Iron-modified rice husk hydrochar and its immobilization effect for Pb and Sb in contaminated soil

Cationic and anionic heavy metal contamination sometimes co-exists in soil systems, such as mining areas and shooting ranges, seriously threatens human health and ecological stability. In this study, iron-modified rice husk hydrochar showed commendable ability to immobilize both heavy metal cation (Pb) and anion (Sb) simultaneously in soils. Iron-modified rice husk hydrochar (HC12.5-180) (5%) amendment reduced the bioavailability (EX- and CB-fraction) of Pb and Sb by 25 and 40%, respectively, which were 8 and 5 times higher than that of pristine rice husk hydrochar (HC0-180) (5%) amendment. The cation (Pb) immobilization mainly depends on cation exchange with mineral components (K⁺, Ca²⁺, Na⁺, Mg²⁺), precipitation with nonmetallic anions (Cl- and SO₄^2-), and complexation. Meanwhile, the iron oxides (FeO, Fe₂O₃, Fe₃O₄), formed during hydrothermal process, can be easily combined with anion (Sb) to form geochemically stable minerals. In conclusion, this work offered a practical and cost-effective technology based on the iron modification rice husk hydrochar for the immobilization of both anionic and cationic heavy metal contaminants in soils.