A flexible layered sulfide captures hydrated rare earth ions from radioactive water, enabling fast and selective cleanup while maintaining chemical stability and radiation resistance in a tunable crystalline framework.
(Nanowerk Spotlight) Radioactive wastewater remains one of the most difficult technical problems in managing nuclear power. Cooling systems, reactor maintenance, and fuel reprocessing all generate water that can contain traces of metallic ions from fission or corrosion. Among these ions are rare earth elements. In nonradioactive form these metals power LEDs, electric motors, and medical scanners. In contaminated water, however, the same chemistry that makes them useful turns them into stubborn pollutants.
Each rare earth ion is small, strongly charged, and surrounded by a shell of water molecules that holds it in solution. This hydrated form makes the ions hard to capture. Conventional cleanup methods such as chemical precipitation, solvent extraction, and membrane filtration struggle to remove them efficiently. These techniques require heavy chemical use, produce secondary waste, or lose performance in complex water mixtures. The result is slow, expensive, and incomplete purification.
The obstacle lies in the structure of the materials used for capture. Most solids that trap contaminants contain rigid pores that can only admit ions after they shed their surrounding water. For rare earth ions that dehydration step consumes extra energy and limits capacity. Researchers have been looking for solids that keep structural order but can also move at the atomic scale to let hydrated ions enter freely.
A study in Advanced Materials (“Wrinkled Layered Sulfide With Tunable Channels Unlocks Precision in Rare Earth Extraction From Radioactive Wastewater”) describes a material that achieves this goal. The researchers created a layered compound of gallium, antimony, and sulfur whose crystal sheets ripple and stack into adjustable channels. These channels widen or narrow as hydrated ions approach, allowing rapid exchange while preserving the overall lattice. The concept is structurally simple yet offers a new path for treating nuclear wastewater and recovering valuable metals from dilute solutions.
The compound, written as [(CH₃)₂NH₂]₂Ga₂Sb₂S₇ and called GaSbS-1, forms in a liquid known as a deep eutectic solvent. This type of solvent is a mixture whose melting point is lower than that of its individual components. It provides both the reaction medium and organic cations, specifically dimethylammonium ions, that occupy spaces between the sulfide layers.
When gallium and antimony sulfides react with hydrazine at about 180 degrees Celsius, the material crystallizes in about 72 percent yield. Lower temperatures or omission of hydrazine lead to amorphous products, which shows that both conditions are essential for forming the ordered structure.
a) Schematic diagram showing the structural aggregation of [Ga2Sb2S10]8− clusters to [Ga2Sb2S8]n4n− chain, [Ga2Sb2S7]n2n− layer, and further to the stacking of layers with the intercalation of [(CH3)2NH2]+ cations in GaSbS-1. b) Zigzag configuration of the adjacent [Ga2Sb2S7]n2n− layers viewed from the b axis and the hydrogen bonds (red dotted lines) between layers and [(CH3)2NH2]+ cations. c) Channel between layers with flexible interlamellar distance. d) Hybrid arrangement of inorganic layers and parallel channels (filled with organic dimethylammonium). e) Schematic diagram of the ion exchange and the expansion along the layer-stacking direction. Depictions of the interlamellar channels in the [Ga2Sb2S7]n2n− layers viewed down the f) b axis, g) layer-stacking direction and h) c axis.
(Image: Reprinted with permission from Wiley-VCH Verlag) (click on image to enlarge)
X-ray measurements explain why GaSbS-1 behaves differently from earlier sulfide exchangers. Within each sheet, gallium atoms link to sulfur atoms in tetrahedral groups, and antimony forms three-sided pyramids. Each antimony atom has a lone pair of electrons that slightly distorts the surrounding bonds, producing microscopic ripples across the layer.
When these layers stack, their peaks and valleys do not align perfectly, creating elongated gaps. Dimethylammonium ions sit in these gaps to balance charge but can move aside when other metal ions approach. Because the sheets are held together by weak forces rather than strong bonds, they can slide and stretch. This movement lets the interlayer channels open enough to admit hydrated rare earth ions, reducing the energy cost of exchange.
