Sunblock can be a lifesaver, screening out cancer-causing ultraviolet radiation. Chemists have gradually improved its effectiveness and water resistance over decades.

But one particular chemical in some of the lotions becomes unloosed in seawater – and can accumulate in amounts great enough to kill coral. Now, a team of scientists at the University of Puerto Rico has unveiled a bead that can soak up the offending compound at beaches worldwide.

The sunblock ingredient is oxybenzone – and the bead uses magnetite nanoparticles wrapped in an envelope of biodegradable materials including algae and fish waste derivatives, they report from the 254th National Meeting an Exposition of the American Chemical Society.

Oxybenzone is water-soluble – and bonds with hydrogen. So the researchers oxidized the sodium oleate within the nanoparticles (which also contain iron oxide), they report. The sodium oleate increases the hydroxyl functions groups on the surface of the nanoparticle.

To test their material, they had an undergraduate student apply the sunblock, then immerse herself at a local beach. She went into the water, and witnessed the sunblock leaching off her body into the ocean. She collected the sample of seawater after 10 minutes and brought it back to the laboratory. The concentration of oxybenzone reached 1.3 parts per million.

The second step of the experiment involved seawater samples dosed with oxybenzone at a much-higher level: some 30 parts per million. The beads were then used to soak up the chemical – and they found that 95 percent of the compound was collected after an hour. (Control groups without the beach showed no change in the oxybenzone levels).

“Our magnetite nanoparticle are magnetic, and we can make them specific to a particular pollutant – in this case oxybenzone – by altering the surface chemistry of these particles,” said Felix R. Roman, leader of the team. “The ideas is that if you dump it in water, you can pull it out with a magnet.”

The next phase of the experiments will involve volunteers swimming in a saltwater pool after liberally applying the oxybenzone block. They will then time how long it takes the beads to remove the chemical, they add.