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

Understanding the role of nanoparticle size and polydispersity in fouling development during dead-end microfiltration

The recent exponential growth of nanotechnology and numerous applications of nanotechnology-based products resulted in water pollution by engineered nanoparticles. Over the last few decades, membrane technology has emerged as one of the most promising and reliable techniques in water purification. Therefore, it is an obvious candidate to remove manufactured nano-sized contaminants and to purify the water. Nanoparticle properties play a crucial role in the performance and effectiveness of membrane filtration. This experimental study investigates the role of nanoparticle size and polydispersity on fouling and rejection development during dead-end microfiltration of electrostatically stabilized silica nanoparticles. Our work on filtration of monodisperse silica nanoparticles (11 nm, 25 nm and 92 nm) smaller than the membrane pore size (~200 nm) demonstrates that an increasing nanoparticle diameter accelerates pore blockage and development of cake. The specific cake resistance of the filtration cake formed decreases with increasing nanoparticle diameter. Filtration of polydisperse nanoparticles (obtained by mixing monodisperse suspensions in various ratios) shows that increasing the fraction of smaller nanoparticles results in delayed pore blockage, and cake filtration occurring at a later stage. The specific cake resistance of the polydisperse nanoparticles is always found to be in between that obtained for the monodisperse nanoparticle suspensions. An increasing weight fraction of larger nanoparticles results in faster development of nanoparticle rejection due to accelerated pore blockage. However, because of the highly porous structure of the filtration cake originating from strong surface charges, the moderate transmembrane pressure applied and cake imperfections, the smallest (11 nm) nanoparticles were rejected only to a low extent, even during the cake filtration stage. An increase in applied transmembrane pressure during filtration of the polydisperse suspension resulted in faster pore blockage and higher specific cake resistance. Nevertheless, rejection of the nanoparticles in the cake filtration stage improved only slightly with increasing transmembrane pressure

Towards controlled fouling and rejection in dead-end microfiltration of nanoparticles – Role of electrostatic interactions

Membrane technology proves to be effective in the removal of nano-sized contaminants from water. However, not much is known on the filtration and fouling behavior of manufactured nanoparticles. The high surface-area-to-volume ratio of nanoparticles, significantly increases the effect of surface interactions on the stability of nanoparticle suspensions. Also, the stability of nanoparticle suspensions and their tendency to aggregate strongly affects the fouling mechanism during membrane filtration of nanoparticles. In this experimental study, fouling development and rejection mechanisms of mono-disperse silica model nanoparticles were investigated in great detail. A microfiltration hollow fiber membrane was employed in dead-end filtration mode for the filtration of commercially available silica nanoparticles under constant pressure. By applying a low concentration of nanoparticles and a large difference between the membrane pore size (∼200 nm) and the nominal size of the nanoparticles (22 nm), a detailed investigation of the fouling mechanisms was allowed. Five subsequent fouling stages were postulated: adsorption, unrestricted transport through pores, pore blocking, cake filtration and cake maturation. Higher concentrations of nanoparticles did not change the behavior of these fouling stages, but were found to lead to their acceleration. Fouling severity and occurrence of dynamic transitions between these fouling stages were quantitatively evaluated. The presence of salts, pH and valency of the cation strongly influenced nanoparticle properties and interactions and thus occurrence and character of the fouling stages. Lower repulsive interactions between the nanoparticles accelerate fouling by faster pore blockage and aggregation on the membrane surface. Porosity and permeability of the formed filtration cake layer are strongly dependent on the repulsive interactions between nanoparticles, with a lower repulsion leading to denser cake layers. This paper clearly shows that fouling development and rejection of nanoparticles by microfiltration membranes easily can be adjusted by tuning the electrostatic interactions between the suspended nanoparticle

Fouling behavior of silica nanoparticle-surfactant mixtures during constant flux dead-end ultrafiltration

The increasing use of engineered nanoparticles in customer products results in their accumulation in water sources. In this experimental study, we investigated the role of surfactant type (cationic, anionic and non-ionic) and concentration on fouling development, nanoparticle rejection and fouling irreversibility during dead-end ultrafiltration of model silica nanoparticles. Our work demonstrates that the type of surfactant influences the nanoparticle stability, which in turn is responsible for differences in fouling behavior of the nanoparticles. Moreover, the surfactant itself interacts with the PES-PVP membrane and contributes to the fouling as well. We have shown that anionic SDS (sodium dodecylsulfate) does not interact extensively with the negatively charged silica nanoparticles and does not change significantly the surface charge and size of these nanoparticles. Adsorption of the cationic CTAB (cetyltrimethylammonium bromide) onto the silica nanoparticles causes charge transition and nanoparticle aggregation, whereas non-ionic TX-100 (Triton X-100) neutralizes the surface charge of the nanoparticles but does not change significantly the nanoparticle size. The most severe fouling development was observed for the silica nanoparticle – TX-100 system, where nanoparticles in the filtration cake formed exhibited the lowest repulsive interactions. Rejection of the nanoparticles was also highest for the mixture containing silica nanoparticles and TX-100

