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Title: Shear-spun polymer nanofibres for water purification
Author: Thompson, Noel
Awarding Body: University of Cambridge
Current Institution: University of Cambridge
Date of Award: 2020
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Access to clean drinking water remains an issue for hundreds of millions of people across the globe, a problem that can be addressed through proper water filtration. Unfortunately, those most severely affected by a lack of access to clean drinking water are also those least able to financially or technologically remedy it. Whilst there are a number of man-portable, low-maintenance filters available, those with consistently high-quality filtration and long lifespans tend to be too expensive for those most in need. Alternately, those that fall within the lowest price ranges are cumbersome, delicate and prone to cracking and failure. Nanotechnology can offer a solution to this issue in the form of polymer nanofibres, a material that was hitherto prohibitively expensive. The studies presented in this thesis explored a novel method of synthesising polymer nanofibres that is capable of large-scale manufacture (done entirely using low-cost materials), the processing of these nanofibres into filters and testing their filtration capability under a variety of conditions. The synthetic technique, shear spinning, was found to be capable of quickly producing polystyrene nanofibres of diameters between 80-2500 nm under variable conditions (polymer concentrations of 10%-20% [by mass] polystyrene in solvent, shear velocity, antisolvent composition), giving the porous nanofibre matrix a hierarchical structure with a variety of pore sizes (50-2500 nm) scaling with the different nanofibre diameters. A popular contaminant adsorbent, powdered activated charcoal, was successfully integrated into some nanofibres, as was silver, a material with antibacterial properties, in the form of nanoparticles. EDX analysis of these doped nanofibres confirmed the retention of the nanoparticles and indicated retention efficiency greater than 80%. A variety of methods of nanofibre deposition (wet-laying deposition from slurry, drying and re-suspending, deposition under vacuum) were tested and examined, with the eventual conclusion that vacuum-deposition of the nanofibres from the slurry in which they were synthesised gave the most even, consistent thickness and coverage of the filtration area. Nanofibre filters were examined by mercury porosimetry and their permeabilities experimentally determined using the Darcy equation. The filters were unusually permeable for materials with their pore size distributions (in the order of 10-13-10-14 m2), and they did not suffer from deformation, cracking or pore collapse at pressures up to 690 kPa, well above the pressure required for a handheld filter. The filters completely retained polymer particles down to 600 nm in diameter, with a loss of total retention occurring between 300-600 nm. This showed that the unusually high permeabilities of the nanofibre filters were not because of continuous channels of wide pores running through the material, and that the pores of different sizes were intermixed. The filters were, however, more prone to clogging than a commercial filter tested for comparison, which was likely to be related to the wide pore size distribution. Testing of the undoped filters with E. coli showed that they could not reach the USEPA’s (United States Environmental Protection Agency) LRV (log reduction value) requirement of 4 (99.99% bacterial retention), reaching LRVs of 2-3. The addition of powdered activated charcoal to the nanofibres was found to give some limited ability to adsorb copper from the water (32-62 mg Cu g-1 of activated charcoal in the filter, analogous to pure powdered activated charcoal). However, this came at the cost of a ~40% loss in permeability. The addition of silver nanoparticles to the nanofibres improved the bacterial retention capabilities of the filters (increasing LRVs by a minimum of 2 to > 4, meeting USEPA requirements) without inducing any negative effects. Silver nanoparticle addition also reduced the growth of bacteria in and around the filters by nearly 90%, lending them self-disinfecting properties. Double-layering of the nanofibre filters, mixing two different nanofibre types (20% undoped polystyrene nanofibre top layer, with either a 10% undoped, charcoal-doped or silver-doped underlayer) improved filter permeability (by a factor of 101, due to the greater permeability of the top layer) and reduced the vulnerability of the filters to clogging whilst also improving bacterial retention capability (due to increased filter thickness), giving consistent bacterial LRVs of 9 and above. Overall, the research presented in this thesis showed that shear spinning is a technique that can quickly and cheaply manufacture polymer nanofibre filters that are highly permeable, capable of satisfying USEPA bacterial filtration requirements and remove nanoparticle contaminants from water.
Supervisor: Cameron, Ruth Sponsor: EPSRC ; Unilever
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
Keywords: purification ; water ; filtration ; nanofibre ; nanofiber ; polymer ; polystyrene ; nanotechnology