Novel insights into released hydrochar particle derived from typical high nitrogen waste biomass: Special properties, microstructure and formation mechanism

Hydrothermal carbonization (HTC) treatment is a promising method to transforming waste biomass into valuable resources and promoting waste recycling, especially for high nitrogen feedstocks. While small-sized hydrochar particle (≥0.45 μm) released from its solid product (hydrochar) application demonstrated large knowledge gaps compared with its original hydrochar and “secondary char” from model biomass (like glucose, sucrose, and starch). Thus, hydrochar particles derived from typical high nitrogen biomass, kitchen garbage (KG), and blue–green algae mud (AM), were collected to investigate their basic properties, microstructures and corresponding formation mechanisms. The results were: 1) the micron-sized hydrochar particles with yields as 3.42–7.86 wt% presented special characteristics, i.e., poor porous structures, moderate pH value, negative surface charge and higher surface hydrophobicity (contact angles as 95.00–117.67°) relative to original hydrochar and secondary char; 2) micronuclei aromatic core and hydrophobic hydrothermal polymers (methoxyl groups/alkyl chain with ether and carboxy groups) were identified in these hydrochar microparticles (HMPs) by jointly using differential thermogravimetry (DTG) analysis, Gaussian fitting model and thermogravimetric analysis combined with Fourier transform infrared spectrometry and mass spectrometry (TG-FTIR-MS) analysis; 3) polycondensation/cyclization reactions and Maillard/Mannich reaction in the KGHMPs, as well as solid–solid conversion and Maillard/Mannich reaction, polymerization reaction in AMHMPs core and its shell were proposed as their dominated formation mechanisms. The conclusions of this study indicated strong binding of HMPs with NH₄⁺, metals, and hydrophobic contaminants, and further reinforcing these application effects as soil fertilizer and decontaminant in soil/water for the N conversion, which also significantly depend on HTC temperature and feedstock.