virus control by enhanced physical separation
Terica Sinclair is a PhD student in the research group Membrane Science & Technology. Her supervisors are prof.dr.ir. H.D.W. Roesink and prof.dr. A.M. de Roda Husman from the faculty of of Science and Technology.
Accessibility to clean drinking water is and will continue to be essential to the quality of life of all humans. Unfortunately, having access to clean and safe drinking water is still a privilege and not a universal right. 2.9 billion people, which accounts for 39% of the global population, lack access to proper sanitation, while more than 1.2 billion live under conditions of physical scarcity of water and another 1.6 billion live in areas of economic water scarcity [1]. An astounding 25% of the global population are drinking from fecally contaminated sources of water [2]. Depending on the water to be treated, techniques are available to remove harmful bacteria and viruses or other contaminants for those who can afford it. However, lack of adequate water treatment infrastructure coupled with poverty has caused limited access to clean and safe drinking water in regions that are at the bottom of the social and economic ladder.
Ultrafiltration (UF) membranes display excellent performance when used for pathogen removal, including ease of maintenance and cost-effectiveness. However, UF membranes require higher pressures to operate and thus utilise pumps and electricity. This makes it impossible for UF membranes to be incorporated in the most simple and cheapest point of use systems, for the poorest and most remote areas. Of the pathogens to remove, bacteria are much larger (microns) than viruses (nanometers) and are readily stopped by most porous membranes by the principle of size exclusion. Contrarily, viruses are much smaller, more robust and very persistent in the aquatic environment, making them a serious problem. In this project, I have investigated how microfiltration (MF) membranes could be modified and utilised in a gravity-driven process, with a focus on the removal of viral pathogens. Such membranes could provide an easy to operate and cheap alternative to UF while attaining the same quality of drinking water with high fluxes.
Undeniably, there is an alarming problem that will persist if nothing is done to reduce the burdens associated with waterborne diseases. Technologies are available that can solve this problem. However, they require substantial investments, maintenance and specialised knowledge. Hence, there is a pressing need to develop alternative approaches, such as Point-of-Use (POU) devices that are inexpensive, require no energy and are easy to use. Also, these POU devices should also lead to safe, virus free drinking water, the focus of this thesis.
In this thesis, the modification of MF membranes for enhanced pathogen removal for drinking water treatment is covered. By using these modified membranes, drinking water purification processes can be simplified compared to UF based treatment systems, a significant advantage for decentralised water treatment systems.
To choose the most appropriate method to modify MF membranes, which would lead to antiviral activity, an extensive review of the literature was systematically performed (Chapter 2). From this systematic review, the modification with a cationic polymer, polyethyleneimine (PEI), was chosen as it has previously demonstrated antimicrobial activity against a range of pathogens when used to modify surfaces.
Using the information from Chapter 2, the antiviral properties of open MF membranes were improved by coating a thin active PEI layer on the membranes outer and inner surfaces using a simple coating procedure (Chapter 3). Here we used readily available commercial flat sheet MF membranes (0.45 micron) as a substrate. Various techniques, such as ellipsometry, zeta potential and Fourier transform infrared spectroscopy (FTIR), were used to monitor the PEI based coating first on model (glass and silicon) surfaces and subsequently on the MF membranes. Layer properties were then correlated to changes in the performance of the membrane, before and after coating.
With these positively charged MF membranes (pore size 0.45 um) the antiviral effects on a surrogate virus (bacteriophage), MS2 (Chapter 3) were investigated. With most viruses being net negatively charged, the positively charged active antiviral layer acted as adsorbent and reduced the viral titer, while maintaining relatively high fluxes in a gravity driven process. Our first generation of modified membranes was able to produce 5000 L/m2 with a ≥3 log10-unit virus reduction. However, some leaching of PEI from the membrane was also observed, leading to doubts on the long-term stability of these membranes.
While a significant reduction in the viral titre was observed with the first generation of modified membranes, there was still a public health risk, based on WHO standards. Hence we focused on finding suitable antiviral moieties to further enhance the reduction capability of our optimised membranes (Chapter 4). Silver and silver nanoparticles (AgNPs) have been used previously in several drinking water applications and are considered as a potent antimicrobial agent. Therefore, we investigated the antiviral effects of AgNPs, with five capping or stabilising polymers, on MS2 bacteriophages. The chosen capping agents polyvinylpyrrolidone (PVP), mercaptoacetic acid (MAA), polyethlene glycol (PEG), citrate and branched polyethyleneimine (PEI), had different surfaces charges at neutral pH. This allowed us to see the effect of charge on the antiviral activity of the different AgNPs. Interestingly there was a substantial synergistic antiviral effect when using the AgNPs in combination with the previously chosen polymer (PEI), in comparison to the other stabilisers. Just like Chapter 3, PEI acts as an adsorbent, which brings the negative viruses near the AgNPs. We also investigated copper nanoparticles (CuNPs) as an economical alternative to silver. For better comparison, we also stabilised the CuNPs PEI based on the results with AgNPs.
In Chapter 5 we clearly demonstrated the capping to be an essential factor in determining the antiviral activity of AgNPs. The lower (less negative) the zeta potential, the more efficient the AgNP, with negative or slightly negative zeta potentials showed ≥3-5 log10-units reductions in time concentration and size dependent studies. However, the Ag/BPEI nanoparticle with a positive zeta potential had a ≥8 log10-units reduction regardless of the test conditions. Moreover, TEM studies indicate that the Ag/BPEI nanoparticles were able to damage the capsid of the MS2 bacteriophages irreversibly. The synergistic effect between PEI and AgNPs was also deemed promising for membrane modification.
In the final experimental Chapter of this thesis (Chapter 6), we improved one the promising results regarding virus reduction discussed in Chapter 3. This was achieved by adding the PEI capped AgNP studied in Chapter 4 to the MF membrane coating approach developed in Chapter 3. The modification process was further enhanced using simple covalent layer-by-layer (LBL) deposition technique. Here one polyelectrolyte, PEI, was used to construct multiple layers by using the chemical crosslinker terepthalaldehyde (TA). Since AgNPs and CuNPs were stabilised with PEI, they too could be covalently coupled during the LBL deposition using TA. The newly designed membranes demonstrated ≥4-5 log10-unit reduction of MS2 bacteriophages over 5000 L/m2 in approximately 2 hours. The presence of the antiviral nanoparticles combined with crosslinked PEI, improved the stability and viral reduction of the coated membrane to meet WHO standards for HWTS systems.
This thesis highlights the possibilities of PEI modified MF membranes for drinking water purification, by being able to tune the final membrane’s antiviral properties not only by changing the coating methodology but also by incorporating antiviral metallic nanoparticles. With an improved understanding of the nature and assembly of the ultrathin layer, the combination of different coating techniques with commercial flat sheet MF membranes can be the starting point in ensuring that there is a reduction in waterborne diseases caused by viral contamination of water. POU systems, based on these membranes, can be developed to produce clean and safe drinking water, to meet the WHO standards for HWTS systems for viral contaminants. This project aimed to contribute to the 2030 agenda for sustainable development: to enable everyone to have universal and equitable access to clean and safe drinking water.
