Three hierarchical porous magnesium borate microspheres: a serial preparation strategy, growth mechanism and excellent adsorption behavior for Congo red

The 3D hierarchical porous 7MgO·2B₂O₃·7H₂O (MBH) microspheres were prepared by a phase transformation of chloropinnoite firstly, and anhydrous α-3MgO·B₂O₃ (MBA) microspheres were obtained by thermal conversion of 7MgO·2B₂O₃·7H₂O, and then β-3MgO·B₂O₃ (MBB) microspheres were obtained by phase conversion of α-3MgO·B₂O₃. All samples were characterized by XRD, FT-IR, TG and SEM. The microsphere nanostructures with a hierarchical porous structure were assembled by nanosheets with a thickness of 20-30 nm, and the growth mechanisms were also proposed. By using N₂ adsorption-desorption, the specific surface areas were measured as 103.62 m² g^-1 for MBH and 46.10 m² g^-1 for MBA. They exhibited excellent selective adsorption performance for Congo red (CR) with maximum adsorption capacities of 202.84 and 170.07 mg g^-1 respectively, and the corresponding adsorption mechanisms were also investigated. The adsorption processes were well fitted with the pseudo-second-order rate equation and Langmuir adsorption model. In addition, the corresponding adsorption thermodynamic parameters were also calculated. It is necessary to highlight that the hierarchical porous microspheres could be considered as promising candidates for removal of CR dye pollutants.