Fe₃O₄ loaded biochar to enhance persulfate activation for tetracycline degradation: Performance and mechanism

Waste sodium lignosulfonate (LS) is widely converted into biochar-based catalysts due to its rich functional groups and high carbon content. However, inefficient peroxydisulfate (PDS) activation by original sodium lignosulfonate biochar (LB) limits its ability to remove organic pollutants from water bodies. Here, LS was employed as a precursor to synthesize Fe₃O₄-loaded biochar (Fe₃O₄@LB) through a one-step pyrolysis process for activating PDS to remove tetracycline (TC). Compared with the original LB, Fe₃O₄@LB exhibited a larger specific surface area (459.78 m²/g), which is advantageous in providing more adsorption and reaction active sites. Quenching experiment and electron paramagnetic resonance (EPR) analysis revealed that ¹O₂ and O₂^•- are the primary active species involved in TC degradation. Characterization results showed that Fe₃O₄ is uniformly distributed on the biochar, providing abundant Fe(II) to activate the PDS to generate reactive species (ROS). The Fe(III) generated after the reaction was reduced by electron-rich biochar to promote Fe(III)/Fe(II) cycling. The Fe₃O₄@LB-3/PDS system displayed excellent performance, degrading 90% of TC within 20 min, with a rate constant k of 0.092 min^-1, which is about three times that of the LB (0.036 min^-1). Based on the liquid chromatography-mass spectrometry (LC-MS) analysis, three possible degradation pathways were proposed. Besides, mung bean growth experiments confirmed the detoxification of TC by the Fe₃O₄@LB-3/PDS system. This work highlights the feasibility of waste LS usage to produce highly efficient biochar catalysts, providing a sustainable and green alternative to address water contamination by antibiotics.