Solidification function of biochar-modified red mud-based geopolymer to heavy metals

This study compared the effects of corn biochar (CB) and bamboo biochar (BB) on the strength development and Cu²⁺/Cd²⁺ immobilization performance of a high-strength red mud-based geopolymer (RMG). Red mud (RM), fly ash (FA), and blast furnace slag (BFS) were used to prepare geopolymer matrices, which were further modified with CB and BB. The results showed that the geopolymer matrix achieved high strength at an RM/FA/BFS mass ratio of 10:4:6, and 3 wt% biochar was selected for the metal immobilization tests based on the compressive strength results. At this dosage, the mean 28-day compressive strengths of CB-RMG and BB-RMG were 43.21 and 41.89 MPa, showing similar strength levels. For BB-RMG specimens containing 3.0 wt% elemental Cu or Cd, the short-term leachate concentrations were 1.98 mg/L for Cu²⁺ and 0.22 mg/L for Cd²⁺. At 42 days, the cumulative leaching parameters of BB-RMG were lower than those of control RMG by 44.15% for Cu²⁺ and 69.32% for Cd²⁺. The lower metal release was consistent with possible contributions from gel encapsulation, pore blocking, retention within biochar pores, and metal binding at biochar surfaces. CB-RMG showed slightly higher compressive strength, while BB-RMG showed lower Cu²⁺ and Cd²⁺ release.