Enhanced degradation of oxytetracycline hydrochloride in neutral heterogeneous electro-Fenton system: Key role of lignin modified Fe-MOFs derived catalyst as cathode

In this study, sulfur-doped iron-based carbon materials were synthesized by calcining Fe-MOF derivative precursors modified with sodium lignosulfonate (SL) during a solvothermal process. Characterizations including SEM, TEM, XRD, BET, and XPS confirmed that SL incorporation introduced sulfur doping and a carbon substrate, enhancing the specific surface area, electrical conductivity and catalytic activity of the electrocatalysts. The materials were applied as cathodes in a heterogeneous electro-Fenton (HEF) system to remove oxytetracycline hydrochloride (OTC), and the degradation mechanism was further investigated. Among the synthesized materials, FeNPs/CS1.5 (the catalyst prepared when both iron salt and organic ligand were used in an amount of 6.0 mmol and sodium lignosulfonate was added in an amount of 1.5 mmol) exhibited the best performance, achieving over 95 % OTC removal and a mineralization rate of 78.2 % under optimal conditions (SL to Fe (NO3)3.9H2O atomic weight ratio of 1:4, catalyst loading of 2.5 mg cm-2, voltage of -1.2 V and pH = 6). Mechanistic studies revealed that OTC degradation occurred through both radical (center dot OH, O2 center dot-, SO4 center dot-) and nonradical (1O2, electron transfer) pathways. Additionally, SL improved electron transfer rates, promoted the Fe (II)/Fe(III) redox cycle, and significantly inhibited iron ion leaching. These results demonstrate the potential of SL-modified catalysts for efficient pollutant removal and their practical applicability in real water treatment systems.