Polyelectrolyte Complex Membranes: Sustainable Preparation and Enzyme Functionalization
Lijie Li is a PhD student in the Department of Molecules and Materials and Department of Membrane Science and Technology. (Co)Promotors are prof.dr.ir. S. Lindhoud and prof.dr.ir. W.M. de Vos from the Faculty of Science and Technology.
Membranes are widely used in industries, agriculture, and medicine fields. Sustainable production and functionalization of membranes are vital and promising. Currently, the dominant NIPS method for membrane preparation relies on unsustainable organic solvents, but using polyelectrolytes as membrane materials can eliminate the use of these organic solvents. PECs are formed in aqueous solutions and have unique properties for membrane preparation. Besides, the properties of polyelectrolyte solutions and the formation of PECs can be tuned by many parameters such as pH and salinity. APS and hot-pressing methods have been developed to utilize polyelectrolyte solutions and bulk PECs to prepare porous membranes and dense saloplastics. In this thesis, we aim to prepare and functionalize membranes with enzymes in a sustainable fashion. We first focus on making PEC membranes via APS using bio-based polyelectrolytes. Then the production of biocatalytic membranes will be explored. Apart from membranes prepared using APS, it will be explored whether non-porous hot-pressed membranes can be functionalized with enzymes by using a salt-annealing approach.
In Chapter 2, the sustainable pH change-induced APS approach was successfully utilized to prepare bio-based PEC membranes. The two most used bio-based polyelectrolytes, cationic CS and anionic CMC were used. Homogenous casting solutions were prepared at pH ~ 1 where CMC was uncharged, and the formation of PECs was induced in an acetate buffer coagulation bath of pH ~ 4. The influence of pH and concentration of the acetate buffer were studied, and the obtained membranes demonstrated tunable structures and microfiltration performance.
In Chapter 3, we successfully prepared biocatalytic PEI-PSS membranes via the pH change-induced APS. The effects of casting solution pH were investigated by adding different amounts of HCl. The lysozyme-functionalized membranes showed temperature-dependent enzymatic activities when the temperature increased from 25 to 45 oC and demonstrated high storage stability. However, the membranes did not show desirable activity at room temperature.
To obtain membranes with biocatalytic activity at room temperature, we further studied the biocatalytic PEI-PSS membrane system in Chapter 4. The polyelectrolytes mixing ratio and lysozyme concentration were varied to tune the membrane structure and the enzymatic activity. Tunable and desirable activities were obtained at room temperature. Besides, the enzyme laccase was successfully introduced and the obtained membranes had a high catalytic ability. The incorporation of laccase further showed the versatility of the APS approach for preparing biocatalytic membranes.
In Chapter 5, biocatalytic membranes were prepared via lysozyme-functionalized saloplastics through salt annealing. PDADMAC-PSS saloplastics that were first prepared via the hot-pressing method. Then the changes in KBr concentration were used for annealing and curing the saloplastics where the temporary opening of pores allowed lysozyme loading. Lysozyme was incorporated via two routes where dynamic incorporation led to higher enzymatic activities. This work provides new possibilities for sustainable saloplastics as a straightforward method for functionalization.
Chapter 6 discusses the findings and remaining problems in this thesis and gives approaches to solve the problems and improve the membranes’ performance. Moreover, this chapter provides clear outlook for future work.