Understanding the Relationship between Osmotic Membrane Structure and Separation Performance

Water scarcity, water-stress, and drought are becoming common terms to describe the world we live in. In order to augment the world's depleting fresh water supply, methods to treat non-traditional water sources to become potable water are being explored. Reverse osmosis is a very promising technology to desalinate seawater, brackish water and even waste water into fresh product water. Forward osmosis (FO) is a developing technology that is currently used to concentrate fruit juices and provide nutrient juices using impaired water sources. Furthermore, FO is also explored as a potential for low energy desalination. However, much research needs to be done to understand how to improve membrane performance, especially for FO applications. The goal of this research was understand whether membranes with high hydrophilicity and large macrovoids will lower internal concentration polarization in FO membranes and increase water permeability and selectivity. Polyaniline (PANi) was used a model material due to its hydrophilicity and tunable pore structure to develop this understanding between membrane structure and performance. First, a review of current membrane materials for RO and FO was provided, along with an overview of membrane formation techniques, and previous research on polyaniline used in membrane filtration. A computational fluid dynamics model study revealed that the base thickness of the membrane dictates the upper bound of a composite membrane. With the same polymer volume, distributing the polymer as an undulating film will always have higher permeability due to the creation of shorter diffusive path lengths. This research showed that the interplay between support membrane and thin film are vast. Then, phase-inverted PANi membranes along with membranes blended with polysulfone were coated with polyamide thin film by interfacial polymerization. Due to the hydrophilicity of PANi, it was found that more water was taken up than the amine during the immersion step, requiring changes in the amine concentration to increase diffusivity into the reaction zone. Through this study, it was found that the membrane structural parameter was not representative of the porous substructure, but rather of the mass transfer limitations of the membrane. As a result, FO membranes were made by phase inversion of pure PANi without thin film coating, and post-treated by wet-curing, acid and base to alter its performance and skin-layer morphology. By post-treating a membrane with camphor-sulfonic acid, enhanced performance was achieved due to denser skin layer and increased pore volume. This study provided an overview of the relationship between composite membrane structure and performance in finding that: 1) support membrane changes flux distribution, 2) thin film polymer distribution can change performance, 3) support membrane dictates thin film formation, and 4) phase inverted membrane skin layer dictates performance