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Researchers at Rice University have developed something akin to the Venus’ flytrap of particles for water remediation. Micron-sized spheres created in the lab of Rice environmental engineer Pedro Alvarez are 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, prolonged exposure is suspected of affecting the health of children and contributing to high blood pressure.
Reactive oxygen species (ROS)-in this case, hydroxyl radicals are bad 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. Close up, the spheres reveal themselves as flower-like collections of titanium dioxide petals. The supple petals provide plenty of surface area for the Rice researchers to anchor cyclodextrin molecules.
Cyclodextrin is a benign sugar-based molecule 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 produced by the spheres degrades BPA into harmless chemicals.
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 developed by the Rice-based and National Science Foundation-supported Center for Nanotechnology-Enabled Water Treatment because the spheres self-assemble from titanium dioxide nanosheets.
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 Rice 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.
Low-pressure microfiltration with a membrane can be used to get these particles back for reuse, 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. This new material overcomes two significant technological barriers for photocatalytic water treatment.
It enhances treatment efficiency by minimizing scavenging of ROS by non-target constituents in water. It also 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.