Menachem Elimelech

Brackish water desalination is imperative for meeting water demands in arid regions far from the seashore. Reverse osmosis, the leading desalination technology, removes nearly all calcium and magnesium ions, which are essential in drinking and irrigation water. Multistep process schemes combining reverse osmosis with ion-selective membrane processes can maintain or reintroduce these minerals without external chemical addition. Previous efforts emphasized membrane processes that retain multivalent ions, focusing primarily on nanofiltration and monovalent-selective electrodialysis. The potential of processes where monovalent ions are retained and divalent preferentially transported through the membrane has not been studied systematically. Here, we explored applying divalent-selective electrodialysis to transfer calcium and magnesium from the influent into the brackish water reverse osmosis permeate …

Pressure-driven membrane desalination (PMD), such as reverse osmosis or nanofiltration, is an energy-efficient technology that addresses water shortages by using saline waters to augment freshwater supplies. This Primer describes several key methodological aspects of PMD, including membrane fabrication, characterization and performance evaluation; system modelling; process configurations; and applications. Thin-film composite polyamide membranes represent the state of the art in reverse osmosis and nanofiltration membranes and are the focus of the membrane development discussion. First, thin-film composite polyamide membrane fabrication using interfacial polymerization and alternative methods is discussed, followed by an exploration of techniques for characterizing the morphological, structural and interfacial properties. Experimental procedures and model frameworks for evaluating membrane …

Membranes consist of pores and the walls of these pores are often charged. In contact with an aqueous solution, the pores fill with water and ions migrate from solution into the pores until chemical equilibrium is reached. The distribution of ions between outside and pore solution is governed by a balance of chemical potential, and the resulting model is called a Donnan theory, or Donnan equation. Including a partitioning coefficient that does not depend on salt concentration results in an extended Donnan equation `of the first kind'. Recently, an electrostatic model was proposed for ions in a pore based on the arrangement of ions around strands of polymer charge, including also ion activity coefficients in solution. That framework leads to an extended Donnan equation `of the second kind', which has extra factors depending on ion concentrations in the pores and salt concentration in solution. In the present work, we set up another Donnan model of the second kind by evaluating the Coulombic interactions of ions in a cylindrical pore, including the interaction of ions with the charged pore walls and between the ions. We assume that counterions are near the pore wall while coions distribute over the center region. Starting from a complete analysis, we arrive at an elegant expression for the chemical potential of ions in such a pore. This expression depends on coion concentration, pore size, and other geometrical factors, but there is no additional dependence on counterion concentration and charge density. This model predicts the Coulombic contribution to the chemical potential in the pore to be small, much smaller than predicted by the electrostatic …

Reverse osmosis (RO) stands as the state-of-the-art desalination technology, owing to its high energy efficiency and low cost. Further improvements in the performance of this technology require a fundamental understanding of transport mechanisms in RO membranes. For decades, the solution-diffusion model has been the prevalent approach to describe water transport mechanism in RO membranes. In this model, water first partitions into the membrane and then diffuses down a concentration gradient of water within the RO membrane. However, recent experimental and theoretical findings pose serious challenges to the fundamental assumptions of this theory. In this perspective, we discuss seven critical flaws in the solution-diffusion model and explain why the model fails to describe water transport in RO membranes. Instead, through our careful analyses, we determine that the pore-flow model, in which a …

During the final revision of this manuscript, a modification to Figure 1 B resulted in an error of a factor of 10 in this figure. Note that the corresponding number quoted in the text (percentage cost of water treatment of< 0.046%) remains correct. However, the numbers represented in the published figure are off by a factor of 10 (and therefore also are inconsistent with the number quoted in the text). The discussion and conclusions remain unchanged, as 0.46% and 0.046% are both still insignificant cost contributions; the correction just further reinforces the conclusions and is consistent with the original numbers quoted in the text. The corrected version of Figure 1 B appears below.

Thin-film composite (TFC) membranes, the backbone of modern reverse osmosis and nanofiltration, combine the high separation performance of a thin selective layer with the robust mechanical support. Previous studies have shown that heat released during interfacial polymerization (IP) can have a significant impact on the physical and chemical structure of the selective layer. In this study, we develop a multilayer transient heat conduction model to analyze how the thermal properties of the materials used in TFC fabrication impact interfacial temperature, focusing on support-free (SFIP), conventional (CIP), and interlayer-modulated IP (IMIP). Using a combination of analytic solutions and computational models, we demonstrate that the thermal effusivities of fluid and material layers can have a significant effect on the temporal evolution of interfacial temperature during IP. In CIP, we show that the presence of a …

In this perspective, we critically assess the competitiveness of salinity gradient energy (SGE) as a renewable energy source. While scientifically intriguing and gaining research attention, SGE encounters formidable challenges in competing with established renewable technologies, such as solar and wind energy. Even the most advanced SGE technology, pressure retarded osmosis, remains far from cost competitive, primarily because of its low energy density and conversion efficiency. These constraints appear to be fundamental, unlikely to be resolved solely through material or process advancements. While integrating SGE in applications such as energy storage or desalination has been actively explored, it still fails to present compelling value propositions in the context of their specific applications. Therefore, we strongly encourage the research community to critically examine the practical impacts of research …

Thin-film composite reverse osmosis membranes have remained the gold standard technology for desalination and water purification for nearly half a century. Polyamide films offer excellent water permeability and salt rejection but also suffer from poor chlorine resistance, high fouling propensity, and low boron rejection. We addressed these issues by molecularly designing a polyester thin-film composite reverse osmosis membrane using co-solvent–assisted interfacial polymerization to react 3,5-dihydroxy-4-methylbenzoic acid with trimesoyl chloride. This polyester membrane exhibits substantial water permeability, high rejection for sodium chloride and boron, and complete resistance toward chlorine. The ultrasmooth, low-energy surface of the membrane also prevents fouling and mineral scaling compared with polyamide membranes. These membranes could increasingly challenge polyamide membranes by …