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Researchers at Rice University have improved micron-size titanium dioxide particles to trap and destroy BPA, a water contaminant with health implications. Cyclodextrin molecules at the surface trap BPA, which is then degraded by the reactive oxygen species (ROS) produced by the light – activated particles. Credit: Danning Zhang
Scientists at Rice University have come up with something that resembles the Venus particle fly trap for water depollution.
The one-micron sized spheres created in Pedro Alvarez's laboratory, an environmental engineer at Rice, are designed to capture and destroy bisphenol A (BPA), a synthetic chemical used in the manufacture of plastics.
The search is detailed in the American Chemical Society Science and technology of the environment.
BPA is commonly used to cover the inside of cans, bottle lids and water supply lines. It was once part of baby bottles. While BPA that infiltrates into foods and beverages is considered safe in low doses, prolonged exposure is suspected to affect children's health and contribute to high blood pressure.
The good news is that reactive oxygen species (ROS) – in this case, hydroxyl radicals – are bad news for BPA. Inexpensive titanium dioxide releases ROS when it is triggered by ultraviolet light. But as the oxidizing molecules disappear quickly, the BPA must be close enough to attack.
That's where the trap comes in.
Up close, the spheres turn out to be collections of titanium dioxide petals in the shape of flowers. The soft petals offer Rice researchers a large surface to anchor the cyclodextrin molecules.
"Petals" of a sphere of titanium dioxide enhanced with cyclodextrin, as seen by scanning electron microscope. When they are triggered by ultraviolet light, spheres created at Rice University are effective at removing contaminants from bisphenol A from water. Credit: Alvarez Lab
Cyclodextrin is a benign sugar-based molecule often used in foods and medications. It has a two-sided structure, with a hydrophobic cavity (avoiding water) and a hydrophilic outer surface (attracting water). BPA is also hydrophobic and naturally attracted to the cavity. Once entrapped, the ROS produced by spheres degrade BPA into harmless chemicals.
In the laboratory, researchers determined that 200 milligrams of spheres per liter of contaminated water degraded 90% of BPA in one hour, a process that would take twice as long with non-enriched titanium dioxide.
The work is part of the technologies developed by the Nanotechnology-based Water Treatment Center funded by the Rice-based Nanotechnology Research Center, as spheres assemble from titanium dioxide nanofilms .
"Most of the processes reported in the literature involve nanoparticles," said Danning Zhang, Rice's graduate student. "The particle size is less than 100 nanometers.Because of their very small size, it is very difficult to recover them in suspension in the water."
The rice particles are much bigger. When a particle of 100 nanometers is 1000 times smaller than a human hair, the titanium dioxide improved is between 3 and 5 microns, about 20 times less than the same hair. "This means that we can use a low-pressure microfiltration with a membrane to recover these particles for reuse," Zhang said. "It saves a lot of energy."
Zhang Danning, a rice graduate student, who led the development of a particle that attracts and degrades contaminants in the water, controls a sample in an environmental lab at Rice. Credit: Jeff Fitlow
Because ROS also depletes cyclodextrin, the spheres begin to lose their trapping ability after about 400 hours of continuous ultraviolet exposure, Zhang said. But once recovered, they can be easily recharged.
"This new material helps overcome two important technological barriers to photocatalytic water treatment," said Alvarez. "First, it improves treatment efficiency by minimizing ROS trapping by non-target water components, in which case ROS is primarily used to kill BPA."
"Second, it allows low-cost catalyst separation and reuse, helping to reduce treatment costs," he said. "This is an example of how advanced materials can help turn academic currents into achievable processes that improve water safety."
Explore further:
Carpet baits, hooks and destroys pollutants in the water
More information:
Danning Zhang et al. TiO easily recoverable, micron size2 hierarchical spheres decorated with cyclodextrin to improve the photocatalytic degradation of organic micropollutants, Science and technology of the environment (2018). DOI: 10.1021 / acs.est.8b04301
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