Pyrolysis Behavior of Gentamicin Fermentation Residue: Product Distribution, Kinetics, and Nitrogen Transformation for Sustainable Antibiotic Waste Valorization

Sustainable management of antibiotic fermentation residues is critical to mitigating environmental and public health risks associated with pharmaceutical waste. Gentamicin fermentation residue (GFR), a challenging antibiotic byproduct rich in nitrogen and organic matter, presents both environmental risks and untapped resource potential. Pyrolysis can convert GFR into high-value products (pyrolysis oil, gas, and char) and eliminate environmental risks through high-temperature treatment. In this study, the product yields, pyrolytic kinetics, and nitrogen transformation pathways at different temperatures (400–800 °C) were investigated to explore the pyrolysis behavior and mechanism of GFR. Results revealed that temperature strongly influenced product distribution: biochar yield was dominant at low temperatures 51.2 ± 0.4% at 400 °C, oil yield peaked at 600 °C 14.6 ± 0.4%, and gaseous products became prevalent above 700 °C, reaching an estimated 78.4 ± 0.2% at 800 °C by mass-balance difference. Furthermore, isoconversional kinetic analysis (FWO and KAS) yielded apparent activation energies of 226.9–249.4 kJ/mol over α = 0.5–0.8, with R2 values of 0.946–0.976, indicating conversion-dependent, multi-step devolatilization behavior. For nitrogen transformation, elemental and spectroscopic analyses showed that nitrogen transitioned from unstable pyrrolic forms in raw GFR to more stable pyridinic and graphitic forms in biochar, enhancing its potential for catalytic and environmental applications. Meanwhile, gaseous nitrogen species such as NH3 and HCN were released at different temperature stages, and a three-stage nitrogen transformation mechanism was proposed linking the decomposition kinetics to the nitrogen migration pathways. These findings highlight pyrolysis as a promising, controllable, and sustainable method to convert antibiotic residues into valuable energy products and functional materials, with its contribution to sustainable pharmaceutical waste management contingent on proper NH3/HCN gas treatment.