The synergistic mechanisms of 2,4-D degradation in the PMS activation by MFB-500. • A novel combination of biochar and FeS was used to complete pesticide degradation. • The coupled kinetic model of interface adsorption and catalysis was established. • The combination of SO 4 •- , •OH and 1 O 2 in MFB-500/PMS dominated 2,4-D degradation. • Fe(II)/Fe(III) cycle was enhanced by electron transfer from sulfur and biochar. • The optimized reaction parameters of MFB-500/PMS were determined. A highly active mediator (magnetic FeS@biochar, MFB) for peroxymonosulfate (PMS) activation was prepared by employing FeSO 4 ·7H 2 O and poplar sawdust as the precursor, for pesticides remediation in soil and groundwater. The magnetic FeS@biochar prepared at 500 °C (MFB-500) did not only showed good performance in activating PMS to degrade 2,4-dichlorophenoxyacetic acid (2,4-D), but also longer lifetime. Due to the introduction of FeS, the defect degree of MFB-500 was higher according to Raman spectra result and favored in PMS activation. The X-ray photoelectron spectroscopy (XPS) and Mössbauer spectra confirmed that sulfur species promoted the regeneration of Fe(II). Moreover, EPFRs also showed an electron shuttle to enhance the recycle of Fe(II)/Fe(III) and increased the PMS activation performance. Electron paramagnetic resonance (EPR) spectroscopy identified SO 4 •- , •OH and 1 O 2 as the reactive oxygen species (ROS) in 2,4-D degradation. The optimized reaction parameters of MFB-500/PMS were determined as [MFB-500] = 700 mg/L, [PMS] = 2.6 mM, [2,4-D] = 0.045 mM. Meanwhile, the interfacial adsorption and catalytic reaction of 2,4-D degradation by MFB-500/PMS is accurately described by a newly mixed order kinetics model with adsorption and decay dominant rate constants k α and k γ , respectively. The effect of pH 0 and coexisting anions was also studied, and it was found that acidic conditions are conducive to the degradation of 2,4-D, while alkaline conditions inhibited. Cl - , NO 3 – and SO 4 2- play a slight inhibitory effect, and HCO 3 – and H 2 PO 4 - will play a significant inhibitory effect. This work provides a promising approach to the rational design of high-performance active mediators for environmental remediation.