Chemical distance model may predict which trace elements plants absorb

Simple chemical cues may soon help scientists predict which trace elements are most likely to enter plants from contaminated soils. In a new study published in New Contaminants, researchers propose that a hydration “fingerprint”—the distance between a dissolved metal ion and surrounding oxygen atoms in water—could act as a practical screening tool when direct measurements are too slow or costly.

When metal cations dissolve, they do not float freely. Instead, they attract nearby water molecules, forming a hydration shell. A key descriptor of this shell is the Me–O distance: the average spacing between the metal atom and oxygen atoms of hydration water. The longer the Me–O distance, the weaker the ion’s interaction with its water environment, which may influence how readily the ion moves through soil and across root interfaces.

Trace metal contamination is a growing issue for agriculture and food safety, driven by mining, industry, electronic waste, and the long tail of fertilizers and land-use change. Yet modeling uptake is notoriously complex. Soil acidity, organic matter, mineral composition, moisture, plant species, and micro-scale chemistry can all shift outcomes, making routine prediction difficult.

To test whether fundamental chemistry could explain uptake, the team compared the Me–O distance to multiple plant-uptake datasets. They also evaluated other candidates, including ionic potential and hydrated radius. Their comparisons spanned three very different systems: a perennial ryegrass pot experiment in Christchurch, field-sampled pasture plants from 39 locations across New Zealand, and oil palm grown in Sumatra, Indonesia.

Across all datasets, plants generally accumulated more of the elements whose hydrated ions showed longer Me–O distances. That consistency across plants and environments suggests the metric captures a basic chemical influence on mobility rather than a site-specific artifact.

The Me–O distance also proved more stable than the alternatives. Ionic potential showed an inverse trend with uptake but failed to remain statistically significant in one dataset. Hydrated radius correlated in some cases, but published values were missing for several emerging contaminants such as gallium, indium, neodymium, and gadolinium.

A notable exception involved lanthanides, the rare earth elements. For these ions, uptake decreased as Me–O distance increased—an inversion that the authors attribute to their unusually large atomic size and distinctive chemistry. The work implies future models may need separate treatment for rare earths.

Ultimately, the researchers emphasize that their approach does not replace soil and plant testing. Instead, it could complement existing assessments by lowering the barrier to estimating risk for poorly studied elements, extending models to new contaminants, and reducing reliance on highly detailed soil partitioning and transport measurements.

Keywords

Trace metals; plant uptake; soil contamination; hydrated cations; Me–O distance; lanthanides; environmental screening; hydration shell; ionic mobility