Protecting our drinking water sources is a global challenge. Many communities rely on natural filtration methods, such as River Bank Filtration (RBF), where river water slowly seeps through soil and aquifer materials, which act as a natural filter. This process is cost-effective, but it isn’t perfect. It often struggles to remove persistent and toxic chemicals like chlorinated phenols (CPs). These pollutants, which enter waterways from industrial discharge and the breakdown of pesticides , can be highly mobile and leach directly into groundwater. A new study in the Journal of Hazardous Materials Advances by Tamara Apostolović, Marijana Kragulj Isakovski, and their colleagues investigates a promising solution: amending these soils with biochar.

The researchers first examined the native alluvial soil, a sandy, highly permeable soil taken from the Danube River bank. They tested how quickly four different CPs would move through columns packed with this soil. The results were concerning. In the native soil, the pollutants moved rapidly, with breakthrough curves appearing in under 10 hours for some layers. The soil’s natural retention ability was very low; the measured retardation factors (Rd​)—a measure of how much a pollutant is slowed down—were minimal, ranging from just 1.0 to 6.15. An Rd​ of 1.0 means the pollutant moves at the same speed as water. The study found this weak retention was primarily driven by hydrophobic interactions (the pollutants “disliking” water) and electron-donor-acceptor interactions. This meant that more hydrophobic, less water-soluble CPs like pentachlorophenol (PCP) were retained slightly better, while more polar CPs passed right through.

Next, the team conducted the same experiment but with soil that had been amended with just 0.5% biochar by mass. The results were transformative. This small addition of biochar, a porous, carbon-rich material, dramatically enhanced the soil’s ability to retain the CPs. The time it took for the pollutants to break through the soil columns increased by a factor of 12 to 40. The retardation factors (Rd​) skyrocketed, landing in a range between 42 and 155. The sorption distribution coefficients (Kd​) were up to a thousand times higher than in the unamended soil. This indicates the CPs were being strongly captured and immobilized by the biochar-soil mixture, preventing their rapid migration.

The most fascinating finding was not just that the biochar worked, but how it worked. The study revealed the biochar fundamentally changed the primary retention mechanism. In the biochar-amended soil, the dominant force of attraction shifted from hydrophobic interactions to polar interactions. The biochar’s highly aromatic structure facilitated strong π−π electron-donor-acceptor (EDA) interactions with the CPs’ phenolic rings. Furthermore, the biochar’s surface had a high point of zero charge (pHpzc​ of 10.8) , giving it a positive charge under the experimental pHpH is a measure of how acidic or alkaline a substance is. A pH of 7 is neutral, while lower pH values indicate acidity and higher values indicate alkalinity. Biochars are normally alkaline and can influence soil pH, often increasing it, which can be beneficial More of ~7. This positive surface attracted the CPs, even non-ionized ones, through dipole-induced dipole interactions. This mechanism shift completely inverted the retention trend: 4-CP, one of the most polar and least retained CPs in the native soil, became the most strongly retained compound in the biochar-amended soil.

This research provides powerful evidence for biochar as a tool for environmental remediation. Amending coarse, sandy alluvial soils—which are typically poor at filtering pollutants—with a small amount of biochar can compensate for their low sorption capacity. By shifting the retention mechanism to favor polar interactions, biochar makes these soils highly effective at capturing a broad spectrum of contaminants, especially the polar ones that are often the most problematic in water. These findings underscore biochar’s potential to significantly improve natural filtration systems, helping to protect the quality of our ground and surface water.

Source: Apostolović, T., Kragulj Isakovski, M., Šolić, M., Beljin, J., Weihermüller, L., & Maletić, S. (2025). Evaluating the Role of Biochar in the Transport Behavior of Chlorinated Phenols in Alluvial Soil Systems. Journal of Hazardous Materials Advances.