Fouling behavior during microfiltration of silica nanoparticles and polymeric stabilizers

Nanotechnology applications give rise to new forms of water pollution, resulting in a need for reliable technologies that can remove nanoparticles from water. Membrane filtration is an obvious candidate. The tendency of nanoparticles to become instable in suspension and form aggregates strongly influences their filtration behavior. This experimental study investigated fouling and rejection during dead-end microfiltration of sterically stabilized nanoparticles. Polyvinylpyrrolidone (PVP) with different molecular weights at different concentrations was used as model steric stabilizer. The large difference between membrane pore size (~200 nm) and the size of the silica nanoparticles (25 nm) allowed a detailed investigation of the filtration process and fouling development. We characterized the feed solution with optical reflectometry, dynamic light scattering, zeta potential measurements and asymmetric flow field flow fractionation (AF4) combined with static light scattering. Subsequently, we looked at the influence of the steric stabilizer (PVP) on nanoparticle fouling development during pore blocking and cake filtration stages.\u3cbr/\u3e\u3cbr/\u3eOur work demonstrates that molecular mass, concentration of the steric stabilizer (PVP) and filtration pressure significantly influence pore blockage and cake filtration. Using a stabilizer with a lower molecular mass generally led to better stabilization of the nanoparticles and the stabilizer contributed less to the fouling. While higher concentrations of the stabilizer enhanced the stability of the nanoparticles, they also caused faster fouling development due to the higher total solute load. Stabilizer with a higher molecular mass was found to contribute more to pore blockage and lead to faster fouling development. Use of a higher transmembrane pressure resulted in compression of the filtration cake, resulting in improved nanoparticle rejection at the expense of permeability\u3cbr/\u3

Fouling behavior during microfiltration of silica nanoparticles and polymeric stabilizers

Nanotechnology applications give rise to new forms of water pollution, resulting in a need for reliable technologies that can remove nanoparticles from water. Membrane filtration is an obvious candidate. The tendency of nanoparticles to become instable in suspension and form aggregates strongly influences their filtration behavior. This experimental study investigated fouling and rejection during dead-end microfiltration of sterically stabilized nanoparticles. Polyvinylpyrrolidone (PVP) with different molecular weights at different concentrations was used as model steric stabilizer. The large difference between membrane pore size (~200 nm) and the size of the silica nanoparticles (25 nm) allowed a detailed investigation of the filtration process and fouling development. We characterized the feed solution with optical reflectometry, dynamic light scattering, zeta potential measurements and asymmetric flow field flow fractionation (AF4) combined with static light scattering. Subsequently, we looked at the influence of the steric stabilizer (PVP) on nanoparticle fouling development during pore blocking and cake filtration stages. Our work demonstrates that molecular mass, concentration of the steric stabilizer (PVP) and filtration pressure significantly influence pore blockage and cake filtration. Using a stabilizer with a lower molecular mass generally led to better stabilization of the nanoparticles and the stabilizer contributed less to the fouling. While higher concentrations of the stabilizer enhanced the stability of the nanoparticles, they also caused faster fouling development due to the higher total solute load. Stabilizer with a higher molecular mass was found to contribute more to pore blockage and lead to faster fouling development. Use of a higher transmembrane pressure resulted in compression of the filtration cake, resulting in improved nanoparticle rejection at the expense of permeability

Fouling behavior of silica nanoparticle-surfactant mixtures during constant flux dead-end ultrafiltration

\u3cp\u3eThe increasing use of engineered nanoparticles in customer products results in their accumulation in water sources. In this experimental study, we investigated the role of surfactant type (cationic, anionic and non-ionic) and concentration on fouling development, nanoparticle rejection and fouling irreversibility during dead-end ultrafiltration of model silica nanoparticles. Our work demonstrates that the type of surfactant influences the nanoparticle stability, which in turn is responsible for differences in fouling behavior of the nanoparticles. Moreover, the surfactant itself interacts with the PES-PVP membrane and contributes to the fouling as well. We have shown that anionic SDS (sodium dodecylsulfate) does not interact extensively with the negatively charged silica nanoparticles and does not change significantly the surface charge and size of these nanoparticles. Adsorption of the cationic CTAB (cetyltrimethylammonium bromide) onto the silica nanoparticles causes charge transition and nanoparticle aggregation, whereas non-ionic TX-100 (Triton X-100) neutralizes the surface charge of the nanoparticles but does not change significantly the nanoparticle size. The most severe fouling development was observed for the silica nanoparticle – TX-100 system, where nanoparticles in the filtration cake formed exhibited the lowest repulsive interactions. Rejection of the nanoparticles was also highest for the mixture containing silica nanoparticles and TX-100.\u3c/p\u3