The impact of field aged biochar (FABC) on the adsorption kinetics and transport behavior of weakly hydrophobic antibiotics in soil is scarcely discussed. This study investigated the impact of FABC on weakly hydrophobic antibiotics (sulfadiazine, SD and florfenicol, FF) transport in purple soil by comparing fresh biochar (FBC), one-year aged biochar (ABC1), and five-year aged biochar (ABC5). Through batch adsorption, soil column experiments, and Hydrus 1D modeling, this study examined the evolution of physicochemical properties of biochar, their effects on soil porosity and dispersion, and antibiotic adsorption. Results showed that aging significantly altered biochar characteristics, with carbon (C) content decreasing by 10.40% while oxygen (O) content increased by 40.52%. ABC1 demonstrated optimal performance with a 99.28% increase in specific surface area (SSA) and enhanced oxygen-containing functional groups, leading to maximum antibiotic retention rates of 16.57% for SD and 24.78% for FF. Although ABC5 showed decreased SSA and adsorption capacity, it maintained stable remediation effects through enhanced biochar–soil interactions, as evidenced by increased dispersivity (λ) and hydrodynamic dispersion coefficient (D). The two-site chemical nonequilibrium model (TSM) revealed that the fraction of equilibrium adsorption sites (f) increased from 0.1164 to 0.3514 after aging, indicating improved antibiotic retention. These findings demonstrate that while one-year aging enhanced remediation capacity, five-year aging stabilized environmental effects through modified soil structure. Graphical Abstract

Biochar amendments show promise for mitigating accelerated antibiotic transport caused by macropore flow in soil, yet their effectiveness in such systems is poorly understood, lacking direct evidence differentiating biochar's role across macropore flow versus soil matrix infiltration. Using a novel macropore and matrix domains (dual-domain) separation apparatus, this study quantified the effect of biochar addition on the transport of weakly hydrophobic antibiotics, sulfadiazine (SDZ) and florfenicol (FFC), in soils under hydraulically isolated and connected domain conditions. Results suggested that biochar's efficacy is significantly amplified when hydraulic connectivity between the dual domains is present. In this state, it actively diverts antibiotics from macropore flow into the soil matrix infiltration, significantly reducing the total cumulative mass fluxes (CMFs) of SDZ from 0.72 ± 0.01 to 0.61 ± 0.00 and FFC from 0.81 ± 0.04 to 0.72 ± 0.02 ( p < 0.05). Partial least squares structural equation modeling (PLS-SEM) subsequently revealed that biochar rewires the system’s causal pathways. The model showed that it leverages mobile carriers (dissolved organic matter and colloids) to create a powerful immobilizing sink in the matrix, while counteracting the advective flux traced by Br ^– . These findings support the new hypothesis that biochar functions as a dynamic “biochar sorption pump” (BSP). This framework reframes biochar from a passive sink to an active flux regulator, providing a basis for designing precision remediation strategies based on soil hydraulic properties to protect vulnerable aquatic ecosystems. Graphical Abstract