In this study, a novel magnetic biochar (BC-Fe_n), synthesized by one-step pyrolysis at 800 °C using platanus leaves and iron-containing sludge in different mass ratios, was used to activate persulfate (PS) for tetracycline (TC) degradation. Results show that BC-Fe_1:2 exhibited excellent degradation performance (91.4%), much better than the raw biochar (BC) (77.1%). And electron paramagnetic resonance (EPR) techniques demonstrated radical ($S{O}_4^{·-}$, ·OH, $O_2^{·-}$) and non-radical (¹O₂) pathways in BC-Fe_1:2/PS, S $O_4^{·-}$ was further proved to be the main active species by quenching experiments. Liquid chromatography-mass spectrometry (LC-MS) was utilized to identify the intermediates of TC and three possible degradation pathways were proposed. Additionally, the BC-Fe_1:2 was weakly affected by pH and the coexisting anions in wastewater, and could still remove 80% of TC in the third cycle experiment. This study provides a new idea for converting backwash iron-containing sludge from groundwater treatment plants into valuable catalysts, and provides new insights into the key role of low-cost magnetic biochar in PS activation.