A study by a team at IIT Bombay and their collaborators reveals how the majority of commercial use plastic degrades into micro and nano plastic particles

Micro and nanoplastics (MNPLs)—plastic fragments smaller than the thickness of a strand of hair—are fast becoming a serious ecological threat. They have been detected almost everywhere, from deep-sea sediments to human organs. Though there is increasing evidence regarding the medical and environmental harm they pose, it's nearly impossible to avoid using plastics in daily life and commercial applications and, thereby, leakage of MNPLs into the environment. This makes it imperative to manage plastic waste better and prevent the formation and release of these tiny fragments into the environment. Understanding how MNPLs form is a crucial step in that direction.

A recent study in the Nature Communications journal by Prof. Kumaraswamy Guruswamy and Vivek Sharma from Indian Institute of Technology (IIT) Bombay, in collaboration with scientists from the Columbia University, USA, University of Vermont, USA, University of the Basque Country UPV/EHU, Spain, Basque Foundation for Science, Spain, and University of Tennessee Knoxville, USA, sheds new light on how MNPLs form through everyday environmental degradation of plastic waste. The study highlights the importance of scientifically informed plastic waste management practices. It suggests some changes to the structure of plastic materials, which may help reduce the formation rate of MNPLs.

Most commercially available plastics have what scientists call a semi-crystalline structure. They are made of crystalline (where the atoms are arranged in a periodic pattern, like in a salt crystal) sheets of plastic molecules (called lamellae) stacked one over another with threads of plastic running between and across these sheets. These threads often act as bridges holding the sheets together. The IIT Bombay team and their collaborators studied three of the most widely used plastics, namely polypropylene (PP), polyethylene terephthalate (PET), and polystyrene (PS), and simulated degradation under controlled lab conditions that mimic wear and tear caused by real-world environments like landfills. By accelerating the degradation process, they could observe in days what would naturally take years.

Across all three plastics studied, researchers observed that the threads degrade first. Once these break down, the stacked lamellae become unstable and fragment. While the fragments from ‘threads’ degrade quickly, the tiny lamellar fragments—the micro and nano plastics—persist for very long periods, potentially posing ecological and medical threats.

As Prof. Guruswamy says, “Once NPLs form and are dispersed in the environment, it is virtually impossible to intervene. It is not realistic to collect and filter out NPLs to clean up the environment.” One potential mitigation strategy suggested by the study is to increase the number of thread-like molecular bridges to strengthen the plastics. While this can slow down the formation rate of MNPLs, it cannot entirely prevent them from forming. The usefulness of semi-crystalline plastics also means that we cannot stop using them. Thus, better waste management strategies are essential to prevent particulate pollution. As Prof. Guruswamy explains, “While we think of plastics as persistent and indestructible, they are not. For example, a plastic chair left out in the sun will become brittle and degrade, releasing MNPLs. It would be best to collect and scientifically recycle or dispose of such objects before they degrade under poorly controlled conditions.”

Another key observation from the study is that there is a wide variability in the size and shape of MNPLs formed through the degradation of plastics. This challenges the common practice in toxicity studies of MNPLs that use nanoplastic particles of similar size and shapes as stand-ins for real-world nanoplastics formed through plastic waste degradation. As Prof. Guruswamy explains, “[The mimics used in laboratory studies on effects of MNPLs] differ significantly from nanoplastics produced by wear and degradation in terms of surface chemistry and size/shape variability. These differences could be critical for understanding toxicity and environmental impact.”

The depth and scope of the study were greatly enhanced by international collaboration. It was conceived by Prof. Guruswamy and his long-time collaborator, Prof. Sanat Kumar, from Columbia University. Prof. Sanat Kumar is also the Rajesh and Nisha Chair Visiting Professor at IIT Bombay. In the study, the IIT Bombay group handled polypropylene and polystyrene, while the Columbia team focused on PET. Currently, Prof. Guruswamy and his collaborators are investigating the implications of their results for recycling plastics. “During reprocessing, polymers can undergo molecular degradation, possibly making them more vulnerable to NPL formation”, Prof. Guruswamy explains the reason for looking at this particular problem.

While the MNPL samples from the real world are not being studied currently, that is a major area for future research, according to Prof. Guruswamy. However, directly collecting these particles from oceans or landfills remains a challenge. More international collaborations and extensive works will hopefully help us effectively tackle the challenges posed by these tiny plastic fragments, invisible to even normal microscopes, in the near future.

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