Halogen-Bonding Frameworks for Stable Iodine Sequestration: Inspiring Nuclear Waste Management

Radioactive iodine isotopes, like iodine-129 (¹²⁹I) and iodine-131 (¹³¹I), are some of the trickiest and most dangerous byproducts of nuclear fission. For instance, iodine-129 has a staggering half-life of 15.7 million years and with this kind of persistence makes it a huge threat to ecosystems and human health. On the flip side, iodine-131 has a much shorter lifespan—just about eight days—but it’s still dangerous in its own way. It’s biologically active, which means it can get absorbed by living organisms and accumulate, leading to health risks. So, finding a way to trap these isotopes and keep them contained isn’t just important—it’s absolutely critical for making nuclear energy safer and protecting the environment. But here’s the thing: containing radioactive iodine is really, really hard. Scientists have tried using materials like zeolites, metal-organic frameworks, and porous polymers to capture iodine, and while these materials can do the job to some extent, they’re far from perfect. A lot of them rely on weak forces to hold the iodine in place, and unfortunately, those forces just aren’t strong enough to keep volatile iodine from escaping. On top of that, iodine tends to sublimate—it can go straight from a solid to a gas at room temperature. Once it escapes, it’s not just gone; it’s out there, potentially causing contamination. And then there’s the durability problem. Many of these materials can’t handle the high temperatures or tough conditions involved in nuclear waste processing, so they fall apart when they’re needed most. That makes them unreliable for real-world use. Scientists know that solving this problem requires a deeper understanding of how iodine interacts with materials on a molecular level. But that’s no easy feat. Iodine is chemically complex, and finding a material that can both grab onto it tightly and hold up under harsh conditions is a big ask. Most current solutions either capture iodine well but don’t last, or they’re stable but not effective enough at trapping iodine. It’s a frustrating gap that researchers have been working hard to bridge.

This is where the new research paper published in Nano Research Journal and led by Postdoctoral fellow Dr. Yi Xie, Pengling Huang, Qiang Gao, Shiyu Wang, Jianchen Wang & Gang Ye from the Tsinghua University, takes center stage. Their work, published in Nano Research Journal, explores a totally different approach: halogen bonding. Halogen bonds are unique interactions where a halogen atom, like iodine, bonds with an electron-rich site. These bonds are strong, precise, and adaptable, making them an ideal tool for tackling the challenges of iodine containment. By incorporating halogen bonding into specially designed frameworks, the team aimed to create a material that not only captures iodine effectively but also keeps it securely trapped, even in tough conditions.

The researchers developed two materials, ETTA_Cl and ETTA_Br, using a compound called 4,4′,4”,4”’-(ethene-1,1,2,2-tetrayl)tetraaniline (ETTA). To make these materials effective, they added chloride and bromide ions, which were critical for their function. These ions were included to serve as active sites, capable of interacting with iodine molecules through halogen bonds, also known as X-bonds. When the researchers analyzed the materials using single-crystal X-ray diffraction, they found that the frameworks were highly crystalline. What stood out was how well the halide sites were exposed inside the one-dimensional microporous channels, setting the stage for efficient iodine capture. To see how well these materials could trap iodine, the team exposed activated samples to iodine vapor under controlled conditions. Both materials performed remarkably well. ETTA_Cl absorbed up to 1.64 grams of iodine per gram of material, while ETTA_Br captured 0.56 grams. This difference was linked to the fact that chloride ions are more reactive than bromide ions when forming halogen bonds with iodine. As iodine entered the frameworks, the materials turned from pale yellow to black—a clear visual confirmation of successful iodine capture. Additional studies showed that this process was driven by strong chemical interactions, rather than weak physical forces, which explained the materials’ impressive efficiency.

The authors also studied how well these materials could remove iodine from liquid solutions, using iodine dissolved in n-hexane. Both frameworks proved to be highly effective. ETTA_Cl removed a striking 93.75% of the iodine in just 10 hours. What was even more impressive was that the materials retained their structure after capturing iodine, as confirmed by powder X-ray diffraction. This showed how durable and reliable they were, even under challenging conditions. Perhaps the most fascinating part of the study was what happened inside the frameworks after iodine was captured. Crystallographic analysis revealed that iodine wasn’t just trapped; it was stabilized inside the pores through halogen bonds. In ETTA_Cl, iodine molecules lined up in a standing position, forming rare tetrahalide anions called [I₂Cl₂]²⁻. These were held firmly in place by strong linear bonds, with precise bond lengths and angles. ETTA_Br showed similar behavior, though the iodine molecules were arranged in both standing and lying positions, forming [I₂Br₂]²⁻ anions.

The team also tested how well these materials could hold onto iodine at high temperatures. Thermogravimetric analysis revealed that the iodine remained securely trapped up to 150°C, far exceeding the 70°C sublimation point of free iodine. This remarkable stability was due to the strong halogen bonds formed within the frameworks. Further tests, including Raman spectroscopy and X-ray photoelectron spectroscopy, confirmed the presence of intact iodine molecules and demonstrated the robustness of the iodine-framework interactions. This work highlights the potential of these materials to address the longstanding challenge of safely and effectively capturing radioactive iodine.

Dr. Yi Xie and colleagues have made significant advancement in in nuclear waste management. Their innovative work tackles one of the toughest challenges in the field—safely capturing and stabilizing volatile iodine isotopes. By tapping into the power of halogen bonding (X-bonding) and integrating it with hydrogen-bonded frameworks, they’ve developed a cutting-edge solution that redefines what’s possible. These new materials use charge-assisted hydrogen bonds and exposed halide sites to form incredibly strong interactions with iodine, offering a far more reliable way to contain it. What really stands out is their discovery of rare polyhalogen anions, like [I₂Cl₂]²⁻ and [I₂Br₂]²⁻. These anions are notoriously unstable and hard to isolate under normal conditions, but the confined environments of the frameworks made it possible to both create and stabilize them.