(499a) Magnetic Nanoparticles on Membrane Surface As Micromixers and Heaters

It is well known that the two 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 to suppress concentration polarization². Therefore, there is still a need to find an efficient way to improve fouling resistance of membranes and to reduce concentration polarization.

Here, we report a novel approach to solve these problems by attaching magnetically responsive nanoparticles to the membrane surface. Here polymers (polyHEMA and poly NIPAAm), were grafted to membrane surfaces by surface initiated atom transfer radical polymerization. After that, the bromine groups at chain ends were converted to primary amine groups by Gabriel synthesis and then used for attaching carboxyl 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. 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. On the other hand, with polyNIPAAm on the surface, nanoparticles were used as heater 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).