Migration and transformation of heavy metals and metalloids in soil and alfalfa (Medicago sativa L.)—mediated by citric acid–lanthanum–modified fly ash—and assessment of ecological safety thresholds

Heavy metal and metalloid contamination in saline–alkali soils pose serious risks to crop safety and ecosystem health. Fly ash is a potential soil amendment, yet its application is limited due to the intrinsic metal content and uncertain long‑term stability. Here, we evaluated citric acid–lanthanum–modified fly ash (CLFA) for immobilizing multiple metals and metalloids in a soil–alfalfa ( Medicago sativa L. ) system and determining ecological safety thresholds. Citric acid increases surface reactivity via complexation, while lanthanum provides strong coordination sites, enabling synergistic adsorption and stabilization. Modification increased the specific surface area of fly ash by 1.87 times and enhanced its surface activity, facilitating selective retention of toxic elements. Pot experiments showed that 5 % CLFA maximized stabilization, increasing residual fractions of contaminants (e.g., cadmium (Cd), Hg, Zn) to > 60 %. For Cd, the potential ecological risk coefficient (Eᵢ) was 329.2 under 10 % CLFA, decreasing to 273.7 under 5 % CLFA, compared with that of 241.7 for CK. Plant accumulation studies revealed that 5 % CLFA inhibited Cd uptake by alfalfa, with root bioconcentration and translocation factors reduced by 9.79 % and 9.82 %, respectively. Overall, these results clarify the mechanisms of surface carboxyl complexation and lanthanide ion exchange in immobilization, offering mechanistic insight and practical guidance for safe fly ash utilization in saline–alkali agroecosystems. • Citric acid-lanthanum modification significantly enhanced heavy metal immobilization. • CLFA at 5 % optimally increased all heavy metal residual fractions to > 60 %. • CLFA effectively reduced ecological risk for the high-risk heavy metals, Cd and Hg. • CLFA significantly inhibited heavy metal uptake and translocation in alfalfa.