Studies on selective adsorbents for oxo-anions. Nitrate ion-exchange properties of layered double hydroxides with different metal atoms

Abstract

Layered double hydroxides (LDHs) with different kinds of metal ions (Mg–Al, Co–Fe, Ni–Fe, and Mg–Fe) in the brucite layers were prepared and their anion exchange properties were studied by measurements of distribution coefficient (K_d) and ion exchange capacity. The basal spacing of LDHs varied depending on the kind of metal ions in the brucite layer. A relatively high K_d value for NO₃⁻ ions was observed on Ni–Fe type LDHs, and a markedly high K_d value for the hydrothermally-treated Ni–Fe type LDHs (Ni–Fe (HT)), prepared at 120 °C. These high K_d values correlated with the basal spacing of 0.81 nm observed for this sample, where the interlayer distance (0.33 nm) is suitable for the stable fixing of NO₃⁻ ions (ionic size = 0.33 nm). A chemical analysis study showed a Cl⁻/NO₃⁻ ion-exchange mechanism for NO₃⁻ adsorption on Ni–Fe (HT). The NO₃⁻ uptake by Ni–Fe (HT) was nearly constant (NO₃/Fe = 0.7) over a pH range between 5 and 10, which also supports the adsorption mechanism of Cl⁻/NO₃⁻ ion exchange. The Ni–Fe (HT) could remove NO₃⁻ ions from seawater effectively (NO₃⁻ uptake = 168 μmol g⁻¹) even though seawater contains an extremely large amount of coexisting anions. This novel adsorbent thus has a NO₃⁻ ion-sieve property and will prove beneficial for the selective removal of NO₃⁻ ions from industrial effluents and seawater to clean the environment.