Pyrolysis of food waste digestate residues for biochar: Pyrolytic properties, biochar characterization, and heavy metal behaviours

The current work has examined the pyrolytic properties, product formation mechanisms, biochar properties, and heavy metal (HMs) safety of biochar during food waste digestate residue (DR) pyrolysis. The results have shown that DR pyrolysis proceeded in five stages. The kinetic model for Stages 1, 3 and 4 was the simple reaction order model, the one-dimensional diffusion model for Stage 2, and the three-dimensional (Jander) diffusion model for Stage 5. Based on thermogravimetric-Fourier transform infrared spectrometry (TG-FTIR) and pyrolysis-gas chromatography-mass spectrometry (Py-GCMS) analysis, the volatile components of the DR pyrolysis were mainly produced by the Maillard, decarboxylation, and deamination reactions as H2O, CH4, CO2, CO, phenol, C--O (anhydride/ketone/aldehyde), C-O and NH3. While there were six main components of the pyrolysis oil, that is, amines and amides, nitriles, N-hybrid compounds, oxides, and sulfides. Appropriate aromatic properties were observed in the prepared biochar, and the biochar obtained at a pyrolysis temperature of 700 degrees C had a relatively high specific surface area. The HMs results showed that the HMs in biochar obtained from DR pyrolysis at 400, 500, 600, 700, and 800 degrees C were predominantly in the oxidizable and residual fractions. The toxicity characteristic leaching procedure (TCLP) tests and the potential ecological risk indices for HMs have indicated a high safety profile for biochar. This work has elucidated the formation process of DR pyrolysis products and the physicochemical properties and safety of biochar. It has also provided an outlet for the application of biochar, which provides a strong contribution to promoting resource use of DR.

Automated summary

The current study investigated the pyrolytic properties of food waste digestate residue (DR), including product formation mechanisms, biochar properties, and heavy metal (HMs) safety. It was found that DR pyrolysis proceeded in five stages, with different reaction kinetic models for each stage. Through analysis, it was determined that the volatile components in DR pyrolysis were mainly produced by various reactions, while the pyrolysis oil contained six main components. The biochar obtained at a pyrolysis temperature of 700 degrees C exhibited appropriate aromatic properties and a high specific surface area. The HMs in the biochar were mainly in the oxidizable and residual fractions, indicating a high safety profile. This research provides valuable insights into the formation process, properties, and safety of biochar, fostering its application and promoting resource utilization of DR.