Insights into the nitrogen transformation process in Maillard reaction of sewage sludge and glucose towards nitrogen resource recovery and environmental sustainability

Sewage sludge (SS), as traditional solid waste, has abundant nitrogen species. It could be utilized cleanly, efficiently and high value-added by co-pyrolysis Maillard reaction with reducing sugar. The nitrogen transformation in co-pyrolysis process of dried sewage sludge (DSS) and glucose was analyzed. An experimental method of thermogravimetric-Fourier transform infrared spectroscopy-mass spectrometry (TG-FTIR-MS) was adopted. Suitable multi-component reaction model was obtained based on the results, and the chemical reaction corresponding to each component was further investigated. Nitrogen heterocyclic compounds (NHCs) were mainly produced in 200-260 °C. Furthermore, it was confirmed that 50 wt.% of glucose addition was beneficial to the production of NHCs. The result of density functional theory (DFT) calculations showed that Maillard reaction of glutamic acid (Glu) and glucose to pyrrole ring was the dominant nitrogen transformation pathway in the co-pyrolysis process. The rate-determining steps (RDS) energy barrier of the pyrrole ring formation reaction involving NH3 is 19.4 kJ/mol higher than that of Glu, indicating that competition may exist between them. The experimental and DFT results revealed the mechanism of interaction and competition of amino acid-N thermochemical parallel reactions, providing essential understanding to nitrogen transformation during the DSS and glucose co-pyrolysis.