Studies on Selective Adsorbents for Oxo-Anions. NO₃⁻ Adsorptive Properties of Ni-Fe Layered Double Hydroxide in Seawater

Abstract

We report the selective adsorptive properties for NO₃⁻ in seawater of Ni-Fe layered double hydroxide (LDH (Ni-Fe)), containing Ni and Fe metal atoms in each layer. LDH (Ni-Fe) with Cl⁻ in the interlayers was synthesized by co-precipitation at constant pH. It showed a higher distribution coefficient (K _d ) for NO₃⁻ than for other anions (HPO₄^{2 −}, SO₄^{2 −} etc.). NO₃⁻ adsorption on LDH (Ni-Fe) was also studied batchwise using NO₃⁻ enriched seawater (NO₃⁻ concentration: 40 μ mol/dm³). The NO₃⁻ uptake attained equilibrium after the adsorption for 4 h. The adsorption isotherm followed Freundlich’s equation. The NO₃⁻ uptake reached 0.33 mmol/g when LDH (Ni-Fe) (0.10 g) was added to NO₃⁻ enriched seawater (1 dm³), corresponding to the removal of 83% NO₃⁻ from seawater. The pH dependence of NO₃⁻ adsorption showed a maximum NO₃⁻ uptake at around pH 8 with low Ni dissolution (less than 0.6%). This adsorbent is therefore promising for the selective removal of NO₃⁻ from seawater for the benefit of the environment.