Nanoparticles are often found in liquid-borne dispersed phases, in addition to the airborne and surface-borne phases. Characterization techniques for nanoparticles are needed for the environmental, health and safety studies of nanomaterials. On the other hand, membrane filtration has been demonstrated to be effective for the removal of liquid-borne nanoparticles. Such technique has found many applications, including: 1) contamination control in liquids used in industries which require high level of cleanliness, 2) control of the release of engineered nanoparticles for environmental health and safety and 3) drinking water purification and disinfection. For evaluating filtration performance of nanometer-rated filters, reliable techniques for counting and sizing liquid-borne nanoparticles are desirable. The objectives of this thesis are to 1) explore methods for characterizing liquid-borne nanoparticles and 2) apply these methods to study nanoparticle filtration problems.
In Chapter 2, calibration results of the Nanoparticle Tracking Analysis (NTA) technique in our lab are reported. The concentration measurements agree well with that estimated by suspension mass concentration within the range of 108-1010 particles/ml. To ensure the concentration measurements are made within the linear valid range, a single sample should be diluted and the NTA concentration measurements for the original and diluted samples should agree. Applying the NTA technique, the size distribution and concentration of nanoparticles in the water used in Abrasive Waterjet Machining (AWM) and Electrical Discharge Machining (EDM) processes were measured. The particles generally have a most probable size of 100-200 nm. The filtration systems of the AWM and EDM processes were found to remove of 70 and 90 % the nanoparticles present, respectively. However, the particle concentration of the filtered water from the AWM was still four times higher than that found in regular tap water. These nanoparticles are mostly agglomerated, according to the microscopy analysis. Since AWM and EDM are widely used, the handling and disposal of used filters collected with nanoparticles, release of nanoparticles to the sewer and potential use of higher performance filters for these processes will deserve further considerations.
The development of an aerosolization technique to measure liquid-borne nanoparticles down to 30 nm and its application to filter evaluation is discussed in Chapter 3. This technique involves dispersing nanoparticle suspensions into airborne form with an atomizer or electrospray aerosol generator, and measuring the size and concentration by a differential mobility analyzer coupled to a condensation particle counter. With the electrospray aerosol generator, residue particles can be controlled to be less than 10 nm, allowing particles as small as 30 nm to be clearly distinguished from the size distribution measurements. Calibrations with 30, 50, 125 and 200 nm polystyrene latex particles showed that liquid-borne and airborne particle concentrations are proportionally related. This provides an effective way to quantify liquid-borne particles as small as 30 nm, which cannot be analyzed by state-of-the-art liquid particle counters. Comparing to NTA, the aerosolization technique gives more accurate representation for polydisperse size distribution.
The aim of Chapter 4 is to study the filtration process of a model membrane filter, the nuclepore filter. Initial filtration efficiency experimentally measured using the aerosolization and NTA techniques are comparable with each other. The capillary tube model modified from that developed for aerosol filtration was found to be useful to represent the experimental results, when a sticking coefficient of 0.15 is incorporated. This suggests that for the polystyrene latex (PSL) particles-nuclepore filter-water system, only 15% of the particle collisions with the filter results in successful attachment. The small sticking coefficient found can be explained by the unfavorable surface interactions between the particles and the filter medium.
The sieving mechanism, in which particles are removed when they are larger than the filter pore size, is primarily used to describe the filtration process. The capture of particles smaller than the pore size, by adsorption via diffusion and interception, becomes effective when the combined electrostatic and van der Waals interactions between particle and filter surface are favorable. In Chapter 5, retention efficiency of a 50 nm- rated Polytetrafluoroethylene (PTFE) filter against nanoparticles of different materials (gold, PSL and silica), sizes (80, 50 and 30 nm), concentrations (2×109 to 4×1011 particles/ml) and size distributions (monodisperse and polydisperse) was measured. The decreasing trend of retention efficiency as a function of particle loading can be readily explained by the sieving mechanism. Among different particle materials, silica shows much lower retention efficiency compared to PSL and gold particles of the same size. This observation can be explained with the DLVO theory, which suggests that higher ionic strength of PSL and gold suspensions causes a decrease in the magnitude of energy barrier and favors their adsorption to the filter surface. In addition, this study observed higher retention efficiency for mixed particles compared to monodisperse ones and there is less than 10% of re-entrainment of particles collected by adsorption.
In Chapter 6, numerical simulations for change in membrane filter's retention efficiency in the presence of a particle previously captured within the filter are reported. Because of the complex porous structure of the membrane, the filter model is simplified into two cases: 1) capillary tube model and 2) single fiber model. Simulations for the capillary tube model show that a particle captured near the pore opening can increase the collection efficiency of 30 nm particles due to impaction and interception by about two times, depending on the relative size of the particles to the pore. From the single fiber model, a particle attached on the fiber can increase the combined impaction-interception single fiber efficiency by 10 times, depending on the location of the particle attached. The capillary tube simulation results can be incorporated into pore blockage model which considers filters as multiple layers of parallel pores. The single fiber simulations can explain the higher collection efficiency due to a previously captured particle. However, the single fiber theory alone cannot account for the observed decreasing trend with particle loading.
University of Minnesota Ph.D dissertation. August 2013. Major: Mechanical Engineering. Advisor: Dr. David Y.H. Pui. 1 computer file (PDF); ix, 144 pages, appendix p. 142-144.
Ling, Tsz Yan.
Liquid-borne nanoparticle characterization and its application to nanometer-rated liquid filter evaluation.
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