Many factors influence the fate of pharmaceutical residues in the soil

A comprehensive Hungarian study has revealed that the behavior of pharmaceutical residues in soil does not depend on a single factor, but is shaped by several interacting processes. The researchers emphasized that, based on their findings, it would be worth revising current practices for assessing soil quality.

Many people have already heard about the problem of pharmaceutical residues in drinking water, but it is perhaps less well known that the medicines we use frequently can also leave traces in the soil. This can pose a serious challenge for agriculture and, in the long run, may also affect human health. But what determines whether these residues remain in place, become bound to soil particles, or move further through the environment?

Pharmaceutical residues such as carbamazepine (an antiepileptic drug), diclofenac (a non-steroidal anti-inflammatory), and the estrogen derivative 17α-ethinylestradiol can enter the environment in various ways — for instance, through treated or untreated wastewater, irrigation, or the application of sewage sludge. Their fate is primarily determined by the factors acting upon them in the soil, which influence their sorption.

The main concern is that, through sorption, these substances can accumulate locally, posing a risk to agricultural crops, as plants may take up the residues together with nutrients — potentially allowing these compounds to enter the food chain.

Researchers from Eötvös Loránd University (ELTE) and the HUN-REN network have recently demonstrated what determines whether these compounds bind to soil or become mobilized, and how root exudates, organic matter, and temperature influence these processes.

The first study examined the effect of organic acids produced during root exudation on the sorption of three pharmaceutical residues. The experiments showed that low-molecular-weight organic acids can enhance the sorption of the pharmaceutical residues carbamazepine and diclofenac, leading to their accumulation in the topsoil. This effect is particularly pronounced in soils with low organic matter content, since in organic-rich soils, the high sorption capacity of organic matter would otherwise dominate the sorption dynamics. In contrast, the estrogen derivative showed strong binding to soil particles even in the absence of organic acids.

Because the concentration of organic acids in the rhizosphere changes over time, pharmaceutical residues may remain near plant roots for varying periods — a factor that can ultimately influence how much of these substances enter plants and, in the long run, the food chain.

The second study investigated how temperature affects the behavior of pharmaceutical residues in the rhizosphere. The results revealed that temperature fundamentally determines the energetic relationships between soil and pharmaceutical molecules — that is, whether adsorption (binding) or desorption (release) becomes dominant. While diclofenac exhibited more stable sorption under warmer conditions, the estrogen derivative and lidocaine tended to bind more strongly in cooler soils. The simultaneous presence of multiple pharmaceutical residues further complicated these thermodynamic interactions. The research clearly demonstrated that temperature-dependent thermodynamic equilibrium is one of the key factors governing the environmental fate of pharmaceutical residues.

Building on the previous two studies, the third investigation focused on the temporal dynamics of interactions between soil organic matter transformations and micropollutants. The researchers found that the decomposition of soil organic matter plays a decisive role in determining the persistence and mobility of pharmaceutical residues. According to the results, pharmaceutical molecules behave quite differently at the beginning of the growing season compared to the end, when the composition of soil organic matter has already changed. The study highlights that the environmental risk posed by pharmaceutical residues varies over time; therefore, the state of soil organic matter must always be considered when collecting samples.

Taken together, the results of the three studies paint an important picture: the behavior of pharmaceutical residues in soil is not determined by a single factor, but by multiple interacting processes. Root-derived organic acids can increase sorption within a short time, the decomposition of organic matter reshapes adsorption mechanisms over the course of months, while temperature continuously influences the equilibrium of sorption processes.

This complexity means that environmental risk assessments cannot rely solely on isolated laboratory parameters or single-point measurements. Instead, they require long-term evaluations that account for changing environmental conditions. Only through such an integrated approach can we truly understand the risks that pharmaceutical residues in soil pose to ecosystems — and ultimately, to human health.

Main contributors to the research

László Bauer
Doctoral student and lecturer, Department of Environmental and Landscape Geography, Faculty of Science, Eötvös Loránd University (ELTE), member of the Environmental Geography Research Group
Research Assistant, Institute of Geography, HUN-REN Research Centre for Astronomy and Earth Sciences, member of the Physical Geography Research Group

Lili Szabó
Lecturer, Department of Environmental and Landscape Geography, Faculty of Science, Eötvös Loránd University (ELTE), member of the Environmental Geography Research Group
Research Fellow, Institute of Geography, HUN-REN Research Centre for Astronomy and Earth Sciences, member of the Physical Geography Research Group

Zoltán Szalai
Associate Professor, Department of Environmental and Landscape Geography, Faculty of Science, Eötvös Loránd University (ELTE), head of the Environmental Geography Research Group
Senior Research Fellow and Head of the Physical Geography Research Group, Institute of Geography, HUN-REN Research Centre for Astronomy and Earth Sciences
Head of the SEDILAB Laboratory