Herein, magnetic Fe₃O₄ nanoparticles were synthesized by a relatively simple and fast sono-synthetic route within 2.0 min. The prepared nanoparticles and synergic effect of NaBH₄ and Na₂S₂O₄ (1 : 0.3 mole ratio) were employed as a new magnetic recyclable catalyst system for fast (2–5 s) and effective reduction of nitroaniline to phenylenediamine. The progress of the catalytic reaction was monitored with UV-vis spectroscopy and the products were extracted with CHCl₃ from an ethanol–water solution. Then, silica-coated magnetic nanoparticles were reacted with carbon disulfide and 1,4-phenylenediamine, which transformed from a monomeric form to a magnetic polyphenylthiourea nanocomposite. The prepared material was characterized with FT-IR, TGA, XRD, VSM, SEM, BET, TEM and EDS techniques. According to the characterization results, an average size of 50 nm was obtained for Fe₃O₄ nanoparticles. The specific surface area for nanocomposite and the saturated magnetization of the naked Fe₃O₄ and prepared composite were 47.9 m² g⁻¹, 37.5 emu g⁻¹ and 21.2 emu g⁻¹, respectively. The polymer nanocomposite shows relatively fast adsorption kinetics towards Pb²⁺ ions within 3 min and good adsorption capacity equal to 250 mg g⁻¹. The results of an isotherm study and kinetic modelling showed that the Freundlich isotherm model and a pseudo-second order model could better describe the lead adsorption property of the composite. The prepared catalyst and composite had good stability as the reduction efficiency and recovery for trace lead adsorption remained constant after 10 and 5 cycles, respectively.