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

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 degreesC. 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 mumol 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.