Polyelectrolyte Multilayer Nanofiltration Membranes - Advancing insights towards asymmetric multilayers
Jurjen Regenspurg is a PhD student in the Department Membrane Science & Technology. (Co)Promotors are prof.dr.ir. W.M. de Vos and dr. E. te Brinke from the Faculty of Science & Technology and prof.dr. H.D.W. Roesink from NXFiltration.
Within this thesis, we have aimed to obtain a more complete overview on how specific parameters influence both symmetric and asymmetric PEM NF membranes. Here, the focus was on obtaining more knowledge and understanding on the connection between multilayer build-up and the subsequent performance of the formed PEMMs. This has been achieved by investigating a broad spectrum of PEs and parameters, and a fair comparison of membrane performance.
From the early 2000s, the topic of PEMMs has been one of high interest. The ability to fabricate PEMs with high control over the final multilayer properties results in high versatility. As a result, most papers dedicated to PEMs use similar polyelectrolyte pairs, nearly always fabricating them under varying conditions and for different applications. This makes it nearly impossible to offer a fair comparison between them. In Chapter 2 we took the work of Tieke and Krasemann, pioneers in the field of PEMMs, as a starting point. Their early work is, to our knowledge, the only work carefully studying a broad selection of polyelectrolyte pairs. They proposed polyelectrolyte charge density to be a key parameter in PEMMs. Building on their work, 9 different polyelectrolyte systems were carefully selected based on their frequent appearance in other studies as determined by Scopus database. To be able to provide a fair comparison between the different systems, all were fabricated under the exact same conditions. Comparing our results against the results obtained by Krasemann and Tieke, we find that PEMM performance can be connected to PEM properties. However, only using charge density is too limited and cannot solely be used to explain PEMM properties. Rather we show that it is an interplay between multiple parameters, such as the correlation between MWCO and the degree of swelling. Swelling is found to be a key parameter, as the mechanism for charged solute retention is also dependent on the degree of swelling. Strong di-electric exclusion is found for polyelectrolyte systems with a swelling degree of 40% and below, while a higher degree of swelling leads to clear Donnan exclusion for charged solutes. This work again highlights how versatile PEM based membranes are, highlighting several key parameters but also providing a more nuanced look at PEM based membranes.
Something which had not yet been thoroughly studied in relation to PEMM performance is polyelectrolyte molecular weight (Mw). The polyelectrolyte Mw is known to influence the formation of PEMs, affecting e.g. thickness and adsorbed mass mainly as a result of polyelectrolyte mobility. Therefore, it is fair to assume that this will also affect the resulting PEMM performance. As such, Chapter 3 systematically investigates the influence of low and high polyelectrolyte molecular weight on the resulting PEMM performance. We find that not only the Mw plays an important part but also, in the confinement of a membrane pore, the mobility of the systems used appears to be very important regarding PEM build-up. On a model surface a combination of high Mw PAH/PSS (low mobility) shows a high amount of adsorption per layer increment, whereas the mobile high Mw PAH/PAA combination exhibits low adsorption. Translating this towards to a membrane support, we show that low Mw/low mobility and high Mw/high mobility systems are able to close the porous support membrane quicker compared to their counterparts of equal mobility but different Mw. Finally, we demonstrate that for the different PAH/PSS systems (low mobility) no differences are found in terms of performance. However, the more mobile PAH/PAA systems do display a major discrepancy regarding multilayer density and charge. PEMs fabricated from low Mw PAH/PAA are more dense which is shown in terms of MWCO, as compared to their high Mw counterparts. Furthermore, low Mw PAH/PAA displays positive membrane charge whereas high Mw PAH/PAA salt retention indicates an overall negative membrane charge.
The first chapters of this thesis have shown that PEMMs are highly tuneable, versatile and have great potential regarding various applications. The recent introduction of asymmetric PEMMs (APEMMs) has increased this potential even more but also resulted in a knowledge gap between symmetric and APEMMs. In chapter 4 we carefully study the effect of top section chemistry and bottom section thickness. Demonstrating the ability to decouple charged and uncharged solute retention depending on the applied top section in APEMMs. Here, the effective amount of interpenetration shows to play an important role. By tuning the bottom section thickness, permeability is improved while maintaining high selectivity towards organic micropollutants (OMPs). Effectively changing the ratio between the top and bottom section allows for tuning of the charged solute retention while maintaining uncharged solute retention performance. Although salt retention can be tuned, OMP removal is demonstrated to be unaffected, remaining high (≥ 96%) for all APEMMs in this study. Again, this highlights the great potential of APEMMs with regard to the removal of micropollutants.
Chapter 5 takes into consideration how an external stimulus, like solution pH, affects membrane performance during a membrane process. As most PEM NF membranes contain at least 1 weak polyelectrolyte, they are susceptible to changes in solution pH. Four fundamentally different polyelectrolyte pairs have been studied in terms of changes network structure and excess charge upon exposure to different feed solution pH values. In the case of PAH/PSS (weak/strong) PEM membranes, charged solute retention if affected due to the partial deprotonation of PAH and resulting excess negative charge in the multilayer. An even more pronounced effect is found for the PAH/PAA (weak/weak) PEM membranes. Indicated by charged solute retention measurements the multilayer exhibits excess positive charge at pH 4, changing to more neutral at pH 6, and turning excess negative at pH 9. For all systems, the pH induced changes in charged solute retention appear to be reversible. Interestingly, upon exposure to pH 9, PAH/PAA multilayers densify as indicated by an increase in uncharged solute retention and decrease in pure water permeability. The densification, unlike charged solute retention, is shown to be irreversible as confirmed by means of ellipsometry.
Overall, the work in this thesis highlights the vast opportunities which are available regarding tailoring PEMM performance. Here, we have shown that not only during the formation of these multilayers, but also during operation membrane performance can be tailored to specific needs. All of these potential tuning parameters also come with complexity, as we demonstrated these can be interconnected. The field of (A)PEM NF membranes can be pushed forward, however this requires systematic optimization with respect to the specific applications.
