Display omitted •Graphitization degree, defects, and basic sites increased with the temperature of biochar modification.•Acetaminophen degradation was enhanced increased with the temperature of biochar modification.•Graphitization degree, defects, C/O, C=O, and basicity were correlated with catalytic activity.•Persulfate was activated via persistent free radicals and delocalized electrons in graphitic structures.•Different electron donors activated persulfate for the biochars modified under different temperature. In this study, a biochar (BC) derived from the pruning wastes of apple trees was thermally modified and was evaluated as a catalyst to explore an innovative strategy of valorization. The BC was thermally modified at 400, 550, and 700 °C, denoted as BC400, BC550, BC700, respectively. The removal of 50 mg/L acetaminophen (ACT) with 1.0 g/L biochars was <9.0% except for BC700 (19.1%), without an oxidant. However, it was greatly enhanced in the presence of 0.5 g/L sodium persulfate. The surface area normalized ACT removal rate constant increased from 6.56 × 10−5 min−1·m−2 for BC to 28.4 × 10−5 min−1·m−2 for BC700, and it was well-correlated with indicators of the degree of graphitization (sp2/sp3-C content), defects (ID/IG), C/O ratio, and C=O content. They were increased from 72.9% to 96.1%, 2.24 to 2.86, 4.9 to 12.1, and a negligible level to 3.9%, respectively, for BC and BC700. The electron paramagnetic resonance spectra and the characteristics of the biochars indicated that the ACT removal was attributed predominantly to SO4•− generation accepting electrons from persistent free radicals and the graphitic structures. The signal of SO4•− was the highest and dominant for BC700, which had the most developed graphitic structures, defects, and graphitic C=O with abundant (de)localized π electrons and non-bonding electrons. The results suggest that biochars can be excellent, environmentally friendly catalysts and that the activation mechanisms can be controlled by simple thermal modification to customize the performance for a wide variety of applications in various media, including water, soil, and sediments.