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+. [Display omitted] •The biochar-Fe3O4 nanocomposites greatly facilitated Cd2+ transport in soils mainly through biochar-Fe3O4 nanocomposites-Cd2+ complexes•Higher mobility of biochar-Fe3O4 nanocomposites-Cd2+ complexes occurred in the red soil than in the paddy soil•Greater enhancement effect on Cd2+ was observed by biochar-Fe3O4 nanocomposites derived from wheat straw•More complexation with hydroxyls contributed to higher co-transport of Cd2+ by biochar-Fe3O4 nanocomposites