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Magnetic nanoparticles (MNPs) which exhibit magnetic and catalytic bifunctionalities have been widely accepted as one of the most promising nanoagents used in water purification processes. However, due to the magnetic dipole-dipole interaction, MNPs can easily lose their colloidal stability and tend to agglomerate. Thus, it is necessary to enhance their colloidal stability in order to maintain the desired high specific surface area. Meanwhile, in order to successfully utilize MNPs for environmental engineering applications, an effective magnetic separation technology has to be developed. This step is to ensure the MNPs that have been used for pollutant removal can be fully reharvested back. Unfortunately, it was recently highlighted that there exists a conflicting role between colloidal stability and magnetic separability of the MNPs, whereby the more colloidally stable the particle is, the harder for it to be magnetically separated. In other words, attaining a win-win scenario in which the MNPs possess both good colloidal stability and fast magnetic separation rate becomes challenging. Such phenomenon has to be thoroughly understood as the colloidal stability and the magnetic separability of MNPs play a pivotal role on affecting their effective implementation in water purification processes. Accordingly, it is the aim of this paper to provide reviews on (i) the colloidal stability and (ii) the magnetic separation of MNPs, as well as to provide insights on (iii) their conflicting relationship based on recent research findings. Graphical abstract Interrelationship of agglomeration, colloidal stability, and magnetic separability of nanoparticles

Magnetic separation technology is a physical separation method that uses the differences in magnetism between matter to separate them from each other by different motion behaviors in a non-uniform magnetic field. It is highly efficient, green, and environmentally friendly, with little change in the physical and chemical properties of raw materials. Magnetic separation technology is commonly used in the field of mineral processing engineering for magnetite, hematite, titanite, and other magnetic ferrous metal oxide minerals. This paper summarizes the application of magnetic separation technology for resource utilization and environmental treatment in different fields, such as non-metal decomposition, valuable metal recovery, use of magnetic carrier chemical separation, biomedical targeted magnetic separation, and use of magnetic species separation in water and wastewater treatment. We seek to review the application and potential of magnetic separation technology in various fields, emphasize their key role, and explore possible directions for their future development.

Laboratory protocols using magnetic beads have gained importance in the purification of mRNA for vaccines. Here, the produced mRNA hybridizes specifically to oligo(dT)‐functionalized magnetic beads after cell lysis. The mRNA‐loaded magnetic beads can be selectively separated using a magnet. Subsequently, impurities are removed by washing steps and the mRNA is eluted. Magnetic separation is utilized in each step, using different buffers such as the lysis/binding buffer. To reduce the time required for purification of larger amounts of mRNA vaccine for clinical trials, high‐gradient magnetic separation (HGMS) is suitable. Thereby, magnetic beads are selectively retained in a flow‐through separation chamber. To meet the requirements of biopharmaceutical production, a disposable HGMS separation chamber with a certified material (United States Pharmacopeia Class VI) was developed which can be manufactured using 3D printing. Due to the special design, the filter matrix itself is not in contact with the product. The separation chamber was tested with suspensions of oligo(dT)‐functionalized Dynabeads MyOne loaded with synthetic mRNA. At a concentration of cB = 1.6–2.1 g·L–1 in lysis/binding buffer, these 1 μm magnetic particles are retained to more than 99.39% at volumetric flows of up to 150 mL·min–1 with the developed SU‐HGMS separation chamber. When using the separation chamber with volumetric flow rates below 50 mL·min–1, the retained particle mass is even more than 99.99%.

Dry High-Gradient Magnetic Separation (Dry-HGMS) enables the manipulation of fine metal particles through their intrinsic magnetic responses. Research to date has predominantly addressed ferromagnetic powders, while the capture behavior of paramagnetic and diamagnetic particles with weak magnetic susceptibility has received limited examination. In this study, a multilayer magnetic filtration structure consisting of uniformly spaced unidirectional magnetic wires is developed to investigate the response-driven capture of such particles under dry conditions. By controlling the direction of the applied magnetic field, the system enables the selective capture of both paramagnetic and diamagnetic particles without inducing powder clogging. To clarify the capture mechanisms, a finite element method (FEM) framework is established that accounts for magnetic, drag, gravitational forces and Lorentz forces. The resulting capture maps reveal the dependence of particle trajectories on magnetic susceptibility, density, and electrical conductivity. Experiments performed on Al and Cr (paramagnetic) and Bi (diamagnetic) particles show trends consistent with the simulations. These results demonstrate that the proposed filtration system utilizes the magnetic-response characteristics of fine metal particles and extends the applicability of Dry-HGMS to weakly magnetic and diamagnetic materials.

