Magnetically Activated Micromixers for Separation Membranes

Abstract

Presented here is a radically novel approach to reduce concentration polarization and, potentially, also fouling by colloids present in aqueous feeds: magnetically responsive micromixing membranes. Hydrophilic polymer chains, poly(2-hydroxyethyl methacrylate) (PHEMA), were grafted via controlled surface-initiated atom transfer radical polymerization (SI-ATRP) on the surface of polyamide composite nanofiltration (NF) membranes and then end-capped with superparamagnetic iron oxide magnetite (Fe(3)O(4)) nanoparticles. The results of all functionalization steps, that is, bromide ATRP initiator immobilization, SI-ATRP, conversion of PHEMA end groups from bromide to amine, and carboxyl-functional Fe(3)O(4) nanoparticle immobilization via peptide coupling, have been confirmed by X-ray photoelectron spectroscopy (XPS) and field emission scanning electron microscopy (FESEM). These nanoparticles experience a magnetic force as well as a torque under an oscillating external magnetic field. It has been shown, using particle image velocimetry (PIV), that the resulting movement of the polymer brushes at certain magnetic field frequencies induces mixing directly above the membrane surface. Furthermore, it was demonstrated that with such membranes the NF performance could significantly be improved (increase of flux and salt rejection) by an oscillating magnetic field, which can be explained by a reduced concentration polarization in the boundary layer. However, the proof-of-concept presented here for the active alteration of macroscopic flow via surface-anchored micromixers based on polymer-nanoparticle conjugates has much broader implications.

Toward remote-controlled valve functions via magnetically responsive capillary pore membranes

“…Building upon this, an exciting new type of filtration membrane responsive to an external magnetic field was recently developed by our groups [5]. The nanofiltration behavior of these membranes can be altered without affecting the characteristics of the entire feedstream by generating small scale mixing above the membrane surface via the movement of grafted polymer chains in a magnetic field [5].…”

“…The nanofiltration behavior of these membranes can be altered without affecting the characteristics of the entire feedstream by generating small scale mixing above the membrane surface via the movement of grafted polymer chains in a magnetic field [5]. In the presence of an oscillating magnetic field both rejection and flux increased for feed streams containing dissolved salts such as MgSO 4 and CaCl 2 .…”

Magnetic nanoparticles augmented composite membranes in removal of organic foulant through magnetic actuation

“…The superparamagnetic (Fe 3 O 4 ) nanoparticles were then attached to the polymer chain ends. As for membrane filtration, the modified magnetic responsive membranes operated under presence of an oscillating magnetic field manifested improvement of permeation (30% higher flux) and salt rejection (15% higher rejection of CaCl 2 ) than the membrane operated in the absence of an oscillating magnetic field [19].…”

“…Several techniques have been explored to incorporate nanoparticles onto the membrane surface, including radical grafting [19,20], hydrogel coating [15], self-polymerization [18], and polyelectrolyte multilayer modification (PEM) [7]. Himstedt et al [19] and Yang et al [20] demonstrated the integration of magnetite nanoparticles on surface of polyamide nanofiltration (NF) membrane through surface-radical polymerization.…”

“…Himstedt et al [19] and Yang et al [20] demonstrated the integration of magnetite nanoparticles on surface of polyamide nanofiltration (NF) membrane through surface-radical polymerization. In the studies, the hydrophilic polymer chains, poly(2-hydroxyethyl methacrylate), were first grafted on surface of polyamide NF membrane via controlled surface-initiated atom transfer radical polymerization.…”