MIL-100(Fe)@biochar as a dual‑function platform for controlled auxin delivery and heavy metal stabilization in...

Modern agriculture requires multifunctional materials capable of simultaneously improving agrochemical efficiency and mitigating soil contamination. However, most currently developed nanomaterials address only a single function, such as pollutant adsorption or agrochemical delivery, while integrated systems capable of regulating multiple processes within the soil-plant system remain limited. Here, a MIL-100(Fe)@biochar nanocomposite was engineered for the controlled delivery of the synthetic auxin 2,4-D and concurrent mitigation of heavy-metal toxicity in soil. The composite was further functionalized with humic acid (HA) and the cationic surfactant CTAB. The multifunctional behavior of the material arises from several complementary mechanisms, including adsorption of 2,4-D within the porous MIL-100 framework through electrostatic attraction and hydrogen bonding, inner-sphere complexation of heavy metals with Fe-O clusters of the MOF, and additional metal binding and rhizosphere modulation provided by humic acid. CTAB formed a positively charged, hydrophobic surface layer that promoted hydrophobic and electrostatic interactions with 2,4-D, resulting in a high sorption capacity of up to 50 mg/g. Pot experiments with barley grown in heavy-metal-contaminated soils showed that developed composites shifted Cu, Pb, Zn, and Cd from bioavailable fractions to more stable pools. This effect was strongest for the HA-coated composite: at a 1% application rate, bioavailable metal fractions decreased from 8-17% to 3-9%, while metal accumulation in roots and shoots declined by 53-71% and 46-58%, respectively. These effects reflect combined metal complexation within the MIL-100(Fe)-biochar matrix and additional binding sites provided by HA. Composite application significantly improved plant performance under contamination stress: root length increased by 1.3-1.4-fold, shoot height by up to 1.7-fold, and biomass by 1.2-1.8-fold, restoring growth to levels comparable to uncontaminated soil. The 1% HA-coated composite loaded with 2,4-D also increased chlorophyll a and b content (1.5-fold) while reducing carotenoids, indicating alleviation of oxidative stress. Developed materials enhanced Cl and Fe translocation to shoots, whereas direct 2,4-D application caused phytotoxicity. Overall, under controlled short‑term conditions, MIL-100(Fe)@biochar nanocomposites demonstrate potential for safer agrochemical management in contaminated soils. Long‑term stability and environmental fate remain to be evaluated before field application.