Study on the Removal of Cesium from Simulated Seawater using Ammonium Phosphomolybdate (AMP) and Natural Zeolite Alginate Microcapsules

When a nuclear facility operates, or when one is decommissioned, it produces water contaminated with radioactive elements. Among these, cesium is a particular concern. It dissolves easily in water, moves quickly through the environment, and stays radioactive for decades. Removing it is vital for safety, but it becomes a difficult puzzle when that water is seawater. Seawater is packed with common salts like sodium, potassium, calcium, and magnesium. These harmless ions crowd out the radioactive cesium, making it hard for standard cleaning materials to grab onto the dangerous element. Scientists have long known that certain minerals can act like magnets for cesium, pulling it out of the water while ignoring the salt. However, turning these fine mineral powders into something practical for a flowing river or a treatment plant is tricky; the powder is too fine to filter out easily, and the salt in the water often weakens its grip.

Researchers in Taiwan set out to solve this specific problem by testing two different mineral-based materials designed to work inside a seawater environment. They took a highly selective mineral called ammonium phosphomolybdate and a naturally occurring mineral known as zeolite, then wrapped each one inside a soft, jelly-like shell made from seaweed extract. This shell, called an alginate microcapsule, acts like a protective net. It holds the fine mineral powder in place so it can be poured into a column and washed with water without washing away, while still letting the water flow through to touch the mineral inside. The team wanted to see which of these two wrapped minerals would be better at catching cesium from simulated seawater, and whether they could do it quickly enough to be useful in a real-world treatment system.

The study began by creating these tiny capsules. The researchers mixed the mineral powders with a liquid seaweed solution and dropped the mixture into a calcium bath. The calcium acted as a glue, instantly turning the drops into firm, round beads that trapped the minerals inside. They made two batches: one filled with the ammonium phosphomolybdate and another with the natural zeolite. To test them, they poured artificial seawater, mixed with a small amount of cesium, over the beads. They measured how much cesium the beads could hold and how well they kept it out of the water. The results showed a clear difference between the two. The beads containing ammonium phosphomolybdate proved to be the superior catcher. They held onto cesium much more tightly and in greater quantities than the zeolite beads. Specifically, the ammonium phosphomolybdate beads captured about 0.102 millimoles of cesium per gram of material, while the zeolite beads captured about 0.078 millimoles per gram. The difference was even more striking in how strongly they held the cesium; the ammonium phosphomolybdate beads showed a distribution coefficient, a measure of how effectively a material pulls a substance from water, that was roughly seven times higher than that of the zeolite beads.

To see if these materials could work in a continuous flow, similar to how a water treatment plant operates, the researchers packed the beads into a column and pumped the contaminated seawater through them at high speeds. They tested flow rates as fast as 50 milliliters per minute. Even at this rapid pace, the ammonium phosphomolybdate beads removed 99 percent of the cesium. When the flow speed increased slightly, the efficiency dipped but remained above 90 percent, proving that the material could handle the pressure of a real treatment system without losing its effectiveness. The zeolite beads also performed well, removing over 90 percent of the cesium under similar conditions, but they consistently lagged behind the ammonium phosphomolybdate in both capacity and strength of capture.

The team also looked closely at what happened to the beads after they had done their job. Using powerful microscopes and chemical scanners, they examined the surface of the beads before and after the cesium exposure. They found that the beads remained physically stable; their internal structure did not collapse, and their surface area, which is crucial for catching contaminants, stayed almost exactly the same. The scans confirmed that cesium had indeed attached to the surface of the beads. However, the scans also revealed that the presence of all the other salts in the seawater made it harder for the cesium to show up on the surface compared to tests done in pure water. The salty environment reduced the amount of cesium detected on the surface by about half for the zeolite beads and by roughly 45 percent for the ammonium phosphomolybdate beads. This confirmed that while the seawater made the job harder, the ammonium phosphomolybdate beads were still robust enough to overcome the competition from the salt ions.

Ultimately, the research demonstrates that wrapping these specific minerals in a seaweed-based shell creates a durable and effective tool for cleaning radioactive cesium from salty water. The ammonium phosphomolybdate beads emerged as the stronger candidate, offering higher capacity and better retention of the radioactive element even in the presence of high salt concentrations. The study suggests that these composite materials are ready for further development as a practical method for treating nuclear wastewater, providing a way to separate dangerous radiation from the ocean without the materials breaking down or washing away.