Insights into the mechanisms of biochar-derived dissolved organic carbon-facilitated transport of oxytetracycline in saturated porous media

Biochar-derived dissolved organic carbon (BDOC) may influence the environmental fate and behavior of tetracycline antibiotics in subsurface environments. In this study, BDOC derived from the pyrolysis of wheat straw at three distinct temperatures (300 °C, 450 °C, and 600 °C) was used to investigate its influence on the transport of oxytetracycline (OTC) through saturated quartz sand. The findings demonstrated that BDOC enhanced OTC mobility due to steric hindrance caused by organic matter accumulation, competition for retention sites between OTC and BDOC, and increased electrostatic repulsion between anionic species, including OTC⁻ ions and quartz sand. Notably, the mobility-enhancing effects of BDOC became significantly more pronounced at higher pyrolysis temperatures, likely resulting from increased organic matter deposition on sand surfaces and intensified electrostatic interactions. However, the promoting effect of BDOC on OTC transport was attenuated as pH increased from 5.0 to 9.0, which was attributed to reduced competitive deposition and steric effects caused by BDOC retention. Furthermore, cation-bridging, particularly with Cu²⁺ in the background solution, amplified BDOC’s promotion effects. These results highlight that dissolved organic carbon released from biochar exerts a notable influence on the antibiotics' mobility within the aquifers.