Effects of reaction parameters on the microstructure and nutrient content of hydrochar prepared from food waste via hydrothermal carbonization

Food waste has high moisture content and generates large volumes. Traditional disposal methods (e.g., landfilling and incineration) are costly and environmentally unfriendly. This study systematically investigated the effects of reaction parameters, including temperature and residence time, on the microstructure and nutrient content of hydrochar derived from food waste at the laboratory scale, while exploring potential reaction mechanisms. Scanning electron microscopy (SEM) results indicated that during hydrothermal carbonization (HTC) at 200°C, food waste disintegrated and generated numerous microspheres, which then condensed to form hydrochar. X-ray diffraction (XRD) results revealed the formation of intermediate structures in hydrochar similar to fullerene C60 crystals during HTC. The specific surface area results indicate that increasing reaction severity significantly reduced the specific surface area of hydrochar from 11.00 to 3.74 m²/g. HTC at 200°C for 1 h induced substantial structural disintegration of the food waste, significantly altering the reaction pathways, especially nutrient transfer. Nutrient analysis revealed that hydrochar produced at 200°C exhibited the highest nitrogen content (21.6 g/kg), while the maximum phosphorus (3.25 g/kg) and potassium (7.47 g/kg) contents were achieved at 180°C. The intermediates were gradually aromatized with increasing temperature and residence time. The key intermediates, including 5-hydroxymethylfurfural (5-HMF), were completely converted under the reaction conditions of HTC at 220°C for 1 h, and the hydrothermal reaction was shifted from feedstock decomposition to product polymerization. This study offers valuable insights into HTC in food waste treatment and presents a new perspective to elucidate the HTC reaction pathway of food waste and the impact of reaction parameters on the nutrient content of the product.