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

Herein we explored the co-transport behaviors of cadmium (Cd) with biochar-FeO 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 Cd (Cd mass recovery = 2.71-10.5%) by 2.5-times in soils, compared to the biochar-alone (Cd 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 Cd by the BFNCs (vs. biochar-alone). The BFNCs greatly increased (27.1-95.5 times) Cd transport in soils mainly through BFNC-Cd complexes, compared to the negligible transport of Cd in soils without presence of BFNCs. Higher mobility of BFNCs and BFNC-Cd 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 Cd was observed by BFNCs derived from wheat straw than wood chip, due to the stronger sorption ability of wheat straw biochar towards Cd, likely stemming from more mineral composition such as CaCO. 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 Cd.