(379a) Magnetically Responsive Membranes As Micromixers and Nanoheaters

Magnetically
Responsive Membranes as Micromixers and Nanoheaters

Qian YANG¹, Heath
H. HIMSTEDT², Xianghong QIAN¹, S.
Ranil WICKRAMASINGHE¹

¹ Ralph
E Martin Department of Chemical Engineering, University of
Arkansas, Fayetteville 72701, USA; ²Department OF Chemical and Biological
Engineering, Colorado State University, Fort Collins, CO 80523.

Two
of the most important factors limiting the use of membrane technology are concentration
polarization and membrane fouling.¹
Efforts have been made to solve these problems by means of optimizing module
design or inducing mixing during operation, pretreatment of the feed and
surface modification of the membrane. Feed pretreatment can only postpone the
decline of membrane performance while surface modification is not able to
suppress concentration polarization.²
Therefore, there is still a need to find an efficient way to improve
anti-fouling property of membranes and to reduce concentration polarization.

Herein,
we report a novel approach to solve these problems by attaching magnetically
responsive nanoparticles to the membrane surface. As can be seen from Figure
1, polymers (polyHEMA and poly NIPAAm) were grafted to membrane surfaces by
surface initiated ATRP. After that, the bromine groups at chain ends were
converted primary amine by Gabriel synthesis and then used for attaching
carboxyl groups covered superparamagnetic Fe₃O₄
nanoparticles. The chain/nanoparticle density can be controlled by varying the
ratio of active to inactive ATRP initiator in the initiator immobilization step.
The modification and nanoparticle immobilization were monitored by XPS and SEM.
XPS confirmed each step of modification. SEM images showed that the
nanoparticles were successfully attached to the membrane surface and the
density can be tuned in a broad range (see Figure 2).

With
nanoparticles attached to the end of polyHEMA, the polymer chains acted as
micromixers under oscillating magnetic field and mixing above the membrane
surface was observed. This mixing resulted in a significantly improved membrane
performance (flux and salt rejection) which can be ascribed to reduced
concentration polarization. Tuning the space between these micromixers by
diluting nanoparticle density leaded to optimization of membrane performance.
On the other hand, with polyNIPAAm on the surface, nanoparticles were used as
nanoheater in the presence of magnetic field to exert temperature change below
and above LCST of this thermo-responsive polymer. Subsequently conformational
changes of polyNIPAAm leaded to a dense/loose layer structure transformation on
the membrane surface and changed membrane performance. This result demonstrated
the possibility of using external magnetic field to control the membrane performance
which is promising for industrial applications to realize instantaneous product
control without interrupting the production line.

References

1. R.W. Baker, Membrane
Technology and Applications, 2nd ed.; John Wiley: Chichester, (2004).

2. D. Rana and T. Matsuura, Chem. Rev., 110, 2448 (2010).