Sorption of cadmium by layered double hydroxides: Performance, structure-related mechanisms, and sequestration stability assessment

Layered double hydroxides (LDHs) have been recognized to have great potential for the treatment of heavy metals in wastewater and soil through various mechanisms. Isomorphic substitution is an important mechanism for the sorption of heavy metal cations with LDH reconstruction and highly stable product formation. However, sorption performance, structure-related relationships, and, more importantly, stability are still poorly understood. In this study, a series of LDHs with different structures were synthesized to evaluate their cadmium (Cd) sorption performance and stability concerning the isomorphic substitution mechanism. Divalent cation types in the LDH lattice determined the Cd sorption capacity as well as the isomorphic substitution possibility, following the order of hydroxide solubility of divalent cations (MII): Ca2+>Mg2+>(Cd2+) > Ni2+>Zn2+. In addition, CaAl-LDH exhibited a super-high Cd sorption capacity of 625.0 mg g-1. Cd sorption by LDHs with different interlayer anion types and divalent/trivalent cation molar ratios varied due to crystallite size-related MII release through cation-exchange/isomorphic substitution. Coexisting cations (e.g., Zn2+, Ni2+, Mg2+) influence the sorption performance of MII-LDH mainly through isomorphic substitution mechanism, largely depending on the solubility of MII(OH)2 with a trend of stable product formation. Furthermore, Mg2.9Cd0.1AlCl-LDH was fabricated, and limited Cd dissolution without destruction of the LDH structure was observed under various conditions. For example, only 7.69%, 2.16% and 0.96% of Cd was released from as-prepared Mg2.9Cd0.1AlCl-LDH in NaCl solution (0.02 mol L-1, pH 5), soil extract, and soil matrix, respectively. The very low leaching of Cd from Cd-containing LDHs indicated the high stability of LDH-sorbed Cd via isomorphic substitution and feasible practical application in Cd sequestration in wastewater treatment and soil remediation.