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Item type:Publication, Access status: with files , Untersuchungen von Layer-by-Layer Beschichtungen für polymere Ultrafiltrationsmembranen und Charakterisierung der Membraneigenschaften(2022); The Layer-by-Layer (LbL) technology offers the possibility to modify ultrafiltration membranes in a way that certain retentions for dissolved substances, such as sulfate, DOC or trace substances are possible. Depending on the polyelectrolyte coating of the membrane, not only the performance parameters but also certain membrane properties change. In order to understand these changes and to gain an understanding of the altered retention mechanisms in the modified membranes, polymeric membranes were investigated in this work with respect to their optimal coating, membrane properties and solute retention behaviour. After initial tests, two polyether sulfone ultrafiltration membranes were found to be suitable for LbL modification, a flat sheet membrane and a multicapillary membrane. When coated with the polyelectrolytes poly-(diallyldimethylammonium chloride) (PDADMAC) and poly (sodium 4-styrenesulfonate) (PSS), it became clear that the number of bilayers (DL) deposited and the additional ionic strength in the coating solution were the dominant factors during coating. Coating at 0.1 mol/L NaCl additional ionic strength and a layer number of 8 DL resulted in 70% sulfate retention for both membranes, with the flat sheet membrane having a slightly higher permeability of 15.6 L∙m-2∙h-1∙bar-1 compared to the capillary membrane. Additionally, a stable backwash for the multicapillary membrane could be achieved when coated under pressure (1 bar). After single layer coating, the so-called odd-even effect was observed, in which the coatings terminated with the polyanion exhibited higher retention and slightly lower permeability than the coatings terminated with the polycation, suggesting that the charge of the top polyelectrolyte has an influence on the separation behaviour of the LbL membranes. This effect could be confirmed by zeta potential measurements which showed that, depending on the terminating polyelectrolyte, a zig-zag effect occurred which corresponded to the odd-even effect observed for retention and permeability; the coatings terminated with PSS had a more negative zeta potential, causing a stronger repulsion of the sulfate anion and thus a higher retention. The separation limit of the membrane, the so-called molecular weight cut-off (MWCO) was found to be between 300 and 400 Da after coating with 8 DL, which corresponds to the MWCO of nanofiltration membranes. Despite this low MWCO, no retention comparable to NF membranes could be achieved for sulfate. Further studies on ion retention showed that size exclusion was initially the dominant factor, caused by the reduction in pore size due to the polyelectrolyte coating. When the PE coatings have reduced the pores to the point where some retention is possible, membrane charge is also an important retention mechanism, and a weakly negative surface charge seems to be sufficient to achieve increased retention for a divalent anion. From the results of this work, it can be concluded that increased solute retention is possible by LbL modification of flat and multicapillary membranes. The retention mechanisms here are similarly complex to those of NF membranes, which LbL membranes resemble not only in separation behavior but also in many membrane properties.Publicationtype: Doctoral Thesis258 526 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: with files , Theoretical evaluation of polyelectrolyte layering during layer-by-layer coating of ultrafiltration hollow fiber membranes(Multidisciplinary Digital Publishing Institute (MDPI), 2021-02-02); ; ; ; Layer-by-layer (LbL) modification of porous membranes for water filtration has become an active research field in the past few years. Different mechanisms regarding polyelectrolyte film growth, swelling and smoothing, transport through these films, etc., have been studied. Although there are conjectures, it is not yet fully understood where the polyelectrolyte layering takes place when modifying porous membranes, either within the pores or on top of the porous material. This study presents a theoretical approach to investigate the dominant layer buildup regime between pore-dominated vs. layer-dominated growth of polyelectrolytes on porous membranes without mechanically interfering or damaging the membrane material. For this, fouling mechanism processes are used as an analogy. The presented approach gives a new insight into layering conformation and might be helpful to investigate the interaction between the membrane surface and the PE film. Moreover, the MgSO4 rejection behavior of two types of modified membranes was investigated: one with an initial pore-dominated layer growth followed by a layer-dominated film growth; the other one with a completely layer-dominated film growth. The data confirm that a rejection for MgSO4 could only be achieved in the regime of layer-dominated film growth. Additionally, when layer-dominated film growth prevails from the early stages of the coating process, permeability values are higher at similar MgSO4 rejection rates compared to an initial pore-dominated and then layer-dominated film growth. Accordingly, the interaction between the membrane pore size and molecular weight of the polyelectrolytes in the coating solutions plays an important role during LbL coating.Publicationtype: Journal ArticleScopus© Citations 8 211 342 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: with files , Characterization and performance of LbL-coated multibore membranes : zeta potential, MWCO, permeability and sulfate rejection(Multidisciplinary Digital Publishing Institute (MDPI), 2020-12-10); ; ; The characterization of membranes is suitable to investigate changes in the membrane properties caused by Layer-by-Layer (LbL) modification. Besides permeability, rejection, and molecular-weight cut-off (MWCO), which give information about the modification of the separation behaviour of the membrane, the zeta potential is capable of describing the surface charge of the membrane and its variation impacted by the properties of the polyelectrolyte multilayers (PEM). In this study, a new method for zeta potential measurement of hollow fibre membranes with several capillaries was developed and further investigations on the LbL modification of such membranes were performed. The results showed that an LbL coating with 8 DL PDADMAC/PSS led to a significant increase in the membrane charge of more than 20 mV. The coating with a different number of polyelectrolyte (PE) layers showed a zig-zag behaviour, comparable to data from flat sheet studies. However, in contrast to most flat sheet membranes, the charge curve assumes a totally negative trajectory at neutral pH. Further experiments on the MWCO of the LbL-modified membrane showed a reduction in the pore diameter from approx. 20 nm to less than 2 nm, reaching the range of nanofiltration membranes. With information on both the zeta potential and the MWCO, it was found that the rejection mechanism in LbL-modified multibore membranes is a complex interplay between the sieving effect due to reduction in the pore diameter and the repulsion effect of the charged membrane.Publicationtype: Journal ArticleScopus© Citations 10 212 259