Insight into the Simultaneous Super-Stable Mineralization of AsO₄ ^3-, Cd²⁺ and Pb²⁺ Using MgFe-LDHs

Multiple-heavy-metal contamination in soil, such as the simultaneous presence of AsO₄ ^3-, Cd²⁺ and Pb²⁺, which can reduce crop yields and damage human health, is a serious issue to be addressed. Herein, the MgFe-LDHs (layered double hydroxides) intercalated with carbonate and nitrate (MgFe-CO₃ and MgFe-NO₃) were synthesized by Separate Nucleation and Aging Steps and ion-exchange method, respectively. The MgFe-CO₃ demonstrated the maximum saturation adsorption capacity of 55.86, 543.48 and 1597.4 mg g^-1 for single AsO₄ ^3-, Cd²⁺ and Pb²⁺ in aqueous solution, while MgFe-NO₃ exhibited 92.50, 387.59 and 869.56 mg g^-1, respectively. Kinetic and thermodynamic results for mineralization of single AsO₄ ^3-, Cd²⁺ and Pb²⁺ fitted well with the pseudo-second-order kinetic model and Langmuir isotherm model, indicating the occurrence of chemisorption and monolayer adsorption for both MgFe-CO₃ and MgFe-NO₃. Furthermore, simultaneous mineralization of AsO₄ ^3-, Cd²⁺ and Pb²⁺ with >99.0 % efficiency in 240 min in aqueous solution and >81.1 % efficiency in 14 days in soil can be achieved by both MgFe-CO₃ and MgFe-NO₃. Preliminary red bean seedlings cultivation experiments indicated that the released Mg²⁺ ions from MgFe-CO₃ and MgFe-NO₃ were capable to promote the emergence and growth of red bean seedlings. Detailed XRD and XPS results demonstrated that the AsO₄ ^3- anions were adsorbed on the laminate of LDHs, whereas the Pb₃(OH)₂(CO₃)₂ was the mineralization product for both MgFe-CO₃ and MgFe-NO₃. In terms of Cd²⁺, CdCO₃ was obtained as a mineralization product for MgFe-CO₃, while CdCO₃ and Cd(OH)₂ can be detected due to the slow transformation of MgFe-NO₃ to MgFe-CO₃ in air.