Researchers at Rice University have developed a new "mat" that can adsorb and destroy pollutants from water. The purifier is made up of titanium dioxide nanoparticles embedded into polymer fibers, which bind contaminants and destroy them in response to UV light. The team says the design is faster, safer and more energy efficient than other systems.

Titanium dioxide is a pretty prolific purifying material. When exposed to UV light, it becomes photocatalytic, releasing reactive oxygen species (ROS) that break down contaminants. Over the years that ability has been put to work in microfluidic filters, smog-eating panels for buildings, and fabric coatings that could let us clean clothes just by hanging them out in sunlight.

In this case, nanoparticles of titanium dioxide were embedded into a highly-porous sheet of spun polyvinyl fibers and put to work removing and killing endocrine disruptors in water. The fibers themselves are hydrophobic (water-repelling), meaning they won't absorb the water but they will attract the pollutants, which are also hydrophobic. Once the mats have soaked up the contaminants, hitting them with UV light will trigger the photocatalytic response, oxidizing and destroying them.

Titanium dioxide is already in use in water treatment, but it usually involves adding large amounts of the stuff to wastewater, forming a slurry. After the photocatalytic step, the treated water then needs to be filtered back out of the slurry, which can be difficult and inefficient.

"Current photocatalytic treatment suffers from two limitations," says Pedro Alvarez, one of the authors of the study. "One is inefficiency because the oxidants produced are scavenged by things that are much more abundant than the target pollutant, so they don't destroy the pollutant. Second, it costs a lot of money to retain and separate slurry photocatalysts and prevent them from leaking into the treated water. In some cases, the energy cost of filtering that slurry is more than what's needed to power the UV lights. We solved both limitations by immobilizing the catalyst to make it very easy to reuse and retain. We don't allow it to leach out of the mat and impact the water."

The researchers say their new technique can destroy more pollutants faster, and do so using less electrical energy, particularly when scaling it up for dirtier water. Going from distilled water to effluent from a wastewater treatment plant, the fibrous mat would only need a two-fold energy increase, compared to the 11-fold one that a slurry would require. And where the pollution is too thick for light to reach the nanoparticles, the technique also allows for a two-step process.

"It can be desirable to do that if the water is murky and light penetration is a challenge," says Alvarez. "You can fish out the contaminants adsorbed by the mat and transfer it to another reactor with clearer water. There, you can destroy the pollutants, clean out the mat and then return it so it can fish for more."

The research was published in the journal Environmental Science and Technology.

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