Role of pH catalysis during hydrothermal carbonization of digestate in biogas slurry: Mechanisms of nitrogen speciation and transformation

Abstract

Large amounts of biogas slurry and digestate waste not only impose an environmental burden, but also hinder the further development of anaerobic digestion. Hydrothermal carbonization (HTC) facilitates the valorization of high-moisture solid waste. In this study, the effect of initial pH (2.5–13.5) of biogas slurry on nitrogen (N) transformation during the HTC of livestock manure digestate was investigated. The migration of N was closely related to pH, and acidic environment effectively enriched N in hydrochar (HC). The N element in HC was mainly presented in heterocyclic forms, with pyridine-N and quaternary-N being the most stable. The N content of HC at pH 2.5, 4, and 6 was 2.10 %, 1.81 %, and 2.28 %, respectively. And the corresponding sum proportion of pyridine-N and quaternary-N was 55.08 %, 39.76 %, and 35.53 %, which was significantly higher than in alkaline environment. Compared with HC-8 (HC obtained at pH 8), the C and N contents in HC 2.5–6 increased by 12.80 % - 45.71 % and 17.53 % - 48.05 %, respectively, indicating C and N contents were higher in HC prepared under acidic conditions compared to neutral and alkaline conditions. Results indicated that the acidic environment promoted the formation of pyridine-N and quaternary-N, which strengthened the Maillard and Mannich reactions. In contrast, alkaline environment hindered protein hydrolysis and suppressed the polycondensation of pyrrole-N. The results provide a basis for pH regulation to control N species in HC, optimizing the HTC process for efficient N recovery and high-value waste utilization.