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 Al2O3 as the crystal regulators and different proportions of MgO and Na2CO3. The results show that the MgAl-LDH synthesized with γ-Al2O3 exhibited a CR adsorption capacity of 233 mg/g, significantly surpassing the 197 mg/g achieved by α-Al2O3-derived LDH. The structure and performance are quite different owing to the different processes of synthesizing MgAl-LDH by α-Al2O3 and γ-Al2O3. Mg/Al-LDH formed by α-Al2O3 slow dissolution and Mg2+ precipitation has a high degree of order, whereas Mg/Al-LDH formed by γ-Al2O3 fast forming AlOOH and Mg2+ 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 Al2O3 crystal structure according to the defect-enhanced adsorption mechanism.