Plastic waste found chemically bonded to rocks in China
Researchers have discovered what they think is a new form of plastic pollution: thin films of plastic waste chemically bonded to rocks.
The finding adds to scientists’ growing recognition that plastics have become part of Earth’s geology. In 2020, geologists described sedimentary rocks in Brazil that had plastic-lined bottle caps, plastic earrings and other litter embedded in their layers. They dubbed the rocks anthropoquinas1. Other scientists have turned up “plastiglomerates”, formed when melted plastic glues together rocks, sand and other natural and human-made materials2.
“People in the twentieth and twenty-first century are creating new geological records,” says Deyi Hou, a soil and groundwater scientist at Tsinghua University in Beijing. Hou and his colleagues found the plastic-sheet-coated rocks near a creek in Hechi City, China.
Their report, published in Environmental Science and Technology on 3 April3, is the first to uncover chemical bonds between plastic and rocks in the environment, Hou says. The source of the plastic, he adds, is rubbish that accumulated in and around the creek, including polypropylene films — like those used to make plastic bags — and polyethylene films — like those used by farmers to cover crops.
When the researchers peered at the plastic–rock combos with spectroscopic instruments, they saw that carbon atoms at the surface of the polyethylene films were chemically bonded to silicon in the rock with the help of oxygen atoms. Hou says this bonding might have been driven by ultraviolet light from the Sun, or by the metabolic activity of a thriving community of microbes that the researchers found living on the plastic rocks. The polypropylene films that the team found seemed to be attached to the rocks by physical forces rather than chemical bonds.
A new epoch
Aside from influencing Earth’s geology, plastic rocks are also concerning because they can shed microplastics into the environment. These plastic fragments can be transported long distances through the atmosphere and oceans, can penetrate plant tissues and might be mistaken for food by animals, including fish and birds. To see how much microplastic would be shed by the sheets bound to the rocks they found, Hou and his colleagues detached parts of the films and exposed them to wet–dry cycles in the laboratory, to mimic what might happen when the creek periodically floods. The team reported rates of microplastic generation that are orders of magnitude greater than those reported in lab tests mimicking plastic shedding in landfill, seawater and marine sediment.
Gerson Fernandino, a geoscientist at the Federal University of Rio Grande do Sul in Porto Alegre, Brazil, who is studying sedimentary anthropoquinas, says Hou’s research is intriguing, but that it is not clear whether the plastic–rock complexes truly represent a new kind of interaction between plastics and rocks. Even so, Fernandino says, the complexes are the first of their kind to be formed in a freshwater ecosystem — most other studies have looked at how plastic interacts with materials in landfills or in marine or coastal environments. Fernandino adds that the results “enrich the discussion on plastics interacting with geological processes”, but emphasizes that the results are preliminary.
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Hou acknowledges that his research so far has been based on only four samples. The team is continuing to search for more examples of plastic interactions in terrestrial ecosystems and further characterizing the complexes in the lab.
Some geologists see the growing body of research on plastic rocks as another line of evidence that humans have profoundly changed the planet’s geology since the mid-twentieth century. Some argue that this shift should be recognized as a new geological epoch, the Anthropocene. In the next few weeks, the Anthropocene Working Group, a committee of scientists formed by the International Commission on Stratigraphy, will vote on which spot on Earth marks the start of this geologic period, on the basis of industrial and radioactive materials found there.
Jan Zalasiewicz, a palaeobiologist at the University of Leicester, UK, who was part of a group that proposed the new epoch in 2008, says Hou’s research brings the Anthropocene home to the present: “When you go into the field, go to a stream and pick up plastic-covered rocks, it helps make the Anthropocene tangible.”
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