(626a) Using Polyphenols to Fix and Quantify Defects in Polyamide Membranes

Polyamide membranes for water treatment and desalination are commonly made using the interfacial polymerization process. This process has been the long-established way to create thin film composite membrane comprised of aromatic polyamide. This process is robust and creates high quality commercial membranes. These membranes can be delicate, however, and prone to defects or imperfections when handling or when forming in the lab. This is especially true as the membrane science community explores other formation methods. Some of these methods are nascent and still being refined in order to reduce the prevalence of these imperfections.

In the early days of reverse osmosis (RO) membrane development, tannins or polyphenols. were used to plug the prevalent imperfections inherent to hand-casted membranes. These polyphenols, sometimes extracted from something as mundane as green tea, became somewhat of a legend of early RO membrane development. When processed by an RO membrane, these molecules would plug molecular-scale defects causing an increase in rejection and a decrease in permeance. In many studies on reverse osmosis, this behavior, which is a type of micro-scale fouling that we refer to in this work as “ripening”, was sometimes confused with membrane compaction.

Herein we explore this phenomenon by challenging RO membranes with different polyphenols as a means of probing the size and shape of defects in the membrane. Benchmarking against commercial membranes, we will evaluate newly developed polyamide TFC membranes made from an emergent manufacturing processes known as electrohydrodynamic spray, which are prone to defects. We also explore the direct incorporation of polyphenols into RO membranes using this process to “pre-ripen” membranes prior to use. Polyphenols can react with TMC monomers and form polyester networks, which improve membrane selectivity. The electrospray method of making membranes is also a layer-by-layer method. These materials can be incorporated throughout the polyamide material or added on top as a sealing layer. A two-layer polyamide membrane has a NaCl rejection of 93.57% and is improved to 97.07% by adding three layers of a tannic acid-based polyester as a sealing layer, with a slight decrease in permeance from 0.97 to 0.90 L/m²hbar.