[Reading Science] First Discovery of 'Chemical Bonded' Plastic Rock in China

Tsinghua University Research Team Publishes Paper in International Journal
"Chemical Bonding Between Stone Surface Quartz Sand and Plastic Carbon"
"Plastic Stones Severely Polluted Due to Erosion"

Plastic is an artificial material created by humans. However, it is showing a tendency to become 'geological minerals' as it accumulates in soil over a long period. Recently, a new type of 'plastic rock' was discovered for the first time in history in China. Scientists were surprised because the plastic and rock were not simply mixed but chemically bonded. This indicates how serious the microplastic pollution problem is.

A research team from Tsinghua University in China recently reported the discovery of plastic-coated rocks in a stream in Hechi City, Guangxi Zhuang Autonomous Region, southern China, to the international academic community. In a related paper published on the 3rd in the international journal Environmental Science and Technology, they announced the first case of chemical bonding between rock and plastic in a natural environment. Polypropylene, commonly used for plastic bags, and agricultural plastic (polyethylene) accumulated in the soil inside and outside the stream, corroded, and adsorbed onto the surface of the rocks.

'Plastic Stone' Discovered by Research Team at Tsinghua University in China. The plastic physically enveloping the stone bonded with the surface silicon at the atomic level with the help of ultraviolet rays and microorganisms. Photo by Tsinghua University Website

After discovering these rocks, the research team analyzed them using a spectrometer and found that the carbon atoms in the polyethylene film on the surface were chemically bonded with silicon in the rock with the help of oxygen atoms. Professor Hou Deyi, who participated in the study, explained, "Such atomic bonds appear to be caused by ultraviolet rays from the sun or metabolic activities of microorganisms," adding, "Polypropylene films likely adhered to the rocks through physical forces (naturally) rather than artificial chemical bonding."

These 'plastic stones' are concerning because they affect Earth's geology and, through erosion processes, become sources of microplastic pollution. Microplastics spread over long distances through the atmosphere and oceans, penetrate plant tissues, and accumulate as fish and birds mistakenly ingest microplastics as food. Recent studies have increasingly shown that microplastics can pose serious threats to human health as well.

To verify how much microplastic the plastic stones they found spread in the surrounding environment, the research team created a laboratory environment similar to a periodically flooding stream and exposed plastic film fragments found in the stream to observe them. The results revealed that far more microplastics were generated compared to landfills, seawater, or marine sediments.

Earlier, in 2020, rocks with melted plastic bottle caps and earrings bonded to sedimentary rock were discovered in Brazil and named 'anthropoquinas.' Some scientists called this a new type of geological material and named it 'plastiglomerates.'

Gerson Ferdinando, a professor of Earth Sciences at the Federal University of Rio Grande do Sul in Brazil, who has studied anthropoquinas, said about this study, "It is not certain whether a new type of fusion between plastic and rock has been discovered," but added, "This is the first plastic-rock composite formed in a freshwater environment; until now, most studies on interactions between plastic and rock materials have been conducted in landfills or marine/coastal environments. This can enrich discussions on geological processes and interactions with plastics."