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Customized block polymer membranes may allow seawater desalination at lower pressure than with reverse osmosis.

Engineered materials have smaller, more consistent pore sizes for selective filtration

Nanofiltration membranes are currently used in a wide range of water and wastewater treatment processes, but they are not without problems. Because their pores are irregularly sized, they are somewhat limited when it comes to filtering out specific substances.

But, researchers have developed a new block polymer membrane material that may further refine the fitness of membranes for water filtration applications while at the same time allowing for harvest of selected metals and substances for other uses.

In block polymer membranes, strong covalent bonds connect blocks of different polymers on the nanoscale, allowing for permeability on a finer scale. Their smaller, regular pore structure contrasts to the irregular macroscale pores of the polymer membranes we know today.

Notre Dame and Purdue researchers Yizhou Zhang, Noelia E. Almodovar-Arbelo, Jacob L. Weidman, David S. Corti, Bryan W. Boudouris, and William A. Phillip developed the new material through research funded by the Army Research Office and National Science Foundation.

Customized Filtration

The potential of block polymer membranes extends beyond their smaller, homogenous pores: They also can be customized to block specific particles, providing unprecedented control over what can and cannot pass through.

Unlike size-selective membranes currently in use, block polymer membranes allow for selective separations that can be tailored to the chemical makeup of specific water sources and toward the specifications of the end use. To create block polymer membranes, it’s necessary to design macromolecular architectures for precursor block polymers. Creating block polymers that reliably self-assemble into the intended precursor morphologies is essential for incorporating the materials into membrane units that can be tailored to purification application profiles. Several nanostructures of different morphologies may be used, and it may be possible to stack them in layers.

Envisioned Uses of Block Polymer Membranes

The researchers envision use in both traditional and hybrid filtration systems. Hybrid systems could include several phases, depending on whether they’re being fed by wastewater, brackish water, or a seawater stream. For instance, water could first pass through a nutrient extraction process, move on to a metal-capture process, then into a nanofiltration process, and exit through a desalination process, all facilitated by membranes molecularly engineered for each purpose.

One envisioned process would cut the cost of desalination using nanofiltration membranes (NF) by removing only divalent cations from saltwater. NF membranes do not excel at removing monovalent ions, but their removal is not critical for many water-intensive systems, such as cooling towers. Although the NF water would not be as pure as that produced by current desalination technologies, no scale buildup is associated with block polymer membrane-treated water because the divalent cation ions are gone.

The water can still be used for cooling systems without worry of salt scale buildup, but there is an energy savings because the process requires lower pressure than reverse osmosis systems.

Reducing Pore Size

Over the past decade, feasible pore sizes of block polymer membranes have entered the ultrafiltration range at ~20 nm and reduced even farther to the nanofiltration range of ~2 nm, but the pore sizes will have to be reduced even farther to between the 0.7 nm and 0.9 nm scale to be a viable alternative for desalination. The researchers see this as a challenging transition, but research is demonstrating that more resilient, more selective membranes are on the horizon.

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