(706a) Responsive Ultrafiltration Membranes Composed of N-Isopropylacrylamide (NIPAAM), Superparamagnetic Iron Oxide (SPIO) Nanoparticles and Cellulose Acetate

Membrane fouling occurs when there is reversible or irreversible attachment of macrosolutes present in the water to the membrane surface. Membrane replacements due to the fouling can add significantly to the operating costs of the membrane system.  Reversible fouling can be minimized by crossflow operation, backflushing during filtration and/or by chemical pretreatment, but it may take time and energy and as well as have to meet safety guidelines with respect to chemicals used.  On the other hand, irreversible fouling cannot be corrected by backflushing or crossflow filtration, and it results into permanent flux decline. Among irreversible foulants, natural organic matter (NOM) is considered to be a major contributor. NOM is composed of wide range of hydrophilic and hydrophobic components; hence, any static hydrophobic or hydrophilic membrane can get fouled.  Therefore, a dynamic membrane able to alternate between being hydrophobic and hydrophilic is hypothesized to decrease. Stimuli-sensitive polymers have been gaining attention in recent years due to their unique property that the polymer changes its conformation from a coiled and hydrophobic state to a uncoiled (or globular) and hydrophilic state in the presence of a stimulus. The stimulus can be pH, temperature, ionic strength or electric or magnetic fields. By alternating between the phases membrane surface can be made dynamic.

The purpose of this study is to develop stimuli responsive membranes to control fouling using membrane casting dopes made of cellulose acetate with N-isopropylacrylamide (NIPAAM). NIPAAM is a stimuli-responsive polymer, which offers the potential to collapse or expand the membrane with changes in temperature. As a respond to a temperature decrease, this polymer expands into a hydrophilic state, while a temperature increase causes it collapse into a hydrophobic state. The phase change arises from the existence of a lower critical solution temperature (LCST) such that the polymer precipitates from aqueous solution as the temperature is increased. By continuously activating the film, it is hypothesized that a dynamic stimuli-responsive surface will prevent foulants from attaching to the membrane surface.

However, alternating between hot and cold temperatures to stimulate the polymer is energy intensive, and during heating and cooling, the response time within the membrane is hindered by the heat transfer resistances. These problems are hypothesized to be eliminated if the heating is localized and intensified within the membrane matrix by embedding small superparamagnetic iron oxide (SPIO) nanoparticles within temperature sensitive membranes. SPIO nanoparticle heating is caused by Neel and Brownian relaxations. A temperature increase due to Neel relaxations occurs by the  reorientation of magnetic moment of the particles after being rotated by an external AC field. Brownian relaxation contributes due to the SPIO nanoparticles in a liquid. Thus, temperature activation can be triggered through alternating electromagnetic heating of the embedded SPIO nanoparticles.