Ion exchange and intercalation properties of layered double hydroxides towards halide anions

Abstract

The selectivity towards halides and the mechanism of ion exchange of ZnAl-LDH in the chloride form (X ⁻ = F ⁻, Br ⁻, I ⁻) were investigated using ion exchange isotherms of Cl ⁻/X ⁻.

Abstract

A layered double hydroxide (LDH) obtained by the urea method, having an empirical formula [Zn _0.61Al _0.39(OH) ₂](CO ₃) _0.195·0.50H ₂O, has been converted into the corresponding chloride form [Zn _0.61Al _0.39(OH) ₂]Cl _0.39·0.47H ₂O by making the solid come into contact with a suitable HCl solution. The intercalation of the other halide anions (X ⁻ = F ⁻, Br ⁻, I ⁻) via the Cl ⁻/X ⁻ anion exchange has been attained and the respective anion exchange isotherms have been obtained with the batch method. The analysis of the isotherms indicates that the selectivity of LDH towards the halides decreases with the increase of the X ⁻ ionic radius, the selectivity order being F ⁻ > Cl ⁻ ≥ Br ⁻ > I ⁻. The CO ₃ ²⁻/Cl ⁻ isotherm has also been reported to highlight the extraordinary selectivity of LDH towards carbonate anions. Samples taken from the isotherms at different exchange degrees were analyzed by X-ray diffraction, thermogravimetry and thermodiffractometry to obtain information about the ion exchange mechanism. The Cl ⁻/Br ⁻ and the reverse Br ⁻/Cl ⁻ exchanges occur with the formation of solid solutions, very likely because of the similar ionic radius of the exchanging anions. In contrast, in the Cl ⁻/F ⁻ and Cl ⁻/I ⁻ exchange, the co-existence of the Cl ⁻ and F ⁻ (or I ⁻) phases in the same sample was detected, indicating the occurrence of a first order phase transition, in which the starting phase is transformed into the final phase, as the process goes on. The variation of the interlayer distances of ZnAl–X intercalation compounds with the hydration degree has been interpreted with a structural model based on the nesting of the guest species into the trigonal pockets of the brucite-like layer surface. Rietveld refinements of the phases with the maximum F ⁻, Br ⁻ and I ⁻ content were also performed and compared with the above model, giving indications of the arrangement and order/disorder of the halide anions in the interlayer region.