Detected plastic waste chemically bonded to stone in China

Researchers have discovered ‘plastic rocks’, presumably formed from plastic bags and bottles, in a tributary of a river.

Researchers have discovered what they think is a new form of plastic pollution: thin films of plastic waste that are chemically bound to rocks in nature.

This discovery contributes to the increasing recognition of scientists that plastic can become a part of the earth’s geology. In 2020, geologists described sedimentary rock in Brazil that contained plastic bottle caps, plastic earrings and other carriers that were indented in layers. They suspected that the rocks were anthropoquinas – the term proposed for cohesive sedimentary rocks related to humans, or technofossils (technological fossils). Other scientists have come up with the word plastiglomerates – rocks made up of a mixture of sedimentary particles and other natural debris, and also man-made material, held together by plastic.

“Humans in the 20th and 21st centuries are creating a new geological record,” said Deyi Hou, a soil and groundwater scientist at Tsinghua University in Beijing. Hou and his colleagues found rocks covered with plastic near a water source in the city of Hetri, China.

Their work, published in the journal Environmental Science and Technology is the first study to explore the chemical bond between plastic and rock in the environment ^first. The source of this plastic, Hou said, is waste that accumulates around water sources, including thin films of polypropylene – like the materials used to make plastic bags – and thin films of polyethylene – like what farmers cover crops.

When the researchers looked closely at the rock-plastic combination with spectrometers, they found that the carbon atoms on the surface of the polyethylene film had chemically bonded to the silicon in the rock with the help of atoms. oxygen. This association could be guided by solar radiation, or by metabolic activity by the microbial community living on the plastic rock they found, Hou said. The polypropylene thin film they found appears to be attached to it by physical forces stronger than chemical bonds.

A new era

In addition to the impact on the earth’s geology, plastic rocks also cause concern because they add to the release of microplastics into the environment. Pieces of plastic can be transported long distances through the atmosphere and oceans, can penetrate plant tissue, and can get lost in the food chain by animals such as birds and fish. ². Wanting to imagine how much microplastics could bind to rock, Hou and his colleagues traced portions of the thin film and exposed them to dry-wet cycles in the lab, to mimic what might happen. out when the water flows cyclically. The team found that the proportion of microplastics produced in the sequence was greater than the results obtained in the laboratory, mimicking the release of plastic into waste dumps, seawater and ocean sediments.

Gerson Fernandino, a geologist at the University of Rio Grande do Sul in Porto Alegre, Brazil, who is studying anthropoquinas in sediments, says Hou’s study is fascinating but it’s unclear whether rock-plastic complexes actually present as a new form of rock-plastic interaction. Even so, Fernandino says, these complexes are still the first form of matter to form in a freshwater ecosystem — most studies have looked at how plastics interact with materials in landfills. or ocean, or coastal environment. These results “enrich the discussion of plastic interactions with geological processes,” Fernandino adds, but emphasizes that the results are preliminary.

Hou understood that his research was still only on four specimens. The team is continuing to look for more examples of plastic interactions with soil ecosystems and further characterizing these complexes in the laboratory.

Some geologists are seeing the study of plastic rock as another line of evidence that humans have profoundly altered the geology of the planet since the mid-20th century. Some agree that the transition is change must be recognized as a new geological epoch, the Anthropocene. Over the next few weeks, the Anthropogenic Research Group, a committee of scientists, set up by the International Council on Stratigraphy, will vote on recording the earth as the beginning of the geological epoch, on the basis of industrial materials and radiation.

Jan Zalasiewicz, a paleontologist at the University of Leicester, UK, who joined a scientific group that proposed a new era in 2008, says Hou’s research brings the Anthropocene back to the present. out in the meadow, going to a stream and picking up a rock covered with resin, he made the Human World visible.