A recent study published in the journal Biochar by Xinyu Liu, Yang He, and colleagues investigated how the aging of biochar in a field setting affects its ability to retain weakly hydrophobic antibiotics, namely sulfadiazine (SD) and florfenicol (FF), in purple soil. This research compared fresh biochar (FBC) to biochar aged for one year (ABC1) and five years (ABC5) to understand the evolution of its physical and chemical properties and their impact on antibiotic transport. The findings revealed that aging significantly alters biochar characteristics, which subsequently dictates how antibiotics are retained and transported through the soil system.

Field aging substantially changed the biochar’s composition: the carbon (C) content decreased by 10.40% over five years, while the oxygen (O) content increased by 40.52%. Specifically, one-year aged biochar (ABC1) demonstrated optimal performance, showing a remarkable 99.28% increase in specific surface area (SSA) and enhanced oxygen-containing functional groups. This led to the maximum antibiotic retention rates: 16.57% for SD and 24.78% for FF. Although the adsorption capacity decreased after five years, the five-year aged biochar (ABC5) maintained stable remediation effects by increasing biochar-soil interactions, which modified the soil structure. This stability was evident from the increased dispersivity (λ) and hydrodynamic dispersion coefficient (D). The total retention of SD and FF in the soil column remained significantly higher in both aged biochar treatments (SABC1 and SABC5) compared to the control soil (S).

The change in biochar’s surface over time directly influenced the adsorption mechanism. Fresh biochar (FBC) has a predominantly hydrophobic surface due to 77% hydrophobic aromatic moieties (-C-C- and -C-H bonds), making hydrophobic partitioning the critical driver for adsorption. Over time, oxidative aging introduces more oxygen-containing functional groups, such as -C-O- and -COOH, increasing the surface hydrophilicity and diminishing the original hydrophobic interactions. In ABC1, the increase in SSA and pore diameter, coupled with surface oxidation, provided the maximum number of optimal sorption sites. However, after five years (ABC5), the precipitation of soil minerals within the biochar pores led to occlusion and a reduction in SSA and pore size, causing a decline in pure adsorption capacity.

Modeling the antibiotic movement using the two-site chemical nonequilibrium model (TSM) provided a clear picture of the shift in retention mechanisms. The TSM model demonstrated that the fraction of equilibrium adsorption sites (f), which quantifies the proportion of adsorption sites at equilibrium with the solution phase, increased significantly. The increase in the equilibrium adsorption fraction suggests that biochar aging improves the overall efficiency of antibiotic retention by optimizing the distribution of antibiotics between the soil particles and the solution.

The overall findings indicate a progressive change in the dominant control mechanism for antibiotic transport. While fresh biochar primarily focuses on adsorption-dominated retention due to its strong inherent capacity, the long-term aged biochar increasingly relies on soil-biochar complexation and altered soil structure to stabilize environmental effects. The initial hydrophobic state of FBC limits water-soil contact, leading to unrealized regulatory potential, but aging-induced hydrophilicity and the formation of micro-aggregates enhance water conductivity and facilitate antibiotic transport. Ultimately, the study confirms that field aging transforms biochar into a stable composite system that maintains long-term environmental regulation by shifting antibiotic transport from being adsorption-dominated to being influenced by dispersion and optimized soil structure.

Source: Liu, X., He, Y., Li, J., Li, J., Zhang, J., & Tang, X. (2025). Does biochar field aging reduce the kinetic retention for weakly hydrophobic antibiotics in purple soil? Biochar, 7(69).