Theoretical study on the mechanism of super-stable mineralization of LDHs in soil remediation

The problem of soil/water pollution by toxic heavy metals has become increasingly prominent, which pose a serious threat to human production and life. Layered double hydroxides (LDHs) have been used as an excellent mineralizer for the heavy metal cations to form a super-stable mineralization structure. In this work, the models of MII2Al-CO3-LDHs (M = Mg, Cu, Cd, Ca, Fe, Ni, Zn, Co) are built to investigate the driving force of LDHs super-stable mineralization and the mechanism of mineralization of heavy metal cations by Ca2Al-LDH using density functional theory (DFT) and molecular dynamics (MD) simulation methods. The calculation results exhibit that the reasons for the super-stable mineralization of LDHs are that they have the super-smaller solubility products than those of the corresponding hydroxides and carbonates. The chemical binding, electrostatic binding and isomorphic substitution mechanisms of CaAl-LDH for five common heavy metal cations (Cu2+, Ni2+, Zn2+, Co2+, Cd2+) are calculated, as well as the effects of the introduction of H vacancy, OH vacancy and Ca vacancy into (0 0 3) surface on the mechanism is further explored. The results show that the mechanism of LDHs removing heavy metal cations favor the electrostatic binding mechanism regardless of the introduction of vacancy. Compared with the chemical binding mechanism and electrostatic binding mechanism, the energy barrier of isomorphic substitution of heavy metal cations with Ca vacancy is reduced the most, indicating that the presence of Ca vacancy is conducive to the occurrence of isomorphic substitution reaction. This work provides useful theoretical understanding for the construction of LDHs mineralizer and understanding the mechanism for the removal of heavy metals.