Does biochar field aging reduce the kinetic retention for weakly hydrophobic antibiotics in purple soil?

Abstract 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