Targeted preparation of biochar for the application as composting additive: Influence mechanism of feedstock properties on biochar derived from traditional Chinese herb residue

The addition of biochar derived from traditional Chinese herb residue (TCHRs) into the composting process of herbal waste could help to achieve the carbon neutrality of the traditional Chinese medicine industry. However, because of the influence of the decoction process and the huge difference in the characteristics of herb residue from various parts, the feedstock properties greatly affect the physicochemical properties of biochar, which will influence the composting performance. In this study, the influence mechanism of feedstock properties on biochar derived from root, stem, and leaf parts of TCHRs was clarified. Results showed that the highest weight loss rates of root, stem and leaf residues after the decoction process increased to − 13.07 %/min, − 5.25 %/min and − 5.90 %/min due to the increasing volatile matter and decreasing ash content. In addition, the oxygen-containing functional groups were reduced, and better pore structures of biochar were developed after the decoction process, which would influence the microbial activity in the composting process. Moreover, the maximum specific surface area of root, stem and leaf residue biochar had reached 125.35 m2/g, 236.33 m2/g and 38.77 m2/g, respectively. The aromaticity of root biochar was up to 83.68 %, which mainly consisted of an aromatic C-C structure. And the aromaticity of leaf biochar was 86.74 %, which mainly consisted of aromatic C-O and C-H. Especially, the stem residue biochar bore a rich carbonyl structure, which helped reduce the risk of heavy metals in the composting process. Finally, it is supposed to choose different sorts of herbal biochar to be additive into various composting processes for the promotion of circular economy of the traditional Chinese medicine industry.