Date

2022-4-26

Author

Kocaman, Ceren

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Precise control over membrane performance by tuning fabrication parameters is highly desirable. We developed ultrafiltration membranes by depositing cellulose nanocrystals (CNCs) of 180 nm length and 8 nm diameter on porous support via tangential flow filtration, then irreversibly coagulated the deposited layer permeating highly concentrated AlCl3 solution. At a high enough shear rate on the support surface, CNCs aligned in the flow direction, showing a nematic order. The shear rates for transition to the nematic phase determined from rheology analysis, polarized optical microscopy, and membrane performance were consistent with one another, at ca. 10 sˆ–1. Permeating and AlCl3 solution through the shear-aligned CNC deposit stabilized the CNC layer by screening repulsive electrostatic interactions, and a stable CNC layer was obtained. On changing the surface shear rate from 10 to 50 sˆ–1, the order parameter of CNCs increased from 0.17 to 0.7, and the rejection for Blue Dextran (5 kDa) increased from 82 to 92.7% and that for β-lactoglobulin (18 kDa) increased from 89.6 to 93.7%. By varying electrostatic interactions between nanocrystals during deposition, membrane performance was also tuned via changing ionic strength and pH of CNC suspensions. Increasing NaCl concentration of CNC suspensions from 0 to 25 mM increased Blue Dextran (5 kDa) rejection from 92.7% to 97% at high tangential flow rate and from 82% to 97% at low tangential flowrate. For membranes fabricated at high tangential flow rate, β-lactoglobulin (18 kDa) rejections increased from 93.7% to 98% when NaCl concentration increased from 0 to 25 mM. Increasing pH of CNC suspensions also increased probe molecule rejections, implying a smaller overall interparticle distance between CNCs in the deposited layer. CNC membranes’ pure water permeances were comparable to commercial membranes of similar separation properties. Polarized optical microscopy analyses showed that the alignment of CNCs and their aggregates, or tactoids, affect CNC deposits’ morphology. In all cases, CNC membrane structures and performances were tuned by easy-to-apply parameters. Hence, a simple and scalable method for controlling the rejection properties of ultrafiltration membranes is developed, which uses aqueous CNC suspensions to form the selective layer.

Subject Keywords

cellulose nanocrystals, ultrafiltration, tangential flow filtration, shear alignment, electrostatic interactions, controlled deposition

URI

https://hdl.handle.net/11511/97305

Collections

Graduate School of Natural and Applied Sciences, Thesis