Upcycling coal gangue and biomass into a silicon-rich soil amendment through hydrothermal humification

The valorization of solid wastes like coal gangue and biomass into soil amendments presents a promising sustainable strategy. However, its underlying mineral-organic synergistic mechanisms that govern the simultaneous activation of nutrients and microbial responses remain poorly understood. Here, we demonstrate a one-pot co-hydrothermal humification (co-HTH) strategy to co-convert coal gangue and biomass into silicon- and humic acid-rich co-hydrochar. Mineralogical analyses reveal that co-HTH effectively decomposes inert SiO₂ in coal gangue into more reactive aluminosilicate phases (e.g., KAlSi₃O₈) via biomass-derived intermediates, evidencing true organic-mineral coupling rather than physical mixing. The resulting co-hydrochar exhibits enhanced humic substance content and nutrient availability, which is associated with marked improvements in maize growth, including increases in whole-plant length, root tip number, and total root length of up to 61.0%, 100.0%, and 77.0%, respectively. Microbial community analysis and functional prediction indicate that the co-hydrochar associated with shifts in microbial functional potential could enrich the key bacterial phyla (e.g., Proteobacteria, Actinobacteria) and genera (Pseudomonas, Acinetobacter) which bolster the soil organic nitrogen cycle, including ureolysis, nitrification, and denitrification. Concurrently, co-HTH reduces arsenic concentrations up to 83.3% and shifts As(III) toward the less mobile As(V) by serving as an electron shuttle, providing labile carbon, and upregulating microbial arsenate reductase genes, thereby promoting arsenic detoxification, consistent with hydrothermal fixation and Fe-associated mineral transformation observed by XPS, XRD, and FTIR. This work provides a mechanistic foundation for a circular economy approach to simultaneously manage industrial and agricultural waste, improve soil health, and ensure crop safety.