Insight into the Mechanism of Cd Removal by MgAl Layered Double Hydroxides with Different Host-Guest Interactions.

Layered double hydroxides (LDHs) have shown great potential as adsorbents for the removal of heavy metals. Nevertheless, how the host-guest interactions of LDHs affect the removal mechanism remains to be less explored. Herein, CO /NO /SO /Cl intercalated MgAl-LDHs with different host-guest interactions were fabricated and their removal mechanism for Cd was investigated. The removal capacity increased in the order of MgAl-CO (127.3 mg/g)<MgAl-SO (173.3 mg/g)<MgAl-NO (305.0 mg/g)≈MgAl-Cl (312.5 mg/g). The quasi-in-situ XRD and XAS demonstrated that Cd was removed as CdCO for MgAl-CO , while for MgAl-Cl/NO /SO , Cd was removed as CdAl-LDHs by an isomorphic substitution mechanism. DFT calculations revealed that compared to the CdAl-LDHs formation the Gibbs free energy of the CdCO formation was lower, which made it easier to remove Cd as CdCO on MgAl-CO . Conversely, isomorphic substitution of MgAl-NO to obtain CdAl-LDHs was a free energy reduction process.