Tests confirm that this flexible framework greatly accelerates ion uptake. When solutions containing about six parts per million of rare earth ions were exposed to GaSbS-1, equilibrium was reached in roughly five minutes. Removal efficiencies ranged from 97 to 99 percent for yttrium, cerium, europium, and thulium.
The maximum loadings reached 63 milligrams per gram for yttrium, 125 for cerium, 143 for europium, and 138 for thulium. The data fit a kinetic model where the rate depends on surface interactions rather than slow diffusion through the pores, indicating that the channels are easily accessible.
After exchange, the lattice spacing increases slightly while the overall structure remains ordered. This swelling confirms that the ions enter while still partly hydrated. By avoiding complete dehydration, the process reduces resistance and allows faster capture. The expansion also increases the number of active sites, helping explain the high capacity.
The material remains stable across a wide range of conditions. It maintains strong performance between pH 4 and 9, and its structure stays intact even in solutions as acidic as pH 2 or as basic as pH 12, with only minor surface changes in the most extreme cases. During operation, the material tends to bring the surrounding water near neutral pH, a useful property for continuous systems.
Selectivity is crucial for practical applications where competing ions are present. Common cations such as sodium, potassium, magnesium, and calcium interfere only slightly, even at much higher concentrations. Elements like cesium, strontium, cobalt, uranium, and iron show limited competition, while aluminum reduces uptake somewhat because it binds to similar sites. Among common anions, only carbonate has a significant effect since it forms soluble complexes with rare earth ions. These patterns confirm that the sulfide framework naturally favors trivalent rare earths.
To test real environments, researchers evaluated the material in mineral water, tap water, lake water, and seawater. It removed over 93 percent of target ions from mineral and tap water. In lake water, the efficiency dropped slightly for lighter and heavier elements, while in seawater it peaked near 90 percent for middle elements such as neodymium and europium. The ability to function effectively in highly saline conditions suggests that the material can operate in diverse natural and industrial waters.
Continuous flow experiments examined how the material performs in column systems used for treatment plants. A packed column containing 1.45 grams of GaSbS-1 processed 20,000 bed volumes of water containing all fifteen rare earth elements at one part per million each. Initial removal rates exceeded 93 percent and stayed high for more than half the run before gradually decreasing as the column became saturated.
During operation, the bed expanded by about 60 percent in height, a visible sign of interlayer swelling. Despite this expansion, the solid maintained its crystalline structure, and the captured metals were evenly distributed throughout the material.
For even faster contact, the researchers created a thin membrane by coating GaSbS-1 crystals onto a polytetrafluoroethylene support about 40 micrometers thick. When solutions containing 0.1 part per million of rare earth ions passed through at moderate pressure, over 99.7 percent were captured in less than a quarter of a second.
Even at ten times that concentration, removal stayed above 99 percent for hundreds of milliliters before gradually declining as the surface sites filled. This combination of high speed and deep purification shows how the flexible channels enhance performance without complex processing steps.
Captured metals can be recovered easily. Washing the material with a potassium chloride solution releases the rare earth ions and restores most of the adsorption capacity. After several regeneration cycles, performance decreases modestly but remains high. Radiation tests confirm that the structure endures beta and gamma doses above 150 kilograys, levels typical of radioactive waste streams, with no detectable damage.
The value of this study lies in its design principle. Wrinkling the lattice of a layered sulfide introduces mobility while preserving order. This structural movement lets hydrated ions pass quickly through the solid and exit when conditions change, creating a reversible system that joins the durability of a crystal with the adaptability of a soft material. The concept could be applied to other combinations of elements to reduce cost and tailor properties.
GaSbS-1 demonstrates that flexibility and selectivity can coexist in ion-exchange materials. Its rapid uptake, high capacity, and resistance to harsh environments point to applications in treating radioactive wastewater and recovering valuable metals from industrial sources. The work suggests a new approach to designing solid-state exchangers that respond precisely to ions rather than forcing them through fixed rigid channels.
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