Enhanced immobilization of Cd(II) by successive isomorphic substitution with Ca(II) and Mg(II) from ternary layered double hydroxides.

The challenge with calcium-containing layered double hydroxides (LDHs) for stabilizing Cd(II) lies in the poor release of Ca(II) into solution, which is crucial for isomorphic substitution with Cd(II). To overcome this, we developed a ternary CaMgAl-LDH (OLDH) that enhances cation release and creates more vacant sites for Cd(II). Our findings reveal that OLDH has a significantly higher Cd(II) immobilizing capacity of 5.38 mmol/g compared to CaAl-LDH (4.54 mmol/g) and MgAl-LDH (0.78 mmol/g) at an initial pH of 5.0. This is attributed to its enhanced isomorphic substitution efficiency of 84.6 % and surface complexation of 15.4 %. The solubility product constant (K) of the resulting Cd-containing LDH was 25 orders of magnitude smaller than that of OLDH, indicating a more stable crystalline product due to the presence of Ca(II). Additionally, 100 % of Ca(II) and 35 % of Mg(II) in OLDH were sequentially released to engage in stepwise isomorphic substitution. The inclusion of Mg(II) in OLDH tends to expose under-coordinated divalent metal centers, which was confirmed by experimental observations and density functional theory (DFT) calculations. This research provides a deeper understanding of the mechanism for Cd(II) immobilization by ternary LDHs and offers practical guidance for the preparation of immobilizers with respect to efficient isomorphic substitution.