Desalinization has become a convenient way of sourcing fresh water. The process involves forcing seawater through a reverse-osmosis process using membranes. These devices used here have pores that favor the passage of molecules with the size and properties of water, but not of other substances (e.g., sodium chloride). Therefore, they can deliver the isolation of fresh potable liquid from the normally undrinkable water. Therefore, these molecules, which are made of a high-tech polyamide, are now used at scales that may even be industrial in some locations.
Limitations of Existing Membranes
This polymer also has considerable drawbacks as a material for membranes. The production processes associated with the equipment renders the polyamide rather clumpy and inelegant at a microscopic level. This, as any polymer scientist would indicate, is rarely a desirable quality in any material that is supposed to form a regular pattern such as the one that results in the membranes’ pores.
Furthermore, this clumpy structure has exhibited the ability to accumulate tiny bits of organic matter as it allows raw seawater to pass through it. As a result, the membrane becomes irretrievably clogged with the material after a certain length of time. The people who operate or depend on these membranes then have to replace it with a new one, and so on.
This is a less than ideal situation for communities and individuals who have come to rely on desalinization for simple, fresh water. However, as it currently stands, these people are locked in a continuous cycle of repeat membrane purchases and installations.
There are few realistic, viable alternatives to these polyamide membranes at present. Scientists are still working on the closest analogs we have. For example, researchers at the University of Manchester reported on their version of a desalinization membrane made using graphene in place of the polymers. However, these products are still far from general availability. Therefore, the optimal interim solution would be to improve the polyamide in terms of structure, function or both.
A microscopic image of MOF (metal-organic framework) crystals that have been designed to separate lithium from seawater. (Source: CSIRO)
But, a group of chemical, molecular and materials engineers from the University of Connecticut (Storrs) has recently published a report on work that may solve the desalination-membrane problem.
New and Upgraded Desalination Membrane
These researchers, led by Jeffrey McCutcheon of the Department of Chemical and Biomolecular Engineering at Connecticut, claim that their new process for membrane manufacture could reduce accumulations on them by up to 40% compared to their conventional counterparts.
The reason for this is commercial models are produced by dipping a membrane ‘substrate’ into a vat of chemicals that are the precursors for polyamide. The polyamide then self-assembles onto the substrate in order to form a complete membrane. Accordingly, the polymer may be arranged in the disordered, sub-optimal clumps with which McCutcheon and his colleagues have taken issue.
Their new method, on the other hand, is based on a technique called electrospraying. This process enabled the researchers to lay the polyamide precursors down onto a substrate (thin-layer aluminum, in this case) in more precise monomolecular layers.
Therefore, these precursors sorted themselves into a more complex polyamide in a more regular, optimized conformation.
However, the Connecticut team also reported that the ability of these polymer layers to be selectively permeable to water (or to allow this substance through them in preference to others such as salts) was comparable to that of a commercial membrane.
A conventional polyamide membrane (left) and a new, electrosprayed variant (right) imaged using SEM. (Source: M. R. Chowdhury et al., 2018)
The team also claimed that their method allowed for the creation of ultra-thin membranes as the polyamide could be laid down at a thickness of as little as 4nm per individual layer. In addition, the variability in the membranes’ surfaces (which determines its roughness or unevenness, and the susceptibility of matter to accumulate on it) was determined to be reduced to as little as 2nm.
All in all, these properties resulted in an appearance that was considerably more regular under scanning electron microscopy.
This new technique of desalinization membrane production could also be regarded as a form of 3D printing, as it depends on the gradual building-up of a structure one layer at a time. Therefore, it may be readily relatable to many pre-existing manufacturing processes.
The technique may also make the production of these new, smoother membranes easier and more cost-effective. This is potentially good news in a future that has been estimated to involve increasingly widespread water shortages, even for developed regions of the world.
Top Image: A major desalinization plant on the Gold Coast of Australia. (Source: Spazio Infinito @ Wikimedia Commons)
A filter that turns saltwater into freshwater just got an upgrade, 2018, Science News, https://www.sciencenews.org/article/filter-turns-saltwater-freshwater-just-got-upgrade , (accessed 27 Aug. 18)
Graphene sieves: Helping Convert Salt Water to Drinking Water, 2017, Evolving Science, https://www.evolving-science.com/environment-energy-water-and-waste-management/graphene-sieves-helping-convert-salt-water-drinking-water-00213 , (accessed on 27 Aug. 18)
More than 2 billion people lack safe drinking water. That number will only grow. 2018, Science News, https://www.sciencenews.org/article/future-will-people-have-enough-water-live?tgt=nr , (accessed on 27 Aug. 18)
M. R. Chowdhury, et al. (2018) 3D printed polyamide membranes for desalination. Science. 361:(6403). pp.682-686.