Magnetic silicon-enriched biochar for effectively mitigating As and Sb in soil-rice continuum: from integrated geochemical, microbial, and phytophysiological insights

Abstract Effective co-immobilization of arsenic (As) and antimony (Sb) in contaminated paddy soils remains a persistent challenge for conventional biochar amendments. To address this limitation, a magnetic biochar gel (FeRBG) was synthesized by integrating rice husk biochar, iron oxides, and graphene into a three-dimensional porous network. Its remediation performance and ecological effects were systematically evaluated in Sb-As co-contaminated soil-rice systems. Compared to pristine and Fe-modified biochar, FeRBG decreased (NH 4 )H 2 PO 4 -extractable Sb and As concentrations more significantly, by 23.1% and 22.3%, respectively, primarily by reducing non-specifically adsorbed fractions and promoting transformation into residual phases. Notably, FeRBG was the only amendment that significantly decreased Sb and As accumulation in rice grains by 16.1% and 34.