Recycling gives a PET new lease of life

The world’s landfill and oceans are filling up with plastic waste. In 2015, nearly 80% of the 6300 million tons of plastic produced ended up in landfill, with only 9% recycled. One of the most commonly used plastics for packaging is polyethylene terephthalate or PET. Some approaches have been devised for recycling PET but tend to produce low quality material. “Instead of recycling, we offer plastic bottles a second life as useful absorbents for water purification,” explains Olga Guselnikova, who led an international research effort at Tomsk Polytechnic University in Russia, Wuhan University of Technology in China, University of Chemistry and Technology and J.E. Purkyne University in the Czech Republic, and the Center for Environmental and Energy Research at Ghent University’s campus in South Korea, with Francis Verpoort. The team created a useful new material from waste plastic by growing a layer of a porous metal-organic framework (MOF) on ordinary, shop-bought PET bottles [Semyonov et al., Applied Materials Today 22 (2021) 100910,]. The PET is first hydrolyzed in HNO3 and DMF to release terephthalic acid. Adding ZrCl4 produces a thin layer of the MOF UiO-66 on top of the PET. The resulting composite material, PET@UiO-66, has a very high surface area, over six times higher than PET itself, and is highly porous. “[We found that] PET@ UiO-66 is a cheap, efficient, and sustainable sorbent for water purification with considerable technical advantages towards imidacloprid removal from water,” says Guselnikova. To demonstrate the capabilities of PET@UiO-66, the researchers used it to remove the insecticide imidacloprid, which is one of the most common neonicitinoids, from water. Imidacloprid is believed to negatively affect bees, aquatic and other organisms, with ingestion causing gastrointestinal, cardiorespiratory, and nervous system problems. But its widespread use has led to its detection in water systems around the world. “The post-treatment of PET transforms it into a high-performance absorbent suitable as a water purificator for imidacloprid,” she explains. The team’s experimental analysis indicates that imidacloprid becomes non-covalently bound to the MOF. This chemisorption is the main controlling step in the absorption process. Moreover, PET@UiO-66 can be regenerated by washing in acetone and heating, so it can be reused and recycled multiple times without losing performance. “The material can be used as-is in modern water purification systems [to remove] insecticides,” says Guselnikova. “Moreover, we envision applications for such materials in water purification stations in agricultural districts, where insecticides (or pesticides) are widely applied.” Instead of an ecological problem, the researchers believe their strategy could transform waste PET on an industrial scale into an ecological solution for cleaning up polluted water.