Challenges associated to magnetic separation of nanomaterials at low field gradient

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. (C) 2014 Elsevier B.V. All rights reserved.