Nitrogen distribution and evolution during persulfate assisted hydrothermal carbonization of spirulina

In this study, persulfate was used during hydrothermal processing of spirulina (160℃-220℃) for enhancement of nitrogen conversion. The nitrogen distribution in aqueous phase, hydrochar and biocrude-oil was evaluated, and the elemental composition and chemical forms of hydrochar were investigated. Results suggested that the addition of persulfate during hydrothermal processing of spirulina increased the atomic N/O of hydrochar for 1.2%-2.4%, whereas the NH₄⁺-N concentration in liquid phase increased by approximately 67-155 mg/L regardless of temperature, suggesting that the persulfate could facilitate the organic nitrogen degradation and protein deamination. The N1s XPS analysis indicated that the protein-N, pyrrole-N, and inorganic-N ratio in spirulina were decreased, while more pyridine-N in hydrochar was formed, suggesting that more stable N forms were generated. In addition, the elementary composition also showed that more N was formed on the surface of hydrochar instead of the core.