Ni transformation and hydrochar properties during hydrothermal carbonization of cellulose

The harm of heavy metals to the environment and human health has become a major concern due to their high toxicity, ease of accumulation in the human body, and resistance to degradation. In particular, Ni is widely used in various industrial and consumer products, which is a toxic pollutant posing great harm to humanity and the environment. Hydrothermal carbonization has broad prospects for reducing the ecological toxicity of heavy metals. However, the effects and mechanisms of hydrothermal carbonization conditions on the stabilization of heavy metals still need to be further explored. This research aimed to explore Ni migration and transformation within cellulose throughout the hydrothermal carbonization process. The results indicated that hydrothermal carbonization facilitated the immobilization of heavy metals due to the generation of hydrochars with complex surface structures. In addition, the hydrothermal carbonization process significantly decreased the weakly bound parts of Ni, thereby reducing the environmental risk of Ni. The optimal conditions for the hydrothermal carbonization process of cellulose added with Ni were 250 ℃ and 90 min. However, further increasing the reaction temperature or retention time resulted in negligible or even negative effects on Ni immobilization. In general, this study proposed possible mechanisms for the effects of hydrothermal carbonization on the migration and immobilization of heavy metals, which may provide insights into handling heavy metals in biomass.