Aggregated Nanoparticle Morphology Effects in Membrane Filtration

Nanoparticles, particle sizes in the range of 1-100 nm, have been intensively investigated for the potential applications due to unique physical and chemical properties. However, there are environmental, health and safety concerns associated with nanoparticles and there is a need to understand how to best remove nanoparticles from aqueous environments.

In order to increase nanoparticle removal efficiencies when using membrane filtration technology, nanoparticles must be aggregated and form clusters with sizes larger than the pore sizes of membrane used. While the extent of aggregation and resulting particles sizes are important for obtaining high nanoparticle removal, it is the rate of aggregation used that affects the morphology of the aggregates. This morphology controls the membrane cake layer resistance developed on the membrane surface as particles are removed during membrane filtration processes. Fast aggregation occurs under conditions of completely screened repulsive force at high ionic strength, and nanoparticle collisions form loose shape aggregates of high porosity. Under low ionic strength conditions with repulsive forces present between nanoparticles, aggregation is accomplished only through multiple collision attempts and compact shape aggregates are formed of lower porosity.

In this study, 70 nm alumina oxide nanoparticles were aggregated under conditions of high and low ionic strength to form aggregates of different morphology. Aggregates were filtered using a hydrophilic polyvinylidene fluoride (PVDF) membrane with a pore size 0.22 μm and 100% nanoparticle removal efficiencies were obtained regardless of aggregation conditions used.  Aggregate morphology was quantified by measured fractal dimensions.  Fractal structure differences coincided with measured filtration resistance values. Low porosity aggregates provided a filtration resistance 43% greater than high porosity aggregates of the same effective size.  Model predictions for measured specific resistance values were improved through incorporation of compressibility indexes. In order to obtain a porous structure with less resistance, a fast coagulation process is suggested for nanoparticle removal.