Adsorptive cellulose membranes for fluid separation

Abstract

One of the drawbacks of current solvent stable nanofiltration membranes is environmentally harsh preparation methods which are being used to improve the stability of the membrane materials because of the lack of viable membrane materials stable in organic solvents. This thesis describes research into the utilization of cheap, natural, biodegradable polymer, cellulose, as a material for the development of solvent stable membrane for different liquid separation applications. A simple high temperature dissolution process in an environmentally friendly solvent N-methylmorpholine-N-oxide (NMMO) was used to improve the greenness of the process. Membranes showed significantly high permeances for polar protic and polar aprotic solvents, including acetone, acetonitrile, tetrahydrofuran (THF), ethyl acetate, and alcohols. Dependency of the fluxes on the viscosities of the solvents was explained by the homogenous symmetric membrane structure formed by phase inversion process. The thickness of the membrane was decreased five times and fluxes were improved dramatically without compromising the mechanical strength of the membranes at high pressure and the resistance of them in harsh conditions. SEM images, Hagen-Pouiseille type transport behavior, and drastic increase in the permeances by decreasing thickness confirmed the homogenous symmetric membrane structure. Rejection experiments conducted for water and organic solvents confirmed that the separation mechanism through the membranes is governed by the adsorption taking place on the membrane surface. The adsorption capability depends on the solvent and the charge of the dyes used as markers in rejection experiments. When the membrane is saturated during adsorption, dyes were permeated through it and rejection failed. Some chemical modifications were proposed to modify the membrane surface to improve their efficiency in organic solvent nanofiltration applications. Cellulose membranes showed an exceptional stability in modification conditions while the commercial backing paper was failed. Solvent stable nanocellulose paper (NCP) backing material with very similar chemical stability as the membranes was prepared in order to produce a completely stable and green end product. Since one of the main objectives of this thesis is the development of green membrane fabrication methods, chemical modifications were not being focused in detail, however, they should definitely be investigated in future to open a new perspective and a more sustainable association for OSN applications. The main challenge in this study is to make use of the natural ability of ‘cellulose’ without compromising its green image. Therefore, we reported the usage of cellulose membranes for metal removal (i.e. silver and arsenic) from aqueous solutions by using their high potential on adsorption processes. Very promising results were reported for silver adsorption. Addition of metal organic framework, UIO-66 with high surface area in cellulose matrix improved the adsorption capacity of membranes. If the regeneration of these membranes could be achieved, then large-scale industrial membrane modules could be built especially for silver removal application.