A novel approach to return sludge to nature: Hydrothermal conversion of full sludge components into humic acid and functional minerals

A novel two-stage hydrothermal technology was developed to convert full sludge components into humic acid and functional minerals as key soil compositions, along with heavy metal immobilization in the synthesized minerals for environmental remediation. In the first hydrothermal stage, by simulating the underground formation mechanism of natural humic acid , the organic components in sludge were hydrothermally converted into humic acid, exhibiting significant aromaticity and porous microstructure similar to that of mineral-derived humic acid, with a high yield of 46.7 % at 150 °C for 1 h. Subsequently, the second hydrothermal stage was conducted to further convert the inorganic sludge residue into functional minerals (analcime) through hydrothermal dissolution and reconstitution at 200 °C over 6 h. The study on the flow direction and distribution of heavy metals in sludge-derived products during hydrothermal processing quantitatively revealed that heavy metals predominantly accumulated in the inorganic sludge residue, with over 90 % of Cu, 84 % of Pb, and 46 % of Cr being retained. These multiple heavy metals were found to be stably immobilized within the secondary hydrothermally synthesized analcime structure, with total leaching rates below 1.7 %, thus minimizing their environmental impacts. Additionally, these hydrothermal products exhibited significant benefits for agricultural fertilization and environmental remediation: the humic acid enhanced soil fertility , while the functional zeolites remediated soil and water. As such, the study exhibits great potential to hydrothermally convert full sludge components into eco-friendly regenerated soil products (humic acid and analcime), paving a novel and safe way for returning sludge to nature. • Two-stage hydrothermal conversion of full sludge components was achieved. • Organic matter was converted to aromatic humic acid (47 % yield) at 150 °C for 1 h. • Multiple heavy metals predominantly accumulated in the inorganic sludge residue. • Inorganic sludge residue was converted to analcime for heavy metal immobilization. • Hydrothermal products improved soil fertility and remediated heavy metal pollution.