PhD Defence Edward Kimani | Physicochemical Modeling of Transport Mechanisms in Reverse Osmosis Desalination Membranes

Physicochemical Modeling of Transport Mechanisms in Reverse Osmosis Desalination Membranes

Edward Kimani is a PhD student in the Department of Membrane Science & Technology. (Co)Promotors are prof.dr.ir. W.G.J. van der meer from the Faculty of Science & Technology, dr. P.M. Biesheuvel from WETSUS and dr.ir. S. Porada from the Wroclaw University of Science & Technology.

Reverse Osmosis (RO) is a key technology for desalinating seawater and brackish water, but predicting ion selectivity in multi-ionic mixtures remains a significant challenge. Current models fail to account for complex ion interactions, such as acid-base reactions and ion pairing, which influence membrane charge and ion retention. This limitation hinders the ability to predict membrane performance, optimize processes, and design efficient multi-unit RO plants.

This thesis develops and validates advanced transport theories for RO desalination of multi-component salt mixtures, integrating acid-base reactions and ion interactions. A 1-D framework using the extended Donnan steric partitioning (ext-DSP) pore model was developed to study ion pair formation and its impact on rejection (Chapter 2). Systematic investigations into cation concentration ratios, feedwater pH, and temperature (Chapters 3–5) revealed that cation ratios affect cation rejection more than anions, the slight charge of polymeric membranes critically influences salt transport, and temperature changes predominantly alter salt and membrane properties, with water transport being more temperature-sensitive than salt transport. A 2-D framework for spiral-wound RO modules (Chapter 6) was also developed, analyzing localized transport phenomena and identifying optimization opportunities through parametric studies of feed conditions and membrane geometry.

By integrating acid-base reactions and ion interactions into transport models, this thesis provides a robust theoretical foundation for predicting ion retention in complex mixtures. It highlights the minor but crucial role of the polyamide active layer's charge in ion rejection and demonstrates the importance of considering hydraulic pressure losses in multi-element pressure vessels. These advancements improve membrane design, process optimization, and more efficient RO plant configurations, addressing critical gaps in current desalination technologies.