Development and Tailoring of Low‐Density Cellulose‐Based Structures for Water Treatment

QC 2024-01-31

Abstract

The challenges posed by our limited clean water sources and the well-known global waterpollution demand more efficient water purification technologies. Additionally, the increasingenvironmental awareness has inspired a shift towards eco-friendly and renewable materials andtechnologies. This thesis is focused on developing effective adsorbent materials from renewableresources to eliminate organic solvents, dyes, and metal ions from water. Cellulose, the most abundantbiopolymer in nature, is the main component used to develop new materials in the present study. Itsdistinctive physical and colloidal properties, in the form of nanocellulose, along with tunable surfacechemistry, play key roles in enhancing the effectiveness of the developed materials.

The primary focus of the first part of the thesis was to develop a molecular layer-by-layermodification technique to customize the surface functionality of cellulose aerogels in a uniform andcontrolled manner. Through the sequential deposition of diamine and triacid monomers, exceedinglythin polyamide coatings were formed on the cellulose aerogels, altering the surface properties fromhydrophilic to hydrophobic. This transformation made them well-suited structures for oil-waterseparation.

Following this, a biohybrid aerogel was developed based on cellulose nanofibrils (CNFs) andamyloid nanofibrils (ANFs), the latter derived by heat treatment of β-lactoglobulin proteins. The pHtunablesurface charge of the aerogel, controlled by the amphiphilicity of the protein, allowed for theadsorption of both cationic and anionic contaminants by adjusting the pH of the solutions.Furthermore, the aerogels exhibited remarkable selectivity for lead (II) ions and they could also beregenerated and reused after each adsorption cycle without a significant loss of their adsorptioncapacity. This was to a large extent possible due to the excellent wet stability of these aerogels, whichwas achieved by crosslinking the CNFs during freezing and ice templating, eliminating the need forfreeze-drying. However, a solvent exchange to acetone after melting was still necessary to reduce theinfluence of the capillary forces during drying to avoid the collapse of the aerogels. In a consecutivestudy, the foaming characteristics of the heat-treated β-lactoglobulin system were exploited to createhighly stable Pickering foams with the aid of using CNFs as stabilizers and to physically lock thesystem through a controlled pH reduction. Interestingly, these Pickering foams could be directlyoven-dried without collapsing, yielding low-density foams. Furthermore, it was demonstrated that thefoams can be chemically crosslinked by incorporating chemical crosslinkers in the formulation or bypre-functionalizing the CNFs with dialdehydes. This crosslinking naturally also provided wet stabilityto the oven-dried foams.

Finally, an innovative and environmentally friendly method was introduced to increase the charge of cellulose fibers by radical polymerization of acrylic acid from the fibers, enabling the preparationof fibers with an exceptionally high charge of 6.7 mmol/g. The introduction of these charged groupssignificantly enhanced the interaction of the fibers with methylene blue as a model dye and lead (II),Copper (II), and Zinc (II) ions as model metal ions, showing the huge potential of these fibers asbuilding blocks for a wide range of adsorbent applications. Overall, this thesis demonstrates thedevelopment and characterization of several bio-based adsorbents for water remediation.

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