Hydrothermal carbonization of Chinese medicine residue from licorice: Effects of pore and chemical structures on chromium migration

The Chinese medicine residue (CMR) is composed of wet substances, so using hydrothermal carbonization (HTC) to recover renewable energy from the residue is a suitable treatment method. Chromium (Cr), a kind of heavy metal element, is enriched in hydrochar and severely restricts its effective utilization. An in-depth analysis of the migration path and mechanism of Cr in hydrochar is helpful in promoting energy utilization for CMR. Here, licorice, a significant Chinese medicine, was selected as the example to analyze the evolutions of its pore and chemical structures and their effects on the migration mechanism of Cr during the HTC process. The products obtained under various HTC conditions were analyzed using nitrogen adsorption, FTIR, and 13C NMR. The results show that, considering reaction time and relevant reactions as the primary factors during the HTC process, the migration pathway of Cr in hydrochar undergoes two stages, and they are the accompanying migration stage and the recovery aggregation stage. Active adsorption sites for Cr may exist within the pore structure of hydrochar. In the HTC process, hydrolysis, decarboxylation, and decarbonylation reactions are the direct drivers of Cr migration, while aromatization is the underlying cause of Cr recovery and aggregation. It is hypothesized that Cr catalyzes the acetylene cyclotrimerization reaction, thereby promoting the formation of aromatic structures in hydrochar and integrating into the hydrochar carbon skeleton.