On Friday February 5th 2016, Krzysztof Trzaskus will defend his PhD thesis entitled ‘Filtration of Engineered Nanoparticles using Porous Membranes’. In his thesis, Krzysztof investigated the use of membranes for the removal of nanoparticles from water with a focus on fouling phenomena. He used monodisperse model silica nanoparticles as feed, as well as polydisperse mixtures. Furthermore, the influence of the addition of polymeric stabilizers and surfactants on fouling development was thoroughly investigated.
The defense will start at 16:45 in the Prof. Dr. G. Berkhoff Room (Collegezaal 4) in the Waaier building at the University of Twente. The introduction to the thesis will start at 16.30 in the same room.
Sustainable development requires focus not only on the advantages of a certain technology, but also on its drawbacks and threats. The same is valid for nanotechnology, involving the manufacturing and use of engineered nanoparticles. Therefore, more and more attention is drawn to the fate of engineered nanoparticles after usage and their removal from aquatic environments. Membrane technology is an emerging technology frequently applied in water purification and can be a solution for nanoparticle removal in water sources since porous membranes are designed to retain colloidal particles. However, nanoparticles are a new, unexplored cause of water contamination, and the application range of membranes for nanoparticle filtration and retention combined with the exact mechanisms responsible for their removal using membranes need to be investigated in order to guarantee safe and secure drinking water in the future as well. 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.
In order to investigate the role of electrostatic interactions during membrane filtration of nanoparticles, a microfiltration hollow fiber membrane was used in constant pressure dead-end mode for the filtration of model silica nanoparticles. A low concentration of nanoparticles in the feed solution and a large difference between the membrane pore size (~200 nm) and the nanoparticle size (25 nm) allow determination of the fouling mechanisms. We postulate that for a stable suspension of electrostatically stabilized nanoparticles fouling occurs in five subsequent stages: adsorption, unrestricted transport through pores, pore blocking, cake filtration and finally cake maturation (Chapter 2). 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. Due to the high sensitivity of the stability of electrostatically stabilized nanoparticles to the solution chemistry, the presence of salts, solution pH and valence of the cation strongly influences the duration and severity of the fouling stages. In general, lower repulsive interactions between the nanoparticles accelerate fouling by faster pore blockage and aggregation on the membrane surface. Moreover, porosity and permeability of the formed filtration cake are strongly dependent on the repulsive interactions between the nanoparticles.
In Chapter 3, the role of nanoparticle size and polydispersity on fouling development and nanoparticle rejection during dead-end microfiltration of electrostatically stabilized silica nanoparticles was investigated. 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 (obtained by mixing monodisperse suspensions in various ratios), an increasing concentration of smaller nanoparticles in the suspension causes delayed pore blockage and cake filtration occurs at a later stage. Moreover, due to the surface charge of the nanoparticles and a less ordered structure of the filtration cake formed due to the polydispersity of the particles in the suspension, the filtration cake has a more porous, open structure. This allows transport of smaller nanoparticles through the filtration cake and the polymeric membrane. As a result, nanoparticle rejection is reduced proportional to the fraction of the smaller nanoparticles present in the feed solution. An increase in transmembrane pressure applied during filtration of the polydisperse suspension causes densification of the filtration cake with only a slight improvement in nanoparticle rejection.
Stabilizers or surface-active compounds added to a feed solution containing nanoparticles change both membrane-nanoparticle and nanoparticle-nanoparticle interactions. An improved stability due to enhanced steric repulsions (introduced by polymers) or stronger surface charges (introduced by low-molecular weight compounds, e.g. surfactants), 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.
Chapter 4 shows that the molecular mass, concentration of the steric stabilizer (in our case PVP) and the transmembrane pressure applied significantly influence the pore blockage and the cake filtration stage during filtration of model silica nanoparticles. In general, PVP with a lower molecular mass is a better stabilizer for nanoparticles and contributes less to fouling by delaying the occurrence of pore blockage. On the other hand, at a higher PVP concentration, PVP contributes to the fouling due to an increase of the total solute load. Moreover, stabilizers with a higher molecular mass block the pores more easily, leading to faster fouling- and rejection development. The nanoparticle rejection drops with increasing PVP concentration and this effect is more pronounced for low-molecular weight PVP. Use of a higher transmembrane pressures results in compression of the filtration cake and improved nanoparticle rejection at the expense of permeability.
Low-molecular weight compounds such as surfactants are often added to nanoparticle suspensions in order to influence their surface properties and stability. Chapter 5 demonstrates that the type of surfactant used (anionic, cationic or non-ionic) and its concentration determine the nanoparticle stability. This directly affects the fouling behavior of the nanoparticles and their rejection during dead-end constant flux membrane filtration. Reduced repulsive interactions between nanoparticles due to the addition of non-ionic surfactant (Triton X-100) cause the most severe fouling and the highest nanoparticle rejection. We hypothesize that the difference in nanoparticle rejection in the presence of the investigated surfactants has its origin in the homogeneity and density of the cake layer formed.
Non-homogeneous hydrodynamic conditions over the length of a membrane fiber during inside-out dead-end filtration can result in different fouling development depending on the axial position inside the fiber. Chapter 6 demonstrates that fouling along the fiber length develops irregularly during filtration of model silica nanoparticles. Moreover, the exact fouling behavior along the hollow fiber membrane is strongly influenced by the applied feed flow rate. However, after the occurrence of pore blockage and the formation of nanoparticle deposit on the membrane surface, rejection of the nanoparticles is no longer determined by the position inside the hollow fiber. Extensive concentration polarization as occurs at some parts of the fiber does not influence significantly the rejection of the silica nanoparticles.
Finally, Chapter 7 discusses the challenges in nanoparticle filtration and suggests directions for further studies that can contribute to a better understanding of the mechanism of nanoparticle filtration.
For more information about this work, please contact Dr. Ir. Antoine Kemperman (firstname.lastname@example.org; phone: +31 (0)53 489 2956).