Scientists have created micron-sized spheres built to catch and destroy bisphenol A (BPA), a synthetic chemical used to make plastics.
BPA is commonly used to coat the insides of food cans, bottle tops, and water supply lines, and was once a component of baby bottles. While BPA that seeps into food and drink is considered safe in low doses, scientists suspect prolonged exposure affects the health of children and contributes 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 triggered by ultraviolet light. But because oxidating molecules fade quickly, BPA has to be close enough to attack.
Scientists say that’s where the spheres—something akin to the Venus’ flytrap of particles for water remediation—come in.
titanium dioxide sphere
“Petals” of a titanium dioxide sphere enhanced with cyclodextrin as seen under a scanning electron microscope. (Credit: Alvarez Lab)
Close up, the spheres resemble flower-like collections of titanium dioxide petals. The supple petals provide plenty of surface area for researchers to anchor cyclodextrin molecules.
Cyclodextrin is a benign sugar-based molecule often used in food and drugs. It has a two-faced structure, with a hydrophobic (water-avoiding) cavity and a hydrophilic (water-attracting) outer surface.
BPA is also hydrophobic and naturally attracted to the cavity. Once trapped, ROS the spheres produce degrades BPA into harmless chemicals.
In the lab, researchers determined that 200 milligrams of the spheres per liter of contaminated water degraded 90 percent of BPA in an hour, a process that would take more than twice as long with unenhanced titanium dioxide.
The work fits into technologies the Center for Nanotechnology-Enabled Water Treatment at Rice University develops because the spheres self-assemble from titanium dioxide nanosheets.
“Most of the processes reported in the literature involve nanoparticles,” says graduate student Danning Zhang, lead author of the paper, which appears in Environmental Science & Technology. “The size of the particles is less than 100 nanometers. Because of their very small size, they’re very difficult to recover from suspension in water.”
The new particles are much larger. Where a 100-nanometer particle is 1,000 times smaller than a human hair, the enhanced titanium dioxide is between 3 and 5 microns, only about 20 times smaller than the same hair.
“That means we can use low-pressure microfiltration with a membrane to get these particles back for reuse,” Zhang says. “It saves a lot of energy.”
Because ROS also wears down cyclodextrin, the spheres begin to lose their trapping ability after about 400 hours of continued ultraviolet exposure, Zhang says. But once recovered, researchers can easily recharge them.
“This new material helps overcome two significant technological barriers for photocatalytic water treatment,” says Pedro Alvarez, professor of materials science and nanoengineering and a professor of civil and environmental engineering.
“First, it enhances treatment efficiency by minimizing scavenging of ROS by non-target constituents in water. Here, the ROS are mainly used to destroy BPA,” he says.
“Second, it enables low-cost separation and reuse of the catalyst, contributing to lower treatment cost. This is an example of how advanced materials can help convert academic hypes into feasible processes that enhance water security,” Alvarez adds.
Additional coauthors are from Rice, Yale University, and Ajou University in South Korea. The National Science Foundation supported the research.