Abstract
Magnetic nanoparticles are researched intensively not only for biomedical applications, but also for industrial applications including wastewater treatment and catalytic processes. Although these particles have been shown to have interesting surface properties in their bare form, their magnetisation remains a key feature, as it allows for magnetic separation. This makes them a promising carrier for precious materials and enables recovery via magnetic fields that can be turned on and off on demand, rather than using complex (nano)filtration strategies. However, designing a magnetic separator is by no means trivial, as the magnetic field and its gradient, the separator dimensions, the particle properties (such as size and susceptibility), and the throughput must be coordinated. This is showcased here for a simple continuous electromagnetic separator design requiring no expensive materials or equipment and facilitating continuous operation. The continuous electromagnetic separator chosen was based on a current-carrying wire in the centre of a capillary, which generated a radially symmetric magnetic field that could be described using cylindrical coordinates. The electromagnetic separator design was tested in-silico using a Lagrangian particle-tracking model accounting for hydrodynamics, magnetophoresis, as well as particle diffusion. This computational approach enabled the determination of separation efficiencies for varying particle sizes, magnetic field strengths, separator geometries, and flow rates, which provided insights into the complex interplay between these design parameters. In addition, the model identified the separator design allowing for the highest separation efficiency and determined the retention potential in both single and multiple separators in series. The work demonstrated that throughputs of ~1/4 L/h could be achieved for 250–500 nm iron oxide nanoparticle solutions, using less than 10 separator units in series.
Highlights
Magnetophoresis is a well-known separation concept utilising the motion of magnetic particles relative to their non-magnetic surrounding medium in response to an inhomogeneous magnetic field [1]
Identical separation conditions were simulated using different time steps. These results showed that ∆t = 0.01 s was sufficient, all simulations were carried out using this time step
The simulations provide a conservative estimate of efficiencies as, due to the assumption of low particle concentrations, they did not account for (1) cooperative magnetophoresis, i.e., cooperative motion of strongly interacting MP, and (2) magnetophoresis-induced convection, i.e., the convective motion of particles towards the source of the field gradient induced by the mechanic instabilities originating from initial magnetic nanoparticle (MNP) accumulation [46]
Summary
Magnetophoresis is a well-known separation concept utilising the motion of magnetic particles relative to their non-magnetic surrounding medium in response to an inhomogeneous magnetic field [1]. MPs or MNPs accumulate first at volumes with high field gradients (e.g., close to the surfaces of magnetically soft fibres) This reduces the separation efficiency over time and can lead to plugging if the fine structures used in many HGMS devices become overloaded with magnetic material. Electromagnets exhibit considerably lower magnetic field gradients (with exceptions [31]), but they allow for magnetic field tuning by setting the field strengths (from 0 to a maximum value), as well as, the direction and frequency if required This facilitates controlled MP handling, for example, for automated sorting or isolation of particulate matter based on magnetic properties [32], compact microfluidic instruments enabling automated cell separation [33], as well as magnetic systems driving nano/micro “vehicles” [34], notably including actuating systems for nanorobotics [35,36]. This model is used to investigate the interplay between the magnetic separator design parameters and to identify the optimum design that yields efficient separation even at higher flow rates
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