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An international team of scientists has found toxic metals in mineral phosphate fertilizers around the world, using a new tool to identify the distribution and impact of such contaminants on soil, water resources and food supplies.

“Although mineral phosphate fertilizers are critical to promoting global sustainable agriculture and food security, we have found high levels of toxic metals in many fertilizers around the world,” said Avner Vengosh, chair of the Department of Earth and Climate Sciences at the School of Duke University Environment.

“Our study developed a new method to identify the sources and environmental impact of these metals.” These metals include cadmium, uranium, arsenic, vanadium and chromium.

The use of mineral fertilizers – synthetic or naturally occurring substances with essential nutrients needed for plant growth – has helped increase sustainable crop yields worldwide. But until recently, its contamination with toxic metals has not been systematically assessed. This new study analyzes global phosphate fertilizers from major phosphate-producing countries.

“We measured strontium isotopes in both phosphate rocks and the fertilizers generated from those rocks to show how the isotope fingerprint of the fertilizers corresponds to their original source,” said Robert Hill, lead author of the study and a Ph.D. student at Duke University.

Isotopes are variations of an element, in this case strontium. Chemical analysis of each fertilizer shows a unique isotopic mix that matches the phosphate rock from which it was extracted.

“Given the variation of strontium isotopes in global phosphate rocks, we have created a unique tool to detect the potential impact of fertilizers globally,” said Hill.

To find out whether strontium isotopes are a reliable indicator of trace elements in fertilizers worldwide, the researchers analyzed 76 phosphate rocks, the main source of phosphate fertilizers, and 40 fertilizers from major phosphate rock-producing regions, including the western United States, China, India, North Africa and the Middle East. The researchers collected samples from mines, commercial sources and the Tidewater Research Station, an experimental field in North Carolina. The research team published their findings on May 9, 2024 Environmental Science and Technology Letters.

Metals found in soil and groundwater come from both natural and man-made sources.

“Strontium isotopes are essentially a ‘fingerprint’ that can reveal contamination in groundwater and soil around the world,” Vengosh said. His research team has also used strontium isotopes to trace environmental contamination from landfill leaching, coal mining, coal ash, fracking fluids and groundwater that is drawn to the surface by oil and natural gas extraction.

“The isotope is a surrogate for identifying the source of contamination,” Vengosh said. “Without this tool, it’s difficult to identify, limit and remediate contamination associated with fertilizer.”

Fertilizers in the study showed varying concentrations of trace elements, with higher levels observed in fertilizers from the US and the Middle East compared to those from China and India. As a result, the researchers concluded that phosphate fertilizers from the US and the Middle East would have a greater impact on soil quality due to their higher concentrations of uranium, cadmium, chromium, compared to fertilizers from China and India, which have more high concentrations of arsenic.