Enhanced Congo red adsorption on defective Mg/Al-layered double hydroxides synthesized with varied Al₂O₃ crystal regulators

Water pollution caused by dyes released during modern industrialization and urbanization is a serious problem. In this work, a defective Mg/Al-layered double hydroxide (MgAl-LDH) with a high removal efficiency for Congo red (CR) was fabricated by hydrothermal synthesis under closed conditions using different crystal forms of nano Al 2 O 3 as the crystal regulators and different proportions of MgO and Na 2 CO 3 . The results show that the MgAl-LDH synthesized with γ-Al 2 O 3 exhibited a CR adsorption capacity of 233 mg/g, significantly surpassing the 197 mg/g achieved by α-Al 2 O 3 -derived LDH. The structure and performance are quite different owing to the different processes of synthesizing MgAl-LDH by α-Al 2 O 3 and γ-Al 2 O 3 . Mg/Al-LDH formed by α-Al 2 O 3 slow dissolution and Mg 2+ precipitation has a high degree of order, whereas Mg/Al-LDH formed by γ-Al 2 O 3 fast forming AlOOH and Mg 2+ precipitation has a lower degree of order and oxygen defects. This difference in the degree of ordering and oxygen defects is a key factor for CR adsorption onto Mg/Al-LDH, and can be adjusted by changing the Al 2 O 3 crystal structure according to the defect-enhanced adsorption mechanism.