Role of typical amino acids on structural evolution of hydrochar and nitrogen transformation mechanisms during hydrothermal carbonization of kitchen waste

Abstract

The involvement of specific amino acids will inevitably alter the pathways for the transformation of kitchen waste (KW) components during the hydrothermal carbonization (HTC) process. In this study, the evolution of hydrochar structure and nitrogen transformation mechanisms was systematically elucidated for the first time at the amino acid level during the HTC of KW. The results showed that glutamic acid promoted the formation of non-condensable gases, resulting in hydrochar with the loosest pore structure. The hydrolysis products of histidine (particularly the imidazole ring) polymerized with aromatic heterocycles to form stable secondary char, achieving maximum yield (55.40 %) and nitrogen retention rate (65.43 %). Phenylalanine hydrolysates (phenethylamine, styrene) facilitated the dissolution of weakly polar organics from KW and further generated amorphous solids through hydrophobic interactions with long-chain amides and indoles. This led to the hydrochar with the lowest yield (8.77 %) and surface area (0.00 m² g⁻¹). Furthermore, hydrochar washed with dichloromethane exhibited an improved pore structure but showed reduced practical utilization potential due to its diminished yield (0.28 %), compromised nitrogen retention rate (0.14 %) and decreased defect concentration (indicated by an I_D/I_G ratio as low as 1.83). This research provided novel insights into the tailored synthesis of nitrogen-doped carbon materials from complex biowaste.