Insight into the Simultaneous Super-Stable Mineralization of AsO43-, Cd2+ and Pb2+ Using MgFe-LDHs

Multiple-heavy-metal contamination in soil, such as the simultaneous presence of AsO43-, 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 AsO43-, 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 AsO43-, 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 AsO43-, 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 AsO43- anions were adsorbed on the laminate of LDHs, whereas the Pb-3(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.