Elsevier

Separation and Purification Technology

Volume 123, 26 February 2014, Pages 171-174
Separation and Purification Technology

Short Communication
Challenges associated to magnetic separation of nanomaterials at low field gradient

https://doi.org/10.1016/j.seppur.2013.12.038Get rights and content

Highlights

  • Magnetophoresis of magnetic nanoparticles.

  • Magnetic separation under low field gradient.

  • Dimensionless number analysis on transport behavior of magnetic nanoparticles.

  • Magnetic separation of nanoparticles for water treatment.

Abstract

Magnetic nanoparticles (MNPs) have been proposed as one of the effective tools for pollutant removal from aqueous environment. In most of the new strategies investigated, which involved the use of MNPs, the ability to reharvest back this nanomaterial by an externally applied magnetic field is always being emphasized. In this short communication, we discuss the challenges associated to the magnetic separation of MNPs from its suspending media through magnetophoresis under low magnetic field gradient. We highlight the major constraints, such as thermal energy, Stokes drag and gravitational pulling, which influence the successful separation of MNPs from aqueous environment by low gradient magnetic separation (LGMS). Dimensionless numbers are introduced to provide a more quantitative comparison between the aforementioned constraints with magnetophoresis at low field gradient. Finally, we focus our discussion on the role of (1) guided/self-assembly approaches and (2) on-site LGMS strategy as the most practical routes of using MNP for water remediation.

Graphical abstract

Magnetophoretic collection of magnetic nanoparticles under low field gradient.

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Introduction

In last decade, we have observed a booming of research findings on the huge potential of magnetic nanoparticles (MNPs) in removing dangerous pollutants, such as arsenic [1], heavy metals [2], chlorinated compounds [3], and also organic dyes [4] from water resources. One of the great advantages of this MNP based water treatment technique is the recollection ability of MNPs, which can be easily achieved by using a hand held permanent magnet [1], [4], after the hazardous compound was adsorbed onto the particle surfaces. The underlying principle behind this separation technique is remarkably straightforward. It relies on the simple fact that the magnetic materials experience magnetophoretic force in the presence of magnetic field gradients and thus these materials can be physically separated out from the surrounding fluids by a magnetic source. In addition, MNPs can also be employed to impart a magnetic dipole moment to biological cells, through immobilization on the cell surfaces, which subsequently leading to magnetophoretic separation of biological substances [5].

The rapid magnetophoretic separation of MNPs under low magnetic field gradient (B < 100 Tesla/m), as observed by others is very likely through field-induced reversible aggregation of particles [6]. Under this scenario, the particle clusters formed would migrate to the region where the magnetic field gradient is the highest. Along its migration pathway, the moving MNP clusters collide with each other and integrated into larger aggregates with higher magnetophoretic velocity [6], [7]. This mechanism is the key factor for successful separation of MNPs in real time and revealed the opportunity for the implementation of low gradient magnetic separation (LGMS) for engineering applications.

In contrast to conventional industry practice in which high gradient magnetic separation (HGMS) is normally being employed, the design rules for LGMS is ill-defined and poorly understood. Moreover, the key parameters involved for implementation of LGMS in water treatment technology are also not being fully explored yet. Recently, Mandel and Hutter have briefly discussed the problems related to MNPs separation [8]. They raised an interesting point in which the use of ferrofluid as a nanoemulsion provides better alternative for water treatment purpose compared to easily agglomerated MNP suspension. However, the liquid–liquid interface between the nanoemulsion and the aqueous media can be the major barrier toward the full realization of this noble idea. Nevertheless, the key question here is how the nanosized magnetic particles can be used effectively for water treatment, and more importantly, the recollection of these particles from their suspension. It is the purpose of this paper to illustrate some general rule of thumbs related to the separation of MNPs under low field gradient.

Section snippets

Underlying problems associated to LGMS

It is often being illustrated that by introduction of a magnetic field, the separation of MNPs from aqueous environment can be made possible in real time as shown in Fig. 1. Even though, the separation time involved is very feasible for practical usages at lab scale, the possibility for this kind of setup to be fully implemented for water treatment purposes is as good as none. By taking the example of a cylindrical NdFeB magnet, the magnetic field Bx along its symmetry axis as a function of the

Transport behaviors of MNPs due to “nanosize effects”

By taking non-interacting particles assumption, at magnetic field B, the magnetophoretic force Fmag needed to induce separation of spherical MNPs is [11], [12]:Fmag=43πrpt3(M·)Bwhere rpt and M are the radius and the magnetization (per unit volume) of the MNP, respectively. By equating the Fmag with viscous drag force (Fdrag = 6πηrpt · umag) experienced by a sphere [13], the magnetophoretic velocity umag can be calculated asumag=2rpt29η(M·)Bwhere η is the viscosity of the suspending medium. For

MNPs for engineering applications

For environmental engineering applications are concerned, MNP is typically being surface functionalized by macromolecules. This step is taken to mitigate the nanotoxicity associated to its small dimension [19], and to maintain its colloidal stability in suspension [20]. However, just recently our group has revealed the conflicting role of colloidal stability in suppressing the magnetophoretic separation of MNPs [21], [22]. After achieved good dispersibility, the polyelectrolyte coated iron

Conclusion

A more localized usages of MNP for water treatment purpose, such as in an on-site treatment facility, should be implemented in order to take full advantage of its magnetophoretic property. The idea of releasing enormous amount of MNPs into environment for water treatment purpose and magnetophoretically re-collecting this nanomaterial back is highly unrealistic. Cooperative phenomenon during the MNP magnetophoresis has greatly complicated the mathematical analysis on predicting the MNP velocity.

Acknowledgements

This material is based on the work supported by Research University (RU) (Grant No. 1001/PJKIMIA/811219) from Universiti Sains Malaysia (USM), Exploratory Research Grants Scheme (ERGS) (Grant No. 203/PJKIMIA/6730013) from the Ministry of Higher Education of Malaysia, and eScience Fund (Grant No. 205/PJKIMIA/6013412) from MOSTI Malaysia. JKL and SWL are affiliated to the Membrane Science and Technology Cluster of USM. JK Lim thanks Lee R. More and Maciej Zborowski from Cleveland Clinic and Sara

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