Oxygen vacancy-induced MgFe-layered double hydroxide with enhanced magnetism for efficient pollutant removal via magnetic separation

Abstract

Magnetic nanomaterials have long been considered promising candidates for environmental remediation due to their ease of separation and reusability. However, achieving both strong magnetism and high adsorption capacity in a single material has remained a significant challenge, particularly in balancing magnetic properties with structural integrity. In this study, layered double hydroxides (LDHs) with Mg-Fe composition were synthesized using conventional coprecipitation and subsequently chemically treated with sodium borohydride (LDH-V_O). Both pristine LDH and LDH-V_O exhibited well-crystallized pyroaurite structures without impurities, maintaining comparable morphology. To examine changes in the local electronic configuration, a series of spectroscopic analyses were performed. An increased ligand-to-metal transition, redshift of absorption, and hyperchromic effect were observed, indicating the development of oxygen vacancies. The bandgap of LDH-V_O was found to be narrower than that of LDH, likely due to the modified electronic structure following chemical treatment. Additionally, LDH-V_O displayed a lower symmetry order and partial reduction in Fe, reflecting changes in the electronic and local structures. LDH-V_O contained more defective oxygen sites than pristine LDH. Magnetic measurements demonstrated altered magnetic properties in LDH-V_O due to these local changes. At 300 K and 77 K, pristine LDH exhibited paramagnetic behavior with very low magnetic susceptibility, whereas LDH-V_O showed superparamagnetism at 300 K and ferro/ferrimagnetism at 77 K. It was attributed to oxygen vacancies disrupting the Fe–O–Fe super-exchange pathway that typically supports antiferromagnetic interactions. The enhanced magnetic properties were attributed to the induced structural defects and partial reduction of Fe. Additionally, defect engineering increased the specific surface area (S_BET = 65.644 m²/g for LDH, and S_BET = 90.592 m²/g for LDH-VO) and promoted surface heterogeneity (n = 1.50 for LDH, and n = 3.47 for LDH-V_O). The LDH-V_O exhibited efficient pollutant removal (513 mg/g) through magnetic separation, highlighting the crucial role of defect sites in enhancing both magnetic response and adsorption interactions beyond electrostatic forces.