Efficient stabilization and solidification of molybdenum oxyanions using alkali-activated geopolymer binders

While the use of geopolymers in stabilization/solidification (S/S) processes for contaminated matrices is well established for mitigating the leaching of cationic pollutants, limited research has focused on the immobilization of anionic species. This study aims to address this research gap by evaluating the effectiveness of S/S techniques employing a geopolymer binder to mitigate the leaching of molybdenum (Mo). The investigation is conducted using five distinct geopolymer paste formulations. The metakaolin (MK) and sodium silicate (SS) were used as binders. The pastes were activated with SS powder and a 12 M NaOH solution, while red mud (RM) and/or charcoal (Cc) were tested as immobilization agents. The S/S mechanisms were characterized using Brunauer-Emmett-Teller (BET) analysis, X-ray diffraction, EN-12457, toxicity characteristic leaching procedure (TCLP), and scanning electron microscopy with energy-dispersive spectroscopy (SEM-EDS). Leaching tests showed that molybdenum concentrations remained well below hazardous waste limits, with leaching rates below 3% across all tested mixtures M1, M1(RM), M1(Cc), and M1(RM/Cc), regardless of the initial molybdenum concentration via coprecipitation of Na₂MoO₄. SEM-EDS mapping identified the primary immobilization mechanisms as a synergistic combination of physical encapsulation and chemical stabilization. This study highlights the effective immobilization of the molybdate anion through geopolymer-based S/S techniques, demonstrating their potential for environmental remediation.