Separating valuable heavy metals, such as uranium, copper, and gold, from complex industrial liquids where they are mixed with highly similar competing ions has long been a fundamental challenge. A study published in Nature Nanotechnology reports a bioinspired membrane strategy that overcomes this limitation, enabling highly selective and efficient metal separation using covalent organic frameworks.

Inspied by microscopic sorting, researchers from the Qingdao Institute of Bioenergy and Bioprocess Technology of the Chinese Academy of Sciences designed artificial nanochannels modeled after voltage-gated calcium (CaV) channels in biological cells. These channels act like an exclusive VIP club. They feature a narrow, one-dimensional hallway lined with highly specific binding sites. When the "VIP" calcium ions enter in a single file, they effectively block the door for all other competing ions—a phenomenon scientists call the "anomalous mole fraction effect" (AMFE).

"If these ions are bound so tightly to the channel walls, how do they move through it so incredibly fast?" asked GAO Jun, corresponding author of the study. "The answer is microscopic shoving. Because the ions are crammed into a tiny, single-file space, they strongly repel each other, essentially pushing the ion in front of them rapidly toward the exit."

"We created microscopic channels just wide enough, at about 1.4 nanometers, to force the target heavy metal ions to line up in a single file," said ZHAO Yongye, co-first author of the study. GAO Hongfei, also a co-first author, added that this precise confinement mitigates the standard size exclusion effect while perfectly accommodating the required single-file transport.

The researchers achieved striking results when they coated the insides of these artificial channels with amidoxime groups, a specific chemical "bait" designed to attract uranium. The system successfully mimicked the biological AMFE. Once uranium entered the channel, it blocked out competing elements, such as vanadium. Because the trapped uranium ions repelled one another, they moved through the barrier smoothly and rapidly, bypassing the usual gridlock.

The real-world potential of this mechanism is vast. In a 22-day continuous test using natural seawater, the process efficiently extracted uranium while rejecting other background metals. Using a low-voltage electrodialysis method, the researchers achieved an extraction throughput that was at least an order of magnitude higher than that of traditional adsorbents.

Furthermore, the researchers demonstrated the versatility of this bioinspired strategy. They successfully extracted copper, a crucial component for clean energy technologies, directly from highly acidic simulated mining leachates by modifying the chemical functional groups inside the channels. They also adapted the chemistry using bipyridine to selectively filter and separate gold from complex liquid mixtures.

"By turning passive adsorptive materials into active, single-file highways for specific metals, this approach bridges the gap between traditional chemical adsorption and modern water filtration," said Li Chaoxu, another corresponding author of the study.

This microscopic sorting mechanism could ultimately lead to a much greener, more sustainable global mining and recycling industry.