Co-hydrothermal treatment of industrial solid wastes and organic material: Red mud weakens carbon stabilization

Co-hydrothermal treatment of industrial solid wastes and organic materials has recently attracted increasing attention as an effective strategy to enhance hydrochar yield and modify its physicochemical properties. However, studies on carbon retention during co-hydrothermal treatment of mixed organic materials and solid wastes remain limited. Herein, hydrochar composites were prepared using phosphogypsum (PG), fly ash (FA) and red mud (RM) with earthworm manure fertilizer (EF) and forest humus (FH) at 150 °C for 5 h. Carbon retention of hydrochar composites was analyzed. The results showed that most of the hydrochar was formed on the surface of the composite. RM increased the carbon retention during co-hydrothermal treatment by 6.41-24.39%, but after H₂O₂ oxidation, the carbon retention of RM-amended hydrochars (17.90-54.72%) was substantially lower than that of hydrochars without RM (37.52-74.09%). This suggests that the presence of minerals, particularly RM, may not facilitate long-term carbon stability of hydrochar, potentially affecting the CO₂ balance upon soil application.