Adsorption And Ion-exchange Behavior Of Layered Double Hydroxides In The Uptake Of Halide Anions From Aqueous Solution

Layered double hydroxides (LDHs) or hydrotalcite-like compounds (HTlc) are a class of anionic clays with the general formula [M_1-x²⁺M_x³⁺(OH)₂]^x+(A(n-))_x/n·mH₂O, where M²⁺ and M³⁺ are metal cations that occupy octahedral sites in the hydroxide layers, while A^n- is an exchangeable anion. LDHs possess relatively large surface areas and high anion-exchange capacities. The exchange efficiency of LDHs varies significantly with the properties of the original interlayer anions and incoming anions, particularly their size and geometrical structure. A wide variety of anionic species including monatomic inorganic anions, oxoanions and organic anions can be intercalated into LDHs interlayer galleries.Uptake of F^-, Cl^-, Br^- and I^- from aqueous solution by M(II)Al-NO₃ LDHs was investigated, taking advantage of the exchangeable nature of the nitrate anion. The removal efficiency was found to increase with increasing charge density of the layers, due to the increase in ion-exchange capacity of LDHs. The nature of the ion-exchange phenomenon was investigated by X-ray diffraction, FT-IR spectroscopy, and TG-MS measurements.The maximum uptake capacities of LDHs for F^-, Cl^-, Br^- and I^- were found to be 88.7, 170.6, 483.1 and 295.9 mg/g respectively. The uptake capacity for I^- is much lower than the ion-exchange capacity of LDHs, associated with the large size and low charge density of the anion. The negative values of â–³H⁰ in each case confirm the strong attraction of the CLDH for the halide anions. For uptake of F^- and Cl^- by CLDH, the...