A Cornell University study has revealed the molecular mechanism of how antibiotics from human and farm animal waste become trapped in soils — findings with the potential to explain the behavior and consequences of antibiotics in the environment.

The new study, published Feb. 15 in the Journal of Colloid & Interface Science, reveals how metals that are abundant in natural soils change the arrangement of clay minerals, causing the mineral layers to come apart and create nano-scale pores that trap antibiotics and hold them in place.

Simulation of clay nanopores containing trapped tetracycline antibiotic compounds: water molecules (light blue), metals (dark blue), antibiotics (dark red), clay layer (orange), negative charges in clay structure (yellow). Credit: Ludmilla Aristilde.

The finding is important for predicting the retention of antibiotics in lands based on the chemistry of their soils. It also opens the door for possibly using engineering solutions to remove these potentially harmful residues.

"Antibiotics are heavily used in animal agriculture, especially in veterinary medicine," said the paper’s lead author Ludmilla Aristilde, an assistant professor of biological and environmental engineering in the College of Agriculture & Life Sciences. Co-authors include researchers from the Université Grenoble Alpes and Université de Haute Alsace in France.

Up to 60% of each antibiotic dose is not metabolized and then is excreted in manure, which is spread as fertilizer on farm fields. Human waste sludge is also applied to lands. As a result, antibiotics — including tetracyclines, the focus of this paper — are widely found in fields worldwide.

Scientists and health officials have been concerned that these antibiotic residues may end up in runoff from agricultural lands that flows into streams and rivers and that they may be toxic to soil microbiota or taken up by crops. This paper offers an explanation of the mechanisms that immobilize tetracyclines in soil. The study suggests that when trapped in clay mineral layers, antibiotics may possibly be unavailable for runoff or for interacting with microbes and plants, although more research is required to investigate these implications further.

According to the research, antibiotic compounds carry a negative electrostatic charge, while minerals in the soil also tend to carry negative charges. Since like charges repel each other, a prevailing hypothesis claims that metals with multiple positive charges in the soil provided an electrostatic bridge that could link antibiotic compounds to the surface of minerals in the soil and hold the antibiotics in place.

However, this new study presents a different view by showing instead that calcium and magnesium in the soil change the arrangement and structure of smectite clay minerals there, creating layers and nanopores where antibiotics become trapped.

"If these layers don't come apart, then the antibiotics cannot be trapped," Aristilde said. "These metals facilitate the structures to come apart."

Aristilde and her research group are continuing their efforts to evaluate the mechanisms that drive how different types of antibiotics and herbicides behave in the environment.