Metal-Organic Sponges Could Clean Water And Solve Feared Lithium Shortage

Metal Organic Frameworks just look like ordinary crystals, but it is their molecule-sized gaps that make them important. CSIRO

Few needs are more urgent than clean water and clean power, and crystals known as metal-organic frameworks (MOFs) could help with both. Besides drinkable water, they could give us lithium for batteries, newly published research demonstrates.

MOFs have interested chemists for some time because their sponge-like structures have unmatched internal surface areas for very small volumes. All that space inside can be used to capture and store molecules and ions, and holes in the lattice can be tuned to perfectly match a target molecule's size, allowing MOFs to act as filters or storehouses.

While the potential has been obvious for a while, making the right MOF for a specific job can be challenging. Now, however, a paper in Science Advances reveals MOFs ready for primetime, with the development of two membranes with unprecedented capacity to select alkali metals from water passed through them.

To remove the salt from seawater we currently either boil it, or push it through membranes using what is known as reverse osmosis. Both are hugely energy intensive, but better membranes could dramatically improve the energy efficiency. Indeed, our own cells remove ions with an energy demand nothing we have made can match. It is hoped MOFs can fill this gap. If you doubt the importance, ask a resident of Cape Town.

Seawater contains lithium, sodium, potassium, and rubidium from the alkali metal group, and while sodium dominates, we need to take them all out to get fresh water. Conventional membranes select the largest atoms for transport most easily, but the MOFs described in the paper, like biological membranes, do the reverse.

Dr Anita Hill of CSIRO told IFLScience that purification of water with many different salts is likely to require multiple MOF membranes, but as the electricity required is so much lower than for existing methods, this is still likely to be a practical solution. Hill acknowledged even the MOFs described in the paper are less efficient than those found in biology but said; “We're getting closer. Cell membranes have perfectly positioned and sized pores for ions to go through and separation to take place.”

Besides the production of clean water, water purification can extract elements valuable in their own right. Mobile electronic devices sent demand for lithium-ion batteries sky high, but electric cars could take that into space. The capacity to remove lithium from brines produced in mining could make a substantial contribution to meeting this need.

Materials science can be a slow field to bring products to the market, Hill told IFLScience, so it could be 10-15 years before MOF-based water filters become available. However, all three research institutions involved in the work already have spin-off companies working in the area, which Hill hopes will shorten that process.

Schematic of an iron-based metal-organic framework with gaps that molecules and ions can pass through. With the right-sized gaps, these can sort molecules based on size. National Institute of Standards and Technology/Public Domain

 

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