Chitosan-polyethyleneimine synergistic coating enables pH-responsive adaptive immobilization of sporadically released Cd/Zn in intensive farmland.

Excessive fertilization and tillage are intensive management practices that induce cropland acidification, which in turn triggers sporadic mobilization of heavy metals. The unpredictable nature of soil acidification poses an intermittent risk of metal remobilization despite the capacity of conventional alkaline amendments (e.g., lime) to suppress metal toxicity. Here, we synthesized a pH-responsive composite (CPFM) comprising amorphous hydrated metal oxides (AHMOs) coated with a chitosan-polyethyleneimine (CS-PEI) sheath. In response to pH fluctuations, CPFM selectively encapsulated or released AHMOs by dynamically modulating its pore architecture (∼10%) during acid-base cycling. CPFM effectively sequestered and stabilized Cd(II) and Zn(II) in soil by leveraging the surface functionality of the polymer coating and the nucleation behavior of released AHMOs. The detoxification mechanism of PEI mediated by the coprecipitated mineral phases was further elucidated by density functional theory (DFT) calculations. In simulated cycling experiments, CPFM achieved remarkable passivation efficiencies of 98.6% for Cd(II) and 97.8% for Zn(II), facilitating long-term immobilization through the formation of acid-resistant Fe-Mn nodules. Crucially, the efficacy of this adaptive strategy was validated across multiple scales, from pot experiments to field trials. This environmentally triggered approach provides a new paradigm for designing advanced functional materials to manage pollution in complex, dynamic ecosystems.