Layered double hydroxides as potential chromate scavengers

The LDH of Ni with Fe, having the formula Ni(1-x)Fe(x)(OH)2(A(n-))(x/n)yH2O (A = NO3-, Cl-; x = 0.25, 0.33), scavenges CrO4(2-) ions from solution throughout the concentration range examined (0.00625-0.25 N). The CrO4(2-) uptake capacity is independent of the anion in the starting LDH but is higher when x = 0.25 (3.60 meq g(-1)) as compared to x = 0.33 (2.40 meq g(-1)). These values are higher than those observed for control compounds beta-Ni(OH)2 (1.86 meq g(-1)) and FeO(OH) (1.26 meq g(-1)), which do not have any interlayer chemistry, showing that chromate uptake takes place by its incorporation in the interlayer region by a stoichiometric anion-exchange reaction, rather than by adsorption. Nevertheless, the interaction between the LDH and the chromate ions is weak. The weak interaction is due to the mismatch between the symmetry of the chromate ions and the symmetry of the interlayer site, which introduces turbostratic disorder in the chromate-intercalated LDHs. The chromate ions can be completely leached out by soaking the LDH in a sodium carbonate solution.