Sustainable Separation of Actinides and Rare Earth Elements Using Sorbitol-Oleic Acid Derived Bio-Based Solvents: Synthesis and Extraction Performance

In the modern world, a quiet revolution is taking place in the materials that power our technology. From the magnets in electric motors to the screens in our devices, a group of elements known as rare earths is essential. These elements are often found mixed together in the earth's crust, and because they are so chemically similar, pulling them apart is a difficult and messy job. To do this, scientists use a process called solvent extraction, which is essentially a way of washing a mixture of metals with a special liquid that grabs onto specific elements and leaves others behind. For decades, the liquids used for this washing have been made from petroleum, a fossil fuel. These traditional chemicals work well, but they are toxic, flammable, and difficult to dispose of safely at the end of their life, leaving a heavy environmental footprint on the very industries trying to build a cleaner future.

A team of researchers at Ege University in Turkey has set out to solve this problem by creating a new kind of washing liquid made from nature. Instead of using petroleum, they turned to sorbitol, a sugar alcohol found in fruits, and oleic acid, a common fat found in olive oil. By reacting these two natural ingredients together, they created a bio-based solvent that can separate valuable metals from radioactive ones. The goal was not just to find a greener alternative, but to see if a substance made from food-grade materials could perform the delicate task of separating thorium and uranium—elements used in nuclear energy—from the rare earths needed for green technology. The results show that while these new bio-solvents are not as powerful as the old chemical giants, they offer a promising, sustainable path forward for a specific type of separation that is crucial for the future of clean energy.

The researchers began by mixing the sugar alcohol and the fatty acid in a glass reactor, heating the mixture to high temperatures to encourage them to bond. As the reaction progressed, they monitored the chemical changes, watching as the acidic parts of the oil molecules transformed into ester bonds, a type of chemical link that gives the new liquid its unique properties. They created two versions of this new solvent, one with a slightly different ratio of ingredients than the other, and named them SOA2 and SOA3. To confirm they had successfully created the new substance, they used advanced imaging tools to look at the molecular structure, finding clear evidence of the new ester bonds that were not present in the original sugar or oil. They also measured how acidic the liquid was, noting a sharp drop in acidity as the reaction completed, which signaled that the transformation was successful.

Once the new solvents were ready, the team put them to the test in a series of experiments designed to see how well they could pull specific metals out of a water-based solution. They created a mixture containing thorium, uranium, and several rare earth elements, all at the same concentration, and mixed it with their new bio-solvents. They carefully adjusted the acidity of the water and the time the liquids were allowed to mix, watching to see which metals would jump into the new solvent and which would stay behind. The results were revealing. At lower acidity levels, the new solvents struggled to grab the metals, as the hydrogen ions in the water competed too strongly. However, once they raised the pH to a moderate level, the solvents began to work effectively.

The most striking finding was the ability of these bio-solvents to distinguish between the radioactive actinides and the rare earths. When the conditions were optimized, the new solvents were able to pull thorium out of the mixture with high efficiency, leaving the rare earth elements behind in the water. In one of the best cases, the new solvent was able to separate thorium from the rare earths with a selectivity factor of over 1,800, meaning it was vastly more likely to grab the thorium than the rare earths. This is a significant achievement because these two groups of elements are notoriously difficult to tell apart. The solvent also showed a strong preference for uranium over the rare earths, though it was less effective at separating uranium from thorium.

To understand how these new solvents compared to the industry standard, the researchers pitted them against two well-known chemical extractants, Cyanex and D2EHPA, which are made from petroleum. The comparison showed a clear trade-off. The traditional petroleum-based chemicals were much more aggressive; they grabbed onto almost every metal in the mixture, regardless of type, and did so very quickly. This high capacity is useful for some industrial tasks, but it makes it hard to separate one metal from another. The new bio-based solvents, by contrast, were more selective. They did not grab as many metals overall, but they were much better at choosing the right ones. They acted more like a gentle filter, allowing the radioactive elements to pass through while holding back the rare earths, or vice versa, depending on the conditions.

The researchers proposed that this difference comes down to how the chemicals interact with the metals. The traditional petroleum chemicals act like strong magnets, forming tight, ionic bonds with the metal ions. This strong grip allows them to pull in almost anything, but it makes it hard to be picky. The new bio-based solvents, however, work through a softer mechanism. They surround the metal ions and replace the water molecules that usually cling to them, a process driven by weaker forces. Because this grip is gentler, the solvent is less likely to pull in metals that are tightly bound to water, which happens to be the case for many of the rare earth elements. This subtle difference in how they hold onto metals is what gives the bio-solvent its ability to be selective, even if it means it cannot hold as much metal at once.

When the team looked at how their new solvents stacked up against other bio-based or neutral solvents reported in scientific literature, the performance was competitive. The separation factors they achieved were comparable to other advanced, non-petroleum options, suggesting that these sugar-and-oil derivatives are a viable contender in the field. However, the study also highlighted a limitation. While the new solvents were excellent at being selective, their overall capacity to hold metal was lower than the aggressive petroleum-based chemicals. This means that for large-scale industrial use, more of the new solvent might be needed to process the same amount of material, or the process might need to be adjusted to handle concentrated solutions differently.

The study concludes that while these sorbitol-oleic acid solvents are not a perfect replacement for every industrial need, they represent a significant step toward sustainable metal refining. They offer a way to separate critical materials without relying on fossil fuels and without generating the same level of toxic waste. The researchers suggest that the future of this technology lies in refining these bio-solvents to increase their capacity without losing their selectivity. One potential path forward is to design polymer versions of these solvents that combine the gentle, selective nature of the bio-material with stronger binding groups. Until then, this work proves that it is possible to create high-performance separation tools from renewable resources, offering a cleaner alternative for the industries that power our modern world.