Cointercalation of Zero-Valent Iron for Improving Spatiotemporal Selectivity toward Heavy Metals in Wastewater

Zero-valent iron (ZVI) has been extensively utilized for heavy metal sequestration. However, its limited spatiotemporal selectivity—defined as the ability to selectively remove coexisting metals at distinct locations and times—often leads to unbalanced reactivity and rapid passivation. Herein, we develop ZVI cointercalated with sulfur and aluminum (SA-ZVI) to achieve the simultaneous and selective removal of Cr(VI) and complexed Cu(II). The rate constant ratio (R = k_Cu/k_Cr) increases from 0.56 for ZVI to 1.33 for SA-ZVI, demonstrating enhanced Cu(II) selectivity under Cr(VI) stress. Cross-sectional FIB-SEM imaging reveals spatially decoupled reactivity, with Cr predominantly enriched at the surface and Cu deposited in the core. Further correlation analysis shows that Cu immobilization is closely linked to sulfur-enriched FeS_x domains, while Cr removal correlates with Al-modified adsorption sites. Depth-resolved XPS analysis suggests that sulfur forms conductive FeS_x domains, facilitating Cu(II) reduction, while aluminum promotes selective adsorption of Cr(VI) and mitigates Fe–Cr passivation. Finally, the Kirkendall effect and galvanic replacement induce Fe diffusion and inward Cu growth, leading to enhanced Cu enrichment within SA-ZVI. This cointercalation-driven interface engineering effectively balances reactivity with selectivity and provides a mechanistic framework for designing multifunctional iron-based materials with programmable spatiotemporal selectivity for wastewater treatment and resource recovery.