A new study from Iowa State University shows that the washing process used to clean plastic flakes before they become new products can leave behind concerning levels of toxic chemicals in the water, especially when certain industrial methods are applied.
Researchers with the university’s Polymer and Food Protection Consortium tested standard ways of cleaning polypropylene, the No. 5 plastic commonly found in yogurt tubs, margarine containers, and other food packaging. They discovered that some cleaning approaches released high amounts of phthalates into the wash water. These chemicals are additives used in plastics and have been tied to increased cancer risks as well as hormone-related problems affecting reproduction and child development.
The findings appear in the journal Advances in Materials Science and Engineering. They come as the United States pushes to raise its overall plastic recycling rate, which remains low for many materials, including polypropylene, at around 3 percent in some estimates. The Environmental Protection Agency has set a national goal of 50 percent recycling by 2030, yet current household recycling sits closer to 21 percent.
Greg Curtzwiler, an associate professor of food science and human nutrition and senior author of the study, said the team is working to understand where these chemicals end up during recycling. “We’re trying to track the fate of these chemicals in the recycling process and figure out how to effectively remove them,” he noted.
In the experiments, plastic was ground into small flakes about the size of coarse salt grains. Different cleaning techniques were applied, including simple physical agitation, agitation with sodium hydroxide (a form of lye), ultrasonic vibration, and a combination of sodium hydroxide with a common industrial detergent.
When researchers used only physical agitation or agitation plus sodium hydroxide, no detectable phthalates or bisphenols (another group of hormone-disrupting chemicals that includes BPA) showed up in the wash water. However, ultrasonic vibration or the sodium hydroxide plus detergent mix produced measurable amounts of two specific phthalates: di(2-ethylhexyl) phthalate, known as DEHP, and di-cyclohexyl phthalate, or DCHP.
The sodium hydroxide and detergent method proved especially problematic when water was reused, as often happens in recycling plants to save resources. After the first wash cycle, DEHP levels in the water reached 10 times the limit set for drinking water. By the 15th cycle, using the same water, concentrations climbed to 25 times the prohibited level. At the same time, the amount of detergent in the water dropped, suggesting the plastic flakes were absorbing some of the cleaner, which itself can interfere with hormonal systems.
Keith Vorst, director of the Polymer and Food Protection Consortium and a co-author, stressed that solutions exist but must balance effectiveness with cost. “It’s important to understand that these are solvable problems,” he said. “You don’t want the cure to be worse than the disease, but we need to do this as cost-effectively as possible.”
Polypropylene recycling has drawn attention because the material is durable and holds value, yet most of it still ends up in landfills or incinerators rather than being reused. The study highlights that the cleaning stage, which is meant to prepare flakes for melting and reforming, remains lightly regulated and not well studied compared with other parts of the recycling chain.
If wash water isn’t treated and is released or leaks from a facility, it could carry these contaminants into rivers, groundwater, or treatment systems.
Broader concerns about plastic recycling already include microplastic releases, but this research points to chemical leaching as another issue that deserves closer attention from plant operators and regulators.
Possible fixes include better wastewater treatment technologies. The Iowa State researchers are exploring several avenues. They’re considering foam fractionation, a process that employs air bubbles to separate out undesirable substances. Another possibility is electro-oxidation, a process that uses electricity to break down chemical compounds.
Furthermore, they’re investigating bio-based approaches that leverage nanomaterials, including carbon nano-onions. A reduction in overall water consumption, perhaps achieved through better sorting techniques or dry-cleaning alternatives, could also prove beneficial.
Recycling companies work with tight budgets, so any changes need to be financially sound while also improving safety. The study doesn’t suggest stopping plastic recycling. Instead, it recommends better management of the washing step to avoid moving pollution from land to water.
As demand grows for more recycled content in packaging and products, researchers say understanding and controlling these chemical flows will become increasingly important. The work was funded in part by the Institute for the Advancement of Food and Nutrition Sciences.
This latest examination adds to ongoing discussions about making plastic recycling truly sustainable rather than simply moving environmental risks from one place to another. Further studies are expected to test these findings at larger scales and explore how different plastic types and contamination levels affect wash water quality.
