Purpose <p>The recalcitrant organic contaminant perfluorooctanoic acid (PFOA) is widely used in industries, and the corresponding contaminated soil has seriously posed environmental risks. Biochar, as a cost-effective adsorbent, has attracted considerable attention for the effective immobilization of PFOA in soils.</p> Methods <p>In this study, KHCO₃-modified biochar (KBC) was synthesized to be employed for the enhancement of PFOA immobilization in two contrasting soil matrices (silty clay loam and sandy loam). Artificial oxidative ageing was used to simulate natural ageing and assess its effects on biochar performance.</p> Results <p>The results showed that KBC remarkably enhanced PFOA immobilization even at a low amendment rate of 0.5% (w/w). KBC-amended silty clay loam exhibited a higher adsorption capacity (3,375.11&#xa0;mg/kg) in comparison to KBC-amended sandy loam (3,190.92&#xa0;mg/kg). However, the immobilization efficiency decreased after ageing due to the attenuation of hydrophobic interaction. Differences in soil properties influenced PFOA immobilization, as soil organic matter competed for adsorption sites through interactions with oxygen-containing groups, while the high cation exchange capacity and pH of sandy loam promoted Ca²⁺ adsorption, weakening electrostatic screening and enhancing the electrostatic repulsion between biochar and PFOA. Desorption and leaching experiments revealed that while KBC effectively immobilized PFOA in silty clay loam, PFOA leaching risk would still be an issue in sandy loam due to increasing electrostatic repulsion over time.</p> Conclusion <p>Therefore, the influence of natural ageing and soil type should be evaluated before applying biochar to remediate PFOA-contaminated soils.</p>