Chlorine-Resistant Reverse Osmosis and Nanofiltration Membranes
Finding solutions to overcome water scarcity is a key challenge of the 21st century, given that half of the global population will live in water-stressed regions by 2050. One way to alleviate water scarcity is to desalinate sea and river waters through the use of membrane technology. Membranes are semipermeable materials through which water can pass, while salts and other dissolved compounds are rejected. State-of-the-art desalination membranes achieve excellent water fluxes and salt rejections thanks to their thin, yet dense polyamide-based selective layer. However, this layer is also a major source of concern in industrial water treatment plants as it is highly reactive towards chlorine, originating from disinfectant added upstream. Even though chlorine is actively removed, low chlorine concentrations often accidentally reach the membrane. In general, this leads to membrane performance losses, premature membrane module replacement and disposal, plant productivity losses and increased costs. Additionally, the exact mechanism behind chlorine-induced performance changes remains unclear.
Aiming at increasing the sustainability of membrane-based water purification, this dissertation focused on the nano- and macro-scale understanding of the chlorine-sensitivity of commercial polyamide-based membranes, and on developing equally performant but chlorine-resistant membranes.
Chlorination of polyamide membranes is a complex process that is highly dependent on the chlorine concentration, contact time and pH. This study was the first to fundamentally investigate low-dose short-time chlorination of state-of-the-art industrial-scale membrane modules as a function of pH. Surprisingly, the chlorination conditions used in this study significantly improved salt rejection, as opposed to rejection deterioration by long-term chlorination in a water-treatment plant. Thus, chlorination can improve or deteriorate membrane performance depending on the conditions. It can, therefore, when executed in a controlled manner, be exploited as an easy and cheap strategy to boost membrane performance at an industrial level. In most cases, the changes in the membrane physicochemical properties were not proportional to the altered membrane performance, substantiating the lack of a clear structure-property-performance relationship for current polyamide-based membranes. Additionally, comparison with lab-scale studies revealed large discrepancies, underlining the urgent need for more adequate experimental design in academia, and more industrial-scale investigations.
To help unravel the structure-property-performance relationship of state-of-the-art water purification membranes, Elastic Recoil Detection (ERD) was introduced to the field as a novel characterization technique. ERD allows elemental depth-profiling of thin films, with nanoscale resolution and parts per million sensitivity, and thereby overcomes many limitations of the traditional characterization techniques. The determination of the complete elemental composition as a function of the membrane thickness allowed to gain knowledge about the membrane depth-heterogeneity, to determine the top-layer thickness and to analytically quantify remnants from synthesis conditions. When ERD was applied to chlorinated membranes, unprecedented information on the chlorine depth-profiles was obtained.
To overcome the intrinsic chlorine-sensitivity of polyamide-based membranes and the accompanied economic and environmental losses, epoxides were introduced as a novel platform chemistry for the synthesis of chlorine-resistant membranes. The knowledge of the well-known monophasic bulk epoxide polymerization, commonly used in the automotive and flooring industry, was transferred to the interfacial synthesis of thin, yet cross-linked poly(epoxyether) films. Owing to the intrinsic high chemical stability of ether bonds, these membranes were chlorine-, acid- and caustic-resistant, and exhibited concurrent high salt rejections and water permeances. These epoxide-based membranes may hence lay the foundation for a new, exceptionally stable generation of membranes for separations in demanding environments.
Overall, this dissertation investigated the chlorine-sensitivity of current state-of-the art membranes through industrial-scale studies and novel nanoscale characterization, and developed a new class of chlorine-resistant, high-performance membranes. This work paves the way for successful and sustainable implementation of membrane technology in current and novel applications under harsh conditions.