Unveiling the intensification role of circulating hydrothermal carbonization: A sustainable way for humic acids production from Chinese medicine residues

Hydrothermal carbonization is recognized as a promising way for converting wet biowastes such as Chinese medicine residues into artificial humic acids with low carbon footprint. However, challenges of massive water input and wastewater output still exist. Herein, circulating hydrothermal carbonization was proposed via reusing process water in next hydrothermal carbonization to mitigate these environmental burdens and enhance humic acids yield. The impact of process water recirculation on the structural variation of Chinese medicine residues, soluble components, and humic acids formation was examined for in-depth understanding of what happened to Chinese medicine residues. Results show that autocatalysis by accumulated organic acids in process water, along with the involvement of organic substances such as 5-HMF and furfural, enhances the yield of humic acids. The reduction in crystallinity index of Chinese medicine residues indicates that alkaline process water further facilitates humic acids formation by promoting the degradation of crystalline cellulose. The highest humic acids yield from Chinese medicine residues reaches 20.70 %. However, 93.2 % of humic acids dissolve in process water, and secondary char microspheres are absent, suggesting that they are likely to share similar precursors and formation pathways. The recirculation of alkaline process water is more favorable for humic acids production from fresh Chinese medicine residues regarding mass yield and chemical structure. This work highlights a sustainable approach for enhancing humic acids production from Chinese medicine residues. • Circulating hydrothermal carbonization enhances humic acids production. • 20.70 % of humic acids can be yielded from Chinese medicine residues. • Alkaline process water facilitates the degradation of crystalline cellulose. • Process water serves as precursors and catalysts to form humic acids. • Formation of humic acids and secondary char shares similar reaction pathways.