A common mineral in red soils tends to retain trace metals over time, according to a study.

Trace metals are nutrients, like zinc, that animals and plants need in small amounts to function properly. Animals typically absorb trace amounts of metals in their diet or through environmental exposures, while plants extract their trace minerals from the soil. If we absorb too little, we can suffer from a deficiency, but the opposite can also be true: too much of a trace metal can be toxic.

Scientists estimate that up to 50% of trace metals found in soils and urban environments may be bound to the surface of mineral grains, making trace metals virtually unavailable for consumption or exposure. Researchers at Washington University in St. Louis wondered what kept them in place.

“When minerals bind trace metals, we often assume that they act like a sponge,” said Jeffrey G. Catalano, professor of earth, environmental and planetary sciences and director of environmental studies in Arts and Sciences. Science. “But sometimes they bind trace metals and don’t let them go. That’s great when they’re contaminants, but bad when they serve as micronutrients.”

In a study published in the journal Environmental science and technology, Catalano and Greg Ledingham, a Ph.D. candidate in his laboratory, discovered that a common mineral called goethite, an iron-rich mineral abundant in the soils that cover the Earth, tends to incorporate trace metals into its structure over time, binding the metals in such a way let him lock them. out of circulation.

The researchers found that the portion of trace metals bound to goethite was proportional to the size of the ions. Up to 70% of nickel, the trace metal with the smallest ionic radii in this study, was nonrecoverable, while only 8% of cadmium was irreversibly bound to goethite.

“In the past, to study how trace metals attach and are retained on mineral surfaces, geochemists had to significantly modify chemical conditions in ways that were neither realistic nor faithful to real-world systems,” he said. said Ledingham, a graduate researcher at the McDonnell Center for Space Science. “Changing pH, for example, affects how particles group together and can influence how metals bind to the surface.

“We used a new approach called isotope exchange that allowed us to track how metals bind, detach, and incorporate into iron oxyhydroxides in real time and under conditions representative of real soils and river systems,” he said. he declared.

“Our study suggests that iron oxyhydroxide minerals, like goethite, may be a much better sink for trace metals than previously thought,” Catalano said.

Knowing that goethite tends to naturally trap trace metals over time could help scientists better predict how certain contaminants move through the environment, the study authors said. It could also mean that trace metal nutrients added to farm and garden soils could become less effective after a few months.

The results suggest that the environmental impact is mixed: trapping metals acting as contaminants will clean soils and water supplies, but metals serving as essential nutrients are also unavailable to plants and other organisms, the researchers said. researchers.