Sustainable Valorization of Blast Furnace Slag into NaA Zeolite via Selective Acetic Acid Leaching for Efficient Heavy Metal Adsorption

Sustainable management of industrial solid waste is critical for a circular economy. This study presents a novel approach for valorizing blast furnace slag (BFS) into NaA zeolite through selective acetic acid leaching followed by hydrothermal crystallization. The leaching step selectively extracts Ca2+ and Mg2+ while efficiently retaining silicon and aluminum in the solid residue, producing a reactive aluminosilicate precursor that facilitates zeolite nucleation and growth. The effects of the silicon-to-aluminum molar ratio (n(Si)/n(Al)), crystallization temperature, and duration on the phase evolution and morphology were systematically investigated. The results demonstrate that phase-pure NaA zeolite with high crystallinity and a uniform cubic morphology can be obtained from precursor gels with n(Si)/n(Al) ratios of 0.5–1.25. Optimal synthesis conditions were identified as n(Na):n(Si):n(Al):n(H2O) = 6:1:1:240 at 373 K for 8 h. The resulting zeolites exhibit a BET specific surface area of 52.1 m2/g, a micropore volume of 0.016 cm3/g, an average adsorption pore size of 4.7 nm, and an external specific surface area of 12.8 m2/g. It achieved near-complete removal of Cu2+ and high adsorption efficiencies for Pb2+ (77.78%) and Ni2+ (71.79%) from 250 mg/L solutions at 298 K with a dosage of 4.0 g/L, following the affinity sequence Cu2+ > Pb2+ > Ni2+, with all pairwise differences statistically significant at p < 0.001, using one-way ANOVA and Tukey’s HSD tests. The adsorption of three metal ions was most accurately described by the Freundlich isotherm and pseudo-second-order kinetic models, indicating heterogeneous multilayer chemisorption. The theoretical maximum monolayer adsorption capacities (qmax) were 307.67 mg/g for Cu2+, 246.09 mg/g for Pb2+, and 173.79 mg/g for Ni2+, whereas the kinetic equilibrium adsorption capacities (qe) reached 62.69, 48.85 and 41.69 mg/g, respectively. This study demonstrates a value-added strategy for valorizing BFS into a micro-mesoporous adsorbent, advancing both circular resource utilization and environmental remediation.