<p>This study utilized rice husk, an agricultural waste, to produce NaOH-modified biochar (NaOH-BC) for activating persulfate (PS, from sodium persulfate, Na₂S₂O₈) to degrade di-n-butyl phthalate (DBP), a common phthalate ester contaminant, in soil. The morphology and composition of NaOH-BC were characterized using techniques including XRD, FTIR, SEM, EDS, XPS, and BET. The effects of initial pH, NaOH-BC dosage, and the molar ratio of PS to DBP on the degradation efficiency were investigated. Optimal conditions (pH = 6, NaOH-BC dosage = 0.8&#xa0;g/L, and a 1:1 molar ratio of DBP to PS) achieved a high DBP degradation rate exceeding 92%. Adsorption experiments indicated that while NaOH-BC could adsorb DBP from soil, adsorption was not the primary removal pathway. Electron paramagnetic resonance (EPR) analysis confirmed that the NaOH-BC/PS system generated significantly higher signals for free radicals (SO₄•⁻, •OH, •O₂⁻, and ¹O₂) compared to the unmodified BC/PS system, demonstrating the superior PS activation capability of NaOH-BC. Quenching experiments identified sulfate radicals (SO₄•⁻) as the dominant reactive species responsible for DBP degradation. Furthermore, density functional theory (DFT) calculations revealed that the initial reaction sites of DBP during radical attack were predominantly located on the butyl side chains and the benzene ring-ester group regions. This study not only confirms NaOH-BC as an excellent biochar material for PS activation but also establishes the NaOH-BC/PS system as an effective strategy for DBP remediation in soil, providing valuable insights for degrading phthalate esters in contaminated soils.</p>