Filtration of engineered nanoparticles using porous membranes

The research presented in this thesis aims at providing a better understanding of the fundamental aspects responsible for nanoparticle removal and fouling development during filtration of engineered nanoparticles. The emphasis is put on the role of interparticle interactions in the feed solution, nanoparticle stability and aggregation in relation to the filtration mechanism. \ud We postulate that for a stable suspension of electrostatically stabilized nanoparticles filtered with the membrane having much larger pores than nanoparticle diameter, fouling occurs in five subsequent stages: adsorption, unrestricted transport through pores, pore blocking, cake filtration and finally cake maturation. After the pore blockage stage, nanoparticle rejection is enhanced from approx. 10% to 90-95%. An increase of the nanoparticle concentration does not change the filtration behavior but only accelerates fouling. Electrostatic interactions between nanoparticles in a suspension are responsible for the duration and severity of the proposed filtration stages.\ud Moreover, we demonstrate that bigger monodisperse silica nanoparticles block membrane pores easily, accelerating pore blockage and cake layer formation, acting as secondary membrane responsible for nanoparticle rejection. In the case of polydisperse silica nanoparticles, an increasing concentration of smaller nanoparticles in the suspension causes delayed pore blockage and cake filtration occurs at a later stage. \ud This thesis proves that polymeric stabilizers or surface-active compounds such as surfactant added to a feed solution containing nanoparticles change both membrane-nanoparticle and nanoparticle-nanoparticle interactions. An improved stability due to enhanced steric repulsions or stronger surface charges, reduce aggregation of nanoparticles. This facilitates their transport through the porous membrane and increases porosity of the filtration cake formed. On the other hand, stabilizers can also act as foulants, and as such can increase the thickness of the filtration cake and occupy the voids between the nanoparticles in the filtration cake.\ud Furthermore, this thesis demonstrates that fouling along a hollow fiber length develops irregularly during filtration of model silica nanoparticles. The exact fouling behavior along the hollow fiber membrane is strongly influenced by the applied feed flow rate