Abstract

With increasing water scarcity and water demands, conventional methods in which rainfall and river runoff are relied on are no longer sufficient to meet human demands. Desalination of seawater and highly brackish water resources has been increasingly viewed as the most considerable option to meet water demands. Moreover, wastewater recycling and reuse has gained more attention in recent years. Reverse osmosis (RO) has emerged as one of the most important technologies for water desalination and water treatment due to its excellent water quality and low energy consumption. Although commercial seawater reverse osmosis (SWRO) membranes achieve high salt rejection (98 – 99.8%), the presence of interchain network and aggregated sub-nanopores in the RO selective layer allow the permeation of small neutral solutes (SNS) such as boric acid, B(OH)3, and urea, (NH2)2CO, through the membrane, rendering commercial RO membranes insufficient to effectively remove SNS with rejection rates of 30 – 80% and 10 – 60% for boric acid and urea, respectively. Industrially, desalination and wastewater treatment plants employ hybrid treatment processes to achieve the required boron and urea levels in the product water, which significantly increase the capital, operation, and maintenance costs. The elimination of the costly additional treatment steps typically employed for boron and urea removal require the development of RO membranes with exceptional size-sieving properties. 

The objective of this dissertation was to create a thin-film composite (TFC) RO membrane with a highly cross-linked, and tightly structured polyamide (PA) layer that can achieve maximum removal rates of boric acid and urea, and by extension other water-soluble SNS, in a single-pass RO process. Herein, two strategies were adapted to enhance size-sieving properties: (1) water-curing as a post-treatment of in-lab made TFC membranes to increase the densification of the polymeric structure by “sealing” free-volumes with newly formed in-situ MPD–TMC PA, and (2) utilizing residual amine groups present in the PA matrix by reaction with an acyl chloride-containing coating monomer to further in-situ “plugging” remaining pores, achieving further enhancement in the size-sieving properties. The results showed significant improvement in rejection rates, achieving > 90% and > 88% rejection for boric acid and urea, respectively, in a single pass RO process.