To enhance dry magnetic separation of fine-grained materials, our research team developed the pneumatic drum magnetic separator (PDMS), an airflow-aided magnetic separator. Different positions at the separation surface of PDMS have varied separation angles, so particles at different positions may be subjected to varying composite forces, resulting in a mismatch between airflow velocity and magnetic field intensity. However, because the separation process of PDMS is continuous and the separation of particles at a certain position is instantaneous, the separation performance of PDMS at a specific separation angle cannot be investigated. To evaluate optimal operating features at different separation angles, a laboratory dry pneumatic flat magnetic separator (DPFMS) was manufactured, which also makes the airflow pass through the separation plane in the opposite direction to the magnetic force. The separation performance of PDMS was revealed by separation tests for −0.15 + 0.074 mm artificial mixed ore with 0–0.6 m/s airflow on DPFMS at various separation angles. At separation angles of 70° and 90°, the separation efficiency increases with an increase in airflow velocity from 16.68% and 33.09% to 77.72% and 76.54%, respectively; at separation angles of 110°, the separation efficiency increases initially from 89.53% to 90.69%, then decreases to 88.22% and keeps decreasing. The synergistic relationship between airflow drag, magnetic force and gravity were investigated by analyzing the composite force and the motion trajectory of a single particle. The results show that the proper airflow velocity aids in enhancing the distinctions between magnetite and quartz particles in resultant force and movement. However, throughout a wide range of air velocity, while the airflow can improve magnetite and quartz separation efficiency of at small separation angles, it may diminish the separation efficiency at large separation angles.

The global demand for rare earth elements (REEs) is expected to increase significantly because of their importance in renewable energy and clean storage technologies, which are critical for drastic carbon dioxide emission reduction to achieve a carbon-neutral society. REE ore deposits around the world are scarce and those that have been identified but remain unexploited need to be developed to supply future demands. In this study, the Khalzan Buregtei deposit located in western Mongolia was studied with the aim of upgrading low-grade REE ore via magnetic separation techniques. The total REE content in this ore was ~6720 ppm (~3540 ppm light REE (LREE) + ~3180 ppm heavy REE (HREE)) with bastnaesite, pyrochlore, synchysite, and columbite-(Fe) identified as the main REE-bearing minerals. As the particle size fraction decreased from −4.0 + 2.0 mm to −0.5 + 0.1 mm, the recovery by dry high-intensity magnetic separation (DHIMS) increased from 20% to 70% of total rare earth oxide (TREO) while the enrichment ratio reached 2.8 from 1.3. Although effective, gangue minerals such as quartz and aluminosilicates were recovered (~22%) due most likely to insufficient liberation. Meanwhile, the wet high-intensity magnetic separation (WHIMS) could produce a magnetic concentrate with TREO recovery of ~80% and enrichment ratio of 5.5 under the following conditions: particle size fraction, −106 + 75 μm; feed flow rate, 3.2 L/min; magnetic induction, 0.8 T. These results indicate that combining DHIMS and WHIMS to upgrade the low-grade REE ore from the Khalzan Buregtei deposit is an effective approach.

