Intercalation of azamacrocyclic crown ether into layered rare-earth hydroxide (LRH): secondary host-guest reaction and efficient heavy metal removal.

A carboxyethyl substituted azacrown ether (CSAE) derivative was intercalated as a second host into a parent host of layered gadolinium hydroxides (LGdH) by an anion-exchange reaction. The influence of intercalation temperature and starting material ratios of CSAE/LRH on the structures and compositions of CSAE-LRH nanocomposites were investigated. Higher temperature and larger initial CSAE-LGdH weight ratios favor of higher degree of ion exchange at a certain range, while lower temperature gives good morphology for the composites. The adsorptive properties for transition and heavy metal ions were studied using the 20 °C-reacted composite, which showed higher adsorptivity toward transition and heavy metal ions, accompanied by the introduction of nitrate anions. The adsorptive capacity for transition metal ions was in the sequence of Cu(2+) > Zn(2+)∼Ni(2+)∼Co(2+) with a high selectivity to Cu(2+). For the heavy metal ions Ag(+), Hg(2+), Pb(2+), and Cd(2+), the composite showed markedly high selectivity for Ag(+) and Hg(2+). When putting Cu(2+), Ag(+), Hg(2+), Pb(2+), and Cd(2+) together, Ag(+) and Hg(2+) still have higher adsorptive selectivity over Pb(2+) and Cd(2+), and Cu(2+) has also relatively high selectivity but not as high as Ag(+) and Hg(2+). The nanocomposites with a second host in the interlayer are one promising kind of material because of the synergy of the steric effect of the parent host (LRH layer) and the particular characteristics of the secondary host (interlayer crown ether anions).