Facilitated transport of cadmium by biochar-Fe₃O₄ nanocomposites in water-saturated natural soils

Herein we explored the co-transport behaviors of cadmium (Cd²⁺)with biochar-Fe₃O₄ nanocomposites (BFNCs)(and biochar-alone for comparison)in water-saturated natural soil (paddy soil and red soil)packed columns. The BFNCs promoted the transport of Cd²⁺ (Cd²⁺ mass recovery = 2.71–10.5%)by 2.5-times in soils, compared to the biochar-alone (Cd²⁺ mass recovery = 1.28–4.07%). Greater interplays via electrostatic attraction, complexation with hydroxyls, and π-π interaction with the aromatic complexes altogether contributed to the higher adsorption capacity and transport potential towards Cd²⁺ by the BFNCs (vs. biochar-alone). The BFNCs greatly increased (27.1–95.5 times)Cd²⁺ transport in soils mainly through BFNC-Cd²⁺ complexes, compared to the negligible transport of Cd²⁺ in soils without presence of BFNCs. Higher mobility of BFNCs and BFNC-Cd²⁺ complex occurred in the red soil than in the paddy soil due to the lower contents of Fe/Al oxides in the red soil. Greater enhancement effect (~2.5 times)on Cd²⁺ was observed by BFNCs derived from wheat straw than wood chip, due to the stronger sorption ability of wheat straw biochar towards Cd²⁺, likely stemming from more mineral composition such as CaCO₃. Our findings suggest that the potential co-transport risks should not be simply ignored particularly when the next-generation of multifunctional biochar‑iron oxide nanocomposites are employed for in-situ remediation of soils contaminated with organic/inorganic contaminants like Cd²⁺.