Linking α-Fe₂O₃-modified zeolite, amendment structure, and PTE fractionation during municipal solid waste...

The environmental mobility of potentially toxic elements (PTEs) in municipal solid waste incineration (MSWI) bottom ash is strongly influenced by their partitioning among labile and less labile solid-phase fractions rather than by total concentrations alone. Although Fe-modified zeolites have been investigated for PTE stabilization, how Fe₂O₃ loading influences the evolution of amendment structure and PTE fractionation during curing remains insufficiently resolved. Here, α-Fe₂O₃-modified zeolite composites containing 4, 8, and 12 wt% Fe₂O₃ were synthesized and combined with Ca₃(PO₄)₂ to stabilize PTEs in real MSWI bottom ash. Structural and textural properties were examined using XRD, FTIR, BET, SEM, and elemental mapping, while PTE fractionation was evaluated by Tessier sequential extraction and multivariate analysis. Among the investigated loadings, 8 wt% Fe₂O₃ provided the best overall stabilization performance within the tested range, reducing the combined labile fractions (F1 + F2) of Cd, Cr, As, and Pb from 41, 42, 35, and 22% to 21, 26, 27, and 13%, respectively. Increasing the loading to 12 wt% produced no further stabilization benefit and was associated with lower BET-derived surface area and pore volume. The observed stabilization performance followed the sequence Pb > Cd > As > Cr > Zn > Cu, accompanied by redistribution from exchangeable and carbonate-bound fractions towards Fe-Mn oxide-associated and residual fractions. Batch-leaching tests further showed 47.8-74.3% and 46.2-68.6% reductions in leachable PTEs under deionized-water and simulated acid-rain conditions, respectively. Collectively, the structural, fractionation, and leaching results are consistent with contributions from zeolite-mediated ion exchange, Fe-associated surface interactions, phosphate-associated processes, and solid-phase evolution during four-week curing. The results show that increasing Fe₂O₃ loading alone does not necessarily improve stabilization, emphasizing the need to consider amendment structure, PTE fractionation, and leachability jointly. Further component-resolved experiments, standardized regulatory leaching tests, and long-term ageing studies are required to quantify individual contributions, assess stabilization persistence, and evaluate broader applicability.