Efficient Removal of Lead, Cadmium, and Zinc from Water and Soil by MgFe Layered Double Hydroxide: Adsorption Properties and Mechanisms

Both biochar and layered double hydroxide (LDH) have drawbacks in regard to the removal of heavy metals. The combined application of biochar and LDH not only solved the problem of the easy agglomeration of LDH but also effectively improved the heavy metal adsorption capacity of biochar. In this work, a MgFe–LDH banana straw biochar composite (MgFe–LDH@BB), with a regular hydrotalcite structure, was synthesized by employing a simple hydrothermal method. The composite showed an ultra-high adsorption capacity for lead (Pb), cadmium (Cd), and zinc (Zn) in water. A series of experiments were conducted to investigate the adsorption characteristics of MgFe–LDH@BB. At pH = 6.0, MgFe–LDH@BB demonstrated the effective adsorption of Pb, Cd, and Zn. In addition, the results showed that the adsorption of Pb, Cd, and Zn by MgFe–LDH@BB was rapid and conformed to pseudo-second-order kinetic and Langmuir models, indicating single-layer chemical adsorption. The maximum adsorption capacity of MgFe–LDH@BB for Pb, Cd, and Zn was 1112.6, 869.6, and 414.9 mg·g−1, respectively. Moreover, the adsorption mechanisms of MgFe–LDH@BB mainly included metal hydroxide/carbonate precipitation, complex formation with hydroxyl groups, and ion exchange. Meanwhile, MgFe–LDH@BB had the ability to immobilize heavy metals in soil. The surface-rich functional groups and cation exchange promoted the transformation of active heavy metal ions into a more stable form.