Parameter Optimization in Preparation of Membranes for Osmotic Processes (POPMOP)

Abstract

The interest in osmotically driven processes such as pressure retarded osmosis (PRO) and forward osmosis (FO) has increased during the last decade. Thus, the synthesis of new membranes specifically designed for the process is essential for the development of these technologies. Conventional thin film composite (TFC) membranes have a relatively hydrophobic support layer e.g. polysulfone (PSf). However, a more hydrophilic support layer is desirable in osmotically driven processes as such supports would give better water flux, less ICP and less fouling. In the current project, we demonstrate that it is possible to coat a hydrophilic support membrane given there are enough functional/reactive groups on the surface. TFC membranes with a cellulose triacetate (CTA) support layer were prepared using a modified interfacial polymerization (IP) method. In this method, a linking molecule covalently binds the polyamide (PA) active layer to the CTA support. The screening of the membranes prepared under various conditions was mainly done in RO mode.

The effects of three linking molecules (trimesoyl chloride (TMC), succinyl chloride and malonyl chloride) on membrane performance were investigated. The membrane prepared using TMC as the linking molecule displayed a strong decrease in salt rejection as a function of time in reverse osmosis (RO) and a strong increase in reverse salt flux as a function of draw solute concentration in forward osmosis (FO). The membranes made with bifunctional succinyl and malonyl chloride displayed more stable performance. Confocal laser scanning microscopy (CLSM) images of the membranes indicate changes in CTA support morphologies upon exposure to high salt concentrations, especially for the TMC membrane. The different behavior of the trifunctional TMC membrane could be attributed to the higher number of charged groups on the support membrane, which causes anisotropic swelling/deswelling in the polymer. Our results suggest that bifunctional linkers can be used to fabricate membranes with performance characteristics less dependent upon salt concentration. The implications of the results for FO and PRO are discussed.

The modified IP method were performed on commercially available hydrolyzed cellulose acetate (CA) UF-membranes and RO grade CTA membranes. These membranes were not customized for the novel IP method. Thus, the preparation of a customized membrane was investigated. A screening of different CTA dope solution compositions (solvents, concentrations) for phase inversion was performed. However, it was difficult to prepare a suitable CTA membrane using common solvents. The attempts to prepare a customized CTA support were not successful.

A pre-study were performed on the preparation of CA hollow fiber support membranes. Several different spinning parameters were investigated and analyzed using principal component analysis (PCA). The hollow fiber CA support membranes prepared in this pre-study displayed fiber diameters in the range of 486-701 µm and membrane thickness of 56-171 µm. These dimensions are in the range of previously reported hollow fiber FO-membranes. The results from the PCA analysis indicated that air gap and bore rate were the most important parameters to control fiber diameter and thickness.