Enhancing the stability and heavy metal immobilization of co-pyrolysis biochar through biomass and red mud co-pyrolysis: A synergistic mechanism

In the research on the preparation of biochar from co-pyrolysis of red mud (RM) and biomass, the synergistic mechanism of immobilizing heavy metals (HMs) and enhancing the stability of biochar is seldom reported. In this study, the effects of different mass ratios of corn stalks (CS) and RM (9:1, 3:1, 2:1, 1:1) and pyrolysis temperatures (300 °C, 450 °C, 600 °C) on the physicochemical properties, carbon stability, HMs speciation, and their potential risks were investigated. Furthermore, the mechanism of the co-pyrolysis process to enhance biochar stability and facilitate transformation of HMs speciation was analyzed. The results showed that co-pyrolysis of CS with RM significantly enhanced biochar's oxidation resistance (I D /I G  = 0.61, R 50,biochar  = 0.52), and simultaneously increased the stabilized state proportion of Ni, Cr, and Cd that exceed standard limits, thereby reduced the biochar phytotoxicity (GI = 193.20%). During the co-pyrolysis process, the metals and metal minerals in RM catalyzed the cleavage of CS, promoting the formation of condensed aromatic carbon structures in biochar, which adsorbed the free HMs to form organic carbon-metal/metal oxide complexes through coordination, achieving the dual roles of immobilizing HMs and enhancing the stability of biochar. In addition, the formation of large aggregates and the conversion of free HMs into metal oxides are important mechanisms for co-pyrolysis to enhance the biochar stability and HMs immobilization, respectively. This study provides a feasible and promising direction for the synergistic and efficient resource utilization of industrial and agricultural waste.