Insight into the Mechanism of Cd2+ Removal by MgAl Layered Double Hydroxides with Different Host‐Guest Interactions

Abstract

AbstractLayered 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, CO32−/NO3−/SO42−/Cl− intercalated MgAl‐LDHs with different host‐guest interactions were fabricated and their removal mechanism for Cd2+ was investigated. The removal capacity increased in the order of MgAl−CO3 (127.3 mg/g)<MgAl−SO4 (173.3 mg/g)<MgAl−NO3 (305.0 mg/g)≈MgAl−Cl (312.5 mg/g). The quasi‐in‐situ XRD and XAS demonstrated that Cd2+ was removed as CdCO3 for MgAl−CO3, while for MgAl−Cl/NO3/SO4, Cd2+ 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 CdCO3 formation was lower, which made it easier to remove Cd2+ as CdCO3 on MgAl−CO3. Conversely, isomorphic substitution of MgAl−NO3 to obtain CdAl‐LDHs was a free energy reduction process.