Synthesis of nitrogen-enriched hydrochar via co-hydrothermal reaction of liquid digestate and corn stalk

Synthesis of carbon material from low-cost and sustainable precursors has been intensively explored in recent years. In this study, a nitrogen (N)-enriched hydrochar was developed via a facile one-step hydrothermal carbonization (HTC) of corn stalk (CS) with liquid digestate (LD) of food waste. The LD substituted water and functioned as the N precursor during HTC. The physicochemical properties of hydrochar derived at different HTC temperatures (180-300 center dot C) were examined and the reaction mechanism was investigated. Intermolecular dehydration and condensation were the primary reactions in the HTC process of CS without LD. The CS-chars maintained the original structure and morphology of the raw corn stalk. The ammonia and inorganic salts in LD promoted the lignin removal, and accelerated the cleavage of the glycosidic linkages of the polysaccharide and hydrogen bonds of cellulose. Benefited from the ammonia and metals in the LD, the recalcitrance structure of the corn stalk was disrupted during the co-HTC even at a low temperature of 220 center dot C. Moreover, carbon spheres were observed in the LDCS-chars, indicating the LDCS-chars were resulted from sequential hydrolysis, dehydration and condensation during co-HTC reactions. Reactions between N compounds in the LD and derivatives from CS contributed to N doping. The N content of LDCS-chars achieved 4.95% at 260 center dot C and 83.94% of the N was presented as pyridinic-N. Co-hydrothermal treatment of CS and LD not only enhanced the characteristics of hydrochar, but also recovered two-thirds of ammoniacal N from the digestate to reduce greenhouse gas emission.

Automated summary

In this study, a nitrogen-enriched hydrochar was synthesized via a one-step hydrothermal carbonization method using liquid digestate as a substitute for water. The physicochemical properties of the hydrochar were examined at different temperatures, and the reaction mechanism was investigated. The presence of ammonia and inorganic salts in the liquid digestate facilitated lignin removal and disrupted the recalcitrant structure of the corn stalk. Moreover, carbon spheres were observed in the resulting hydrochar.