Gecko’s feet for adhesion, lotus leaf for hydrophobicity, moth’s eyes for reduction of reflectivity in solar cells, and electric eels for power generation – these are just a few examples of how we can study nature and apply them in improving current technologies. This technique is called biomimicry.
Researchers in Australia, led by Benny Freeman, have recently discovered a ground-breaking material that mimics the ion selectivity or filtration mechanism of organic cell membranes. The new study, published in the journal Science Advances, proves that the material provides an ultrafast and efficient way to extract lithium, other metals, and minerals from water.
First, how do our own cells filter ions? The answer was discovered by Roderick MacKinnon and Peter Agre and the subject of the 2003 Nobel Prize in chemistry.
Our cell membranes, or the walls that separate our cells from their surroundings, are made up of tiny pores, which are actually functioning as channels to water molecules and metal ions. In particular, the ion channels allow potassium ions to pass through inside the cells, but inhibit sodium ions to enter, making them ion-selective.
How this happens, one might think that potassium ions are smaller than the sodium ions that’s why they can enter the cells. However, that’s not the explanation behind the selectivity or filtration mechanism of our cell membranes.
Ions outside the cells are hydrated, that is, they are surrounded by water molecules. As they enter the ion channels, these water molecules detach and so the ions become dehydrated. The dehydrated potassium ions are close enough to the oxygen atoms within the channels that they get easily attached and thus pass through the channel, and finally, get rehydrated inside the cell. On the other hand, the sodium ions, which have smaller diameter, are distant from the oxygen atoms within the channels such that they can’t bind to the channels and instead, go back to the oxygen atoms of the water molecules outside the cell membrane.
The newly developed filtration membrane uses a zinc-based metal-organic framework (MOF), which mimics the ion selectivity mechanism of our cell membranes. Its high ion selectivity is just one of the reasons it could be a breakthrough in lithium and metal recovery and water desalination.
- Highly Ion-Selective
The zinc-based membrane used in the study is a MOF called ZIF-8, which has angstrom-sized pore windows and pore cavities, the same with biological cell membranes. The ion selectivity of ZIF-8 mimics that of biological cells such that the metal ions also undergo a series of hydration-dehydration-rehydration as they pass through each pore of the membrane. The results of the study show that “the size confinement might play a more significant role in the ion selectivity.”
Although ZIF-8’s ion selectivity is not comparable with that of biological cells, it still has the highest ion selectivity among other artificial filtration membranes. As such, it can efficiently remove specific ions, unlike other filtration processes that remove all ions.
The filtration process using the membrane needs a minimal amount of electricity to make the ions flow to the other side of the filter. Under a very low voltage of 20 mV, the zinc-aluminum-based filtration has a filtration rate or ion mobility of 100,000-1 million ions per second, which is comparable to biological cell membranes. The results of the study show that the interaction between the water molecules and ZIF-8 “apparently plays a key role in the absolute ion mobility.”
The superfast transportation of ions to the other side of the ZIF-8 membrane is also attributed to its very large internal surface area, the largest among any known material so far, owing to its sub-nanometer-scale internal structure.
- Energy Efficient
Reverse osmosis membranes are currently the widely used technology in removing salts from water. In terms of energy consumption, these membranes can still be improved by a factor of 2 to 3. On the other hand, the ZIF-8 membrane, mimicking the selective ion transport mechanism of biological cells, consumes much less energy. “Stripping water molecules from the weakened hydration shell when ions pass through the small ZIF-8 pore should cost less energy,” the researchers explained in their published article.
Unlike reverse osmosis, using a ZIF-8 membrane in water filtration consumes much lesser energy because it does not force water molecules to also pass through it and it does not require high pressure to make water flow through the membrane.
- Environmentally Sustainable
Its energy-efficient and ultrafast filtration properties make ZIF-8 environmentally sustainable, in addition to its abundant raw materials and safe manufacturing. “The prospect of using MOFs for sustainable water filtration is incredibly exciting from a public good perspective while delivering a better way of extracting lithium ions to meet global demand could create new industries for Australia,” Dr. Anita Hill of CSIRO said.
- Efficiently Recovers Lithium and Other Metals from Waste Water
According to the study, zinc-based MOF showed the following selectivity series: Li+ > Na+ > K+ > Rb+, which is contrary to the conventional filtration membranes. This means that ZIF-8 could extract Lithium best among other metal ions from wastewater such as in mining industry. This zinc-based MOF thus offers a new and efficient method of supplying global demand for lithium, which plays a significant role in mobile devices and electric cars.
“The prospect of using metal-organic frameworks for sustainable water filtration is incredibly exciting from a public-good perspective while delivering a better way of extracting lithium ions to meet global demand could create new industries,” said Anita Hill, CSIRO’s chief scientist.
“Also, this is just the start of the potential for this phenomenon. We’ll continue researching how the lithium-ion selectivity of these membranes can be further applied. Lithium ions are abundant in seawater, so this has implications for the mining industry who current use inefficient chemical treatments to extract lithium from rocks and brines. Global demand for lithium required for electronics and batteries is very high. These membranes offer the potential for a very effective way to extract lithium ions from seawater, a plentiful and easily accessible resource.”
For instance, the wastewater generated by hydraulic fracturing in the Barnett and Eagle Ford shale formations in Texas contains high concentration levels of lithium. On a weekly basis, each well can generate up to 300,000 gallons of fracking water, which could be enough resource for lithium to power 200 electric cars or 1.6 million smartphones.
- Efficient for Water Desalination
The research team’s filtration membrane can also be applied to the efficient removal of salts and ions from water. As one of the authors of the study, Monash University’s Professor Huanting Wang said, “We can use our findings to address the challenges of water desalination. Instead of relying on the current costly and energy-intensive processes, this research opens up the potential for removing salt ions from water in a far more energy efficient and environmentally sustainable way.”
The ZIF-8 membranes “have significant potential to perform the dual functions of removing salts from seawater and separating metal ions in a highly efficient and cost-effective manner, offering a revolutionary new technological approach for the water and mining industries.”