Sustainable water via polymeric membranes with highly homogenized nanopores

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Water scarcity poses a challenge to all continents and impacts the lives of almost 40% of the global population. By 2025, the country or territory inhabited by 1.8 billion individuals will face a severe scarcity of water, and approximately two-thirds of the global population may endure catastrophic water shortages. Consequently, it is imperative to reclaim water resources from diverse sewage sources. Membrane technology, which relies on a distinctive rate-governed separation mechanism, offers significant benefits in terms of energy efficiency and environmental sustainability. In particular, nanofiltration (NF) is a process in which very small sieving holes are used to effectively filter water or separate organic substances by precisely separating molecules or ions. NF membranes that have both high water permeance and high solute selectivity are necessary to achieve low energy consumption by reducing the operating pressure, improving product purity by rejecting unwanted substances, and reducing the floor space needed for the separation system, aligning with the strategic goals of sustainable development. Nevertheless, existing nanofiltration membranes are still limited by the inherent compromise between the permeability of water and the ability to selectively filter solutes caused by undesirable nanopores in the selective layer, restricting their effectiveness in energy-efficient applications.

Recent research has shown that density fluctuations in the selective layer of membranes have a negative impact on the transport of water molecules, indicating that improving the homogeneity of the nanopore structure is beneficial for increasing water permeance without sacrificing selectivity. However, the pore size of the selective layer also plays an important role in the transport of water molecules. Therefore, it has become clear that simultaneous control of density fluctuations and pore size in the selective layer is required to obtain an optimal membrane structure for achieving both high permeance and selectivity, but this remains a challenge.

Fig. 1 Schematic illustration of cinnamate-mediated polymerization (CMP). In the typical synthesis of highly homogenized membranes (HMMs), cytidine 5’-monophosphate (CA) molecules can react with the designed reactive monomers of CB-GMA (benzyl cinnamate (CB) and glycidyl methacrylate (GMA)) to form a selective layer on the porous substrate via the CMP reaction.

To achieve the above goals, we comprehensively considered the following aspects and rationally designed HHMs with highly homogenized and well-tailored nanopores. First, how can the diffusion of reactive monomers in situ be controlled to achieve high homogeneity in nanopores? Second, how can the size of nanopores be controlled to achieve sustainable water purification while ensuring selectivity? Finally, how can the inherent trade-off effect between the permeability and selectivity of membrane materials be overcome to achieve an efficient water supply?

Fig. 2 MD simulation results. a, Initial (0 ns) and final (10 ns) morphological diagrams of the P-GMA (left) and CB-GMA (right) systems. The smile faces represent ordered and effective monomer diffusion. The crying faces represent random and ineffective monomer diffusion. b, Number density (ρN) distribution of CA molecules in the z-axis direction. c, The increase in the number (ΔN) of CA molecules in the polymerization reaction region. d, Radial distribution function between CA centroids. e, Number of H-bonds between CB and CA.

In this work, we report a cinnamate-mediated polymerization (CMP) method for synthesizing a highly homogenized membrane (HHM) with well-tailored nanopores. CMP can effectively regulate both monomer diffusion and chain spacing, as corroborated by both computational simulations and experiments. Molecular dynamics simulations revealed uniform cytidine acid (CA) monomer diffusion from the water phase to the polymerization reaction zone due to strong hydrogen bonding interactions between benzyl cinnamate (CB) and CA, which inhibited CA agglomeration. In addition, CB bridges between reactive sites in situ can regulate intermolecular voids via the length of rigid C-C skeletons. Consequently, our HHMs exhibited unparalleled pure water permeance (104.3 L m-2 h-1 bar-1) and excellent molecular sieving ability, overcoming the current upper bound. HHMs can provide an economical and environmentally friendly pathway to enable a sustainable water supply. High-performance and stable HHMs can greatly reduce the size of membrane modules and extend their service life while maintaining highly efficient water treatment, which is beneficial for saving limited resources. More importantly, HHMs can also be extended to the purification of organic solvents, which is expected to promote the efficient recycling of chemicals and a circular economy. We believe that our HHM can also be applied to a wider range of fields, such as carbon capture, crude oil processing, and beyond.

For more information about this highly homogenized membrane technology for sustainable water purification, please refer to the paper by Huang et al. “https://www.nature.com/articles/s41893-024-01371-1”.

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