To trace tiny pieces of plastic, use a metal detector

Small bits of plastic, such as polymers found in some shampoos and rubber from broken-down car tires, pollute the marine environment. They can interfere with algae growth and kill shrimp larvae that eat them. But how those particles travel through sewer systems and the environment remains uncertain. The plastic confounds analyses because it doesn’t separate easily from its traveling companion materials, and its dilute concentration in water is below the detection limit of most analytical methods. To trace nanoplastics, Denise Mitrano and her colleagues adapted techniques from trace metal research. Trapping palladium inside nanoparticles provided the researchers with a tracker to identify and follow the plastic through one step of a laboratory-scale wastewater treatment system.

The nanoplastic particles the researchers made, as shown in the image, consist of a polyacrylonitrile core and a polystyrene shell. The molecules in the core bind around the Pd to form a nanoparticle in solution, and the shell represents one of the most abundant types of plastic found in the environment. Inductively coupled plasma mass spectrometry acts as a metal detector to locate the Pd, which can thus serve as a proxy for the nanoplastic.

Mitrano and her colleagues next analyzed how the trackable particles behave in a model batch reactor of a wastewater treatment plant. An industrial-scale system processes a mixture of 97% liquid and 3% solid that separates into sludge, which undergoes further processing, and an effluent that is released directly into the environment. After stirring the mixture in the model system for a half hour, more than 99% of the plastic was confined in the sludge.

Although the analysis was limited to the laboratory, the observations can serve as a starting point for determining how much nanoplastic may be released into the environment. The main source could be the sludge, which, depending on locality, can either be incinerated or used as fertilizer for agriculture. (D. M. Mitrano et al., Nat. Nanotech., doi:10.1038/s41565-018-0360-3 .)