CHITOSAN/PVA MEMBRANE

Abstract

Chitosan is a highly hydrophilic naturally occurring biopolymer. Membranes made of chitosan thus are highly permeable to water and hold high potential for use in the separation of water from aqueous organic solution. Crosslinked chitosan membranes possess higher selectivity for water but higher degree of crosslinking lowers membrane flux and makes the membrane brittle. In this study, chitosan is blended with polyvinyl alcohol (PVA) and the two components are crosslinked with an aim to improve the membrane properties. Chitosan/PVA membrane was successfully made from chitosan/PVA blends and crosslinked using glutaraldehyde as the crosslinking agent and trace amount of sulfuric acid as catalyst. Mechanical strength and membrane flux were increased by adding PVA into chitosan compared to crosslinked membrane without PVA. The miscibility between chitosan and PVA was first investigated. It was found that' miscibility between chitosan and PVA occurs at the molecular level, as indicated by the existence of a unique single Tg whose value depends on PVA content in the blend. Thin film chitosan/PVA membrane were treated with glutaraldehyde to yield a thin crosslinked surface layer on all sides of the membrane. Thus one piece of this crosslinked chitosan/PVA membrane possesses two functional surface layers for separation. Performance of the crosslinked chitosan/PVA membranes is evaluated in a separation of water from water/ethanol mixture through pervaporation (PV) under various experimental conditions. Experiments showed that membrane separation performance was dependent on its degree of crosslinking, feed concentration, downstream and upstream pressures, type of feeds, membrane thickness, PVA content in membrane and run time of the PV system. With higher degree of crosslinking in a thicker membrane coupled with lower downstream pressure and the lower PVA content, the membrane selectivity for water increases up to a point when only water is allowed to permeate through the membrane and ethanol is totally retained. Baker's model appears to be adequate to explain experimental results successfully. Besides the separation of water from water/ethanol feed, separation of water from water/NMP feed was also tested. Crosslinked chitosan/PVA membrane also possessed high water selectivity in the separation of a water/NMP feed. In both the separation of water from water/alcohol and water/NMP mixtures, membrane flux and selectivity for water can be controlled by modification of the membrane structure such as by changes in PVA content in the membrane and the degree of crosslinking.