Iron-enhanced X-type zeolite made by coal gangue for Pb/Cd-contaminated soil remediation

Purpose Soil contamination and solid waste accumulation pose significant risks to the safety of agricultural produce and residential areas. In China, coal gangue, a typical form of solid waste, is rich in valuable elements like silicon and aluminum, making it an ideal precursor for the synthesis of zeolite molecular sieves. This study focuses on transforming coal gangue into iron-enhanced X-type zeolite (Fe-NaX) molecular sieve and explores its potential in remediating lead (Pb) and cadmium (Cd) co-contaminated soil. Methods Fe-NaX was synthesized from coal gangue in Sichuan using an alkaline melting hydrothermal method. In order to evaluate the effectiveness of Fe-NaX in remediation and assess its ecological risk, soil culture and acid rain leaching experiments were conducted. The microstructures of Fe-NaX were detected to explore its remediation mechanism. Results The optimal Fe-NaX was synthesized at a 110 °C hydrothermal reaction temperature, a 1.5 mass ratio of NaOH and pretreated coal gangue (PCG), a 2.0 Si/Al molar ratio, and a 3.5 M alkalinity. Fe-NaX demonstrated outstanding performance in remediating Pb and Cd contaminated soil, with immobilizing efficiency for high bioavailability fractions of Pb and Cd at 44.4% and 21.9%, respectively. Even under acid rain stress, Fe-NaX was able to decrease the release of Pb and Cd in the soil by 42.3% and 59.6% respectively, with minimal ecological risk. Conclusion This study, based on a “solid waste for soil-remediation” strategy, transforms coal gangue into Fe-NaX for use in soil remediation and holds great promise for the contaminated soil remediation and coal gangue comprehensive utilization. Graphical abstract