Insight into the Mechanism of Cd<sup>2+</sup> Removal by MgAl Layered Double Hydroxides with Different Host‐Guest Interactions
Qian Li, Zhaohui Wu, Sha Bai, Yingjie Liu, Jiaxin Li, Yu‐Fei Song
Abstract
Abstract 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 3 2− /NO 3 − /SO 4 2− /Cl − intercalated MgAl‐LDHs with different host‐guest interactions were fabricated and their removal mechanism for Cd 2+ was investigated. The removal capacity increased in the order of MgAl−CO 3 (127.3 mg/g)<MgAl−SO 4 (173.3 mg/g)<MgAl−NO 3 (305.0 mg/g)≈MgAl−Cl (312.5 mg/g). The quasi‐in‐situ XRD and XAS demonstrated that Cd 2+ was removed as CdCO 3 for MgAl−CO 3 , while for MgAl−Cl/NO 3 /SO 4 , Cd 2+ 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 3 formation was lower, which made it easier to remove Cd 2+ as CdCO 3 on MgAl−CO 3 . Conversely, isomorphic substitution of MgAl−NO 3 to obtain CdAl‐LDHs was a free energy reduction process.