<p>This study investigates the in situ degradation of naphthalene (NAP) in contaminated soil using a biochar-supported nano zero-valent iron (BC@nZVI) composite to activate persulfate (PS). NAP, a persistent polycyclic aromatic hydrocarbon (PAH), poses significant ecological risks due to its carcinogenicity and environmental mobility. The BC@nZVI composite was synthesized via a two-step hydrothermal–carbonization method and characterized using XRD, SEM, TEM, BET, and FT-IR, confirming its enhanced stability and reduced aggregation compared to bare nZVI. Sandbox experiments optimized critical injection parameters, demonstrating that a PS dosage of 2% soil mass and a BC@nZVI:PS mass ratio of 1:2 achieved 76.6% NAP degradation within 4.5 cm of the injection well after 72 h. Pre-injection of BC@nZVI followed by PS at a high flow rate (2.68 L·min⁻¹) enhanced reagent distribution, yielding degradation rates of 44.1–80.3% across a 9 cm radius. Reduced injection volumes (3.2 L) or reversed injection sequences diminished efficiency by 14–25%, while excessive reagents induced radical self-quenching (e.g., SO₄⁻· scavenging) and Fe²⁺ overconsumption. Compared to conventional Fe²⁺-activated systems, the BC@nZVI/PS system exhibited superior soil penetration, sustained radical generation, and reduced PS consumption, effectively addressing challenges in heterogeneous soil remediation. These findings provide actionable strategies for scalable PAH-contaminated site remediation, balancing high efficacy with operational and environmental feasibility.</p> Graphical Abstract <p></p>