Quick look
Researchers from Iowa State's Polymer and Food Protection Consortium found some methods industrial recycling plants use to wash plastic may cause high levels of toxic chemicals in untreated water used in the cleaning process.
AMES, Iowa - Potentially harmful chemicals can contaminate untreated water used in recycling plants to clean plastics before they're processed into new products, according to a new study by an Iowa State University research group.
Researchers from Iowa State's Polymer and Food Protection Consortium tested common industrial plastic-washing practices and found that some methods left the wash water with high levels of two types of phthalates - a class of widely used additives linked to cancer risks and hormone disruptions related to reproduction and development, especially in children.
The findings show the need for additional understanding of how recyclable plastic is cleaned for processing, which is lightly regulated and relatively understudied, said Greg Curtzwiler, a researcher for the Polymer and Food Protection Consortium and senior author of the study, which was published in Advances in Materials Science and Engineering.
"We're trying to track the fate of these chemicals in the recycling process and figure out how to effectively remove them," said Curtzwiler, an associate professor of food science and human nutrition.
Hunting for concerning chemicals
The study focused on polypropylene - No. 5 plastic - which is frequently used for dairy product tubs and other food packaging. Increased polypropylene recycling is a high-priority target for reducing plastic waste because it's a durable, high-value material with a low current rate of recycling, as little as 3% by some estimates.
When investigating different ways of washing polypropylene, researchers noticed the samples - which are ground into flakes 1 to 3 mm wide, roughly the size of coarse salt grains - had reduced levels of some potentially toxic chemicals after being washed.
"We thought, 'OK, these compounds have to be going somewhere.' We knew we needed to take a closer look at the water," Curtzwiler said.
No detectable phthalates or bisphenols - another hormone-disrupting class of plastic enhancers, such as BPA - appeared in wash water when plastics were cleaned with physical agitation alone or agitation combined with sodium hydroxide, a form of lye. But when cleaning with ultrasonic vibration or sodium hydroxide plus a common industrial detergent, researchers found di(2-ethylhexyl) phthalate (DEHP) and di-cyclohexyl phthalate (DCHP) in the wash water.
The sodium hydroxide and detergent method was tested using the same water over 15 cycles, as wash water is often reused in industrial settings. Researchers found the levels of DEHP accumulated over time, from 10 times the level prohibited in drinking water after the first wash to 25 times the drinking-water limit after 15 cycles. The concentration of the industrial detergent fell as wash water was reused, suggesting the plastic flakes were absorbing some of the cleaner - concerning because it can also disrupt hormonal systems.
Finding cost-effective solutions
The study, funded in part by the Institute for the Advancement of Food and Nutrition Sciences, suggests that while further research is needed, recycling companies may need to change how plastic is cleaned and water involved is managed. The risks can be mitigated, said study co-author Keith Vorst, director of the Polymer and Food Protection Consortium.
"It's important to understand that these are solvable problems," said Vorst, an associate professor of food science and human nutrition.
Foam fractionation - passing air bubbles through water to skim off contaminants caught in the resulting foam - is one option for filtering out some toxic chemicals. Iowa State researchers also are studying electro-oxidation (using electricity to break down concerning chemicals) and a bio-based treatment with a nanomaterial called carbon nano-onions, Curtzwiler said.
Reducing how much water is used to wash recyclable plastic also could be part of a multi-pronged solution, whether through improved sorting or cleaning methods that involve far less or even no water, Vorst said.
Changes must be economical in an industry trying to expand while operating on thin margins, Curtzwiler said. The U.S. Environmental Protection Agency has set a goal to boost the national plastic recycling rate to 50% by 2030, but the most recent report by The Recycling Partnership estimated that only 21% of all eligible household material in the U.S. is recycled, with rates even lower for plastic.
"You don't want the cure to be worse than the disease, but we need to do this as cost effectively as possible," he said.
