The use of magnetic composites in wastewater treatment has become widespread due to their high flocculating characteristics and ferromagnetism. This review provides an analysis and summary of the preparation and application of magnetic composites through controllable assembly for use in wastewater treatment. The applications of magnetic composites include the treatment of dye wastewater, heavy metal wastewater, microalgae suspensions, and oily wastewater. Additionally, the recycling and regeneration of magnetic composites have been investigated. In the future, further research could be focused on improving the assembly and regeneration stability of magnetic composites, such as utilizing polymers with a multibranched structure. Additionally, it would be beneficial to explore the recycling and regeneration properties of these composites.

In order to separate the colloidal in high-turbidity water, a kind of magnetic composite (Fe₃O₄/HBPN) was prepared via the functional assembly of Fe₃O₄ and an amino-terminal hyperbranched polymer (HBPN). The physical and chemical characteristics of Fe₃O₄@HBPN were investigated by different means. The Fourier Transform infrared spectroscopy (FTIR) spectra showed that the characteristic absorption peaks positioned at 1110 cm⁻¹, 1468 cm⁻¹, 1570 cm⁻¹ and 1641 cm⁻¹ were ascribed to C–N, H–N–C, N–H and C=O bonds, respectively. The shape and size of Fe₃O₄/HBPN showed a different and uneven distribution; the particles clumped together and were coated with an oil-like film. Energy-dispersive spectroscopy (EDS) displayed that the main elements of Fe₃O₄/HBPN were C, N, O, and Fe. The superparamagnetic properties and good magnetic response were revealed by vibrating sample magnetometer (VSM) analysis. The characteristic diffraction peaks of Fe₃O₄/HBPN were observed at 2θ = 30.01 (220), 35.70 (311), 43.01 (400), 56.82 (511), and 62.32 (440), which indicated that the intrinsic phase of magnetite remained. The zeta potential measurement indicated that the surface charge of Fe₃O₄/HBPN was positive in the pH range 4–10. The mass loss of Fe₃O₄/HBPN in thermogravimetric analysis (TGA) proved thermal decomposition. The –C–NH₂ or –C–NH perssad of HBPN were linked and loaded with Fe₃O₄ particles by the N–O bonds. When the Fe₃O₄/HBPN dosage was 2.5 mg/L, pH = 4–5, the kaolin concentration of 1.0 g/L and the magnetic field of 3800 G were the preferred reaction conditions. In addition, a removal efficiency of at least 86% was reached for the actual water treatment. Fe₃O₄/HBPN was recycled after the first application and reused five times. The recycling efficiency and removal efficiency both showed no significant difference five times (p > 0.05), and the values were between 84.8% and 86.9%.

To enhance the physical and chemical properties and stability of magnetic MOFs, and to improve their adsorption and reuse performance, an innovative modification strategy involving hybridization with three-dimensional (3D) terminal carboxyl hyperbranched polymers was employed. A novel polymer composite material, Fe₃O₄@MOF/HBPC, was synthesized and its morphological characteristics were analyzed. The adsorption properties of Fe₃O₄@MOF/HBPC were tested by removing Malachite Green (MG) from aqueous solutions. Various influencing factors, including dosage, pH, and temperature, as well as adsorption kinetics, isotherms, and thermodynamics were investigated. The results showed that the stretching vibration peaks of Fe₃O₄@MOF/HBPC appeared at 1708 cm⁻¹, 1408 cm⁻¹, 1278 cm⁻¹, 1112 cm⁻¹, and 543 cm⁻¹, corresponding to the CO, –COO⁻, O–H, C–O, and Fe–O bands, respectively. Fe₃O₄@MOF/HBPC exhibited a negative charge and a spherical structure. The composite material contained the elements O, C, Fe, and N, and the assembly process had a minimal impact on the crystal structure. For MG removal, the optimal reaction conditions were a dosage of 20 mg and pH 6. The adsorption process followed a pseudo-first-order kinetic model and a Langmuir isotherm model, and it was found to be endothermic and spontaneous, involving both physical and chemical adsorption. Regarding recycling and reuse, Fe₃O₄@MOF/HBPC demonstrated a recycling efficiency of over 90.8%, with a removing efficiency of MG of above 87.5% after three cycles, indicating excellent stability and reusability.

In order to improve dispersibility, polymerization characteristics, chemical stability, and magnetic flocculation performance, magnetic Fe₃O₄ is often assembled with multifarious polymers to realize a functionalization process. Herein, a typical three-dimensional configuration of hyperbranched amino acid polymer (HAAP) was employed to assemble it with Fe₃O₄, in which we obtained three-dimensional hyperbranched magnetic amino acid composites (Fe₃O₄@HAAP). The characterization of the Fe₃O₄@HAAP composites was analyzed, for instance, their size, morphology, structure, configuration, chemical composition, charged characteristics, and magnetic properties. The magnetic flocculation of kaolin suspensions was conducted under different Fe₃O₄@HAAP dosages, pHs, and kaolin concentrations. The embedded assembly of HAAP with Fe₃O₄ was constructed by the N–O bond according to an X-ray photoelectron energy spectrum (XPS) analysis. The characteristic peaks of –OH (3420 cm⁻¹), C=O (1728 cm⁻¹), Fe–O (563 cm⁻¹), and N–H (1622 cm⁻¹) were observed in the Fourier transform infrared spectrometer (FTIR) spectra of Fe₃O₄@HAAP successfully. In a field emission scanning electron microscope (FE-SEM) observation, Fe₃O₄@HAAP exhibited a lotus-leaf-like morphological structure. A vibrating sample magnetometer (VSM) showed that Fe₃O₄@HAAP had a relatively low magnetization (Ms) and magnetic induction (Mr); nevertheless, the ferromagnetic Fe₃O₄@HAAP could also quickly respond to an external magnetic field. The isoelectric point of Fe₃O₄@HAAP was at 8.5. Fe₃O₄@HAAP could not only achieve a 98.5% removal efficiency of kaolin suspensions, but could also overcome the obstacles induced by high-concentration suspensions (4500 NTU), high pHs, and low fields. The results showed that the magnetic flocculation of kaolin with Fe₃O₄@HAAP was a rapid process with a 91.96% removal efficiency at 0.25 h. In an interaction energy analysis, both the U_DLVO and U_EDLVO showed electrostatic repulsion between the kaolin particles in the condition of a flocculation distance of 30 nm. As Fe₃O₄@ HAAP was employed, kaolin particles could cross the energy barrier more easily; thus, the fine flocs and particles were destabilized and aggregated further. Rapid magnetic separation was realized under the action of an external magnetic field.