fe­(II)/Fe­(III) ratio in GR-Cl is close to 3, a slightly higher ratio was used to prevent the formation of magnetite and to favor the GR formation . After aging for 1 day, the suspension was first washed by centrifuging for 10 min at 3,500 rpm (2,000 g ) and replacing the supernatant by UPW.…”

“…The preparation of iodide-containing GR (GR-I) and of a complete solid solution between the isostructural iodide and chloride GR endmembers has been reported. , However, the concentrations of dissolved iodide used in these studies substantially exceed the amounts expected in the case of canister failure in a DGR. The aim of this study is to investigate the sorption behavior of iodide by preformed GR-Cl at different pH m , contact times, and ionic strengths using more environmentally relevant iodide concentrations.…”