Purification of mRNA with oligo(dT)‐functionalized magnetic particles involves a series of magnetic separations for buffer exchange and washing. Magnetic particles interact and agglomerate with each other when a magnetic field is applied, which can result in a decreased total surface area and thus a decreased yield of mRNA. In addition, agglomeration may also be caused by mRNA loading on the magnetic particles. Therefore, it is of interest how the individual steps of magnetic separation and subsequent redispersion in the buffers used affect the particle size distribution. The lysis/binding buffer is the most important buffer for the separation of mRNA from the multicomponent suspension of cell lysate. Therefore, monodisperse magnetic particles loaded with mRNA were dispersed in the lysis/binding buffer and in the reference system deionized water, and the particle size distributions were measured. A concentration‐dependent agglomeration tendency was observed in deionized water. In contrast, no significant agglomeration was detected in the lysis/binding buffer. With regard to magnetic particle recycling, the influence of different storage and drying processes on particle size distribution was investigated. Agglomeration occurred in all process alternatives. For de‐agglomeration, ultrasonic treatment was examined. It represents a suitable method for reproducible restoration of the original particle size distribution.

Magnetic molecularly imprinted polymers (MMIPs) have fused molecular imprinting technology with magnetic separation technology, emerging as an innovative material capable of recognizing specific molecules and efficiently separating target substances. Their application to the extraction and purification of mycotoxins has great potential, due to the toxicity and economic impact of these contaminants. In this work, MMIP has been proposed as a sample treatment for the determination of main four aflatoxins (B1, B2, G1 and G2) in pig feed. The MMIP was formed through the integration of magnetic material (Fe3O4) with commercial molecularly imprinted polymers, avoiding the synthesis step and, therefore, simplifying the process. The analyses were carried out by high-performance liquid chromatography with fluorescence detection and the method was validated and limits of quantification (LOQs) between 0.09 and 0.47 ng/g were obtained, below the allowed or recommended levels by the European Union. Repeatability and intermediate precision showed relative standard deviations lower than 10% in all cases and trueness ranged from 92 to 111%. Finally, the proposed method was applied to 31 real pig feed samples, detecting aflatoxins with concentrations between 0.2 and 3.2 ng/g.

Fly ash, a substantial byproduct of coal combustion in thermal power plants, poses significant disposal challenges due to its vast generation and limited utilization. The presence of Fe 3 O 4 imparts weak magnetic properties to fly ash, making magnetic separation an attractive approach for its recovery. However, traditional magnetic separation methods often struggle with low efficiency, inadequate segregation between magnetic and non-magnetic components, and the presence of non-magnetic impurities in the final product. This not only limits resource recovery but also hinders efforts towards sustainable waste management. To address these limitations, this study aimed to enhance the recovery of magnetic iron oxides from fly ash using spiral dry magnetic separation. The magnetic circuit of spiral dry magnetic separator was simulated with Ansys Maxwell, and experiments were conducted to optimize separation parameters, such as rotational speed and separation cycles. The optimal conditions identified are 60 rpm, where iron oxide content peaks at 42.7%, and four separation cycles, ensuring maximum recovery. The synthesized magnetic product exhibits high saturation magnetization, indicating its potential for diverse applications. The spiral dry magnetic separation technique offers a practical solution for waste management and fly ash valorization, addressing environmental concerns and promoting resource recovery. Graphical Abstract

The recovery and reuse of waste printed circuit boards (PCBs) has attracted more and more attention from global researchers, as recycling of waste PCB metals is of great significance to the rational utilization of metal material resources. This study puts forward a clean and economical method in which enhanced gravity separation and wet high-gradient magnetic separation were combined to recover waste PCBs with heat treatment at a temperature of 240 °C. The heat treatment could improve the metal liberation effect of the PCBs, and the thermal behavior was measured by thermogravimetric analysis (TGA). The pyrolysis of the non-metal fraction (NMF) began around 300 °C, and the glass transition temperature of epoxy resin was 135.17 °C. The enhanced gravity separation technique was used for the separation of metals and NMF under the compound force field. The metals grade of the gravity concentrates fraction (GRF) was 82.97% under the optimal conditions, and the metals recovery reached 90.55%. A wet high-gradient magnetic separator was applied to classify the GRF into magnetic (MA) and non-magnetic (NMA) fractions, which could achieve iron and copper enrichment. After the three stages combined process, the copper and iron grades of the NMA and MA fractions were 70.17% and 73.42%, and the recovery reached 74.02% and 78.11%, respectively.