Studies on selective adsorbents for oxo-anions.: NO3- adsorptive properties of Ni-Fe layered double hydroxide in seawater

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 (HPO42-, SO42- etc.). NO3- adsorption on LDH (Ni-Fe) was also studied batchwise using NO3- enriched seawater (NO3- concentration: 40 mu mol/dm(3)). 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 dm(3)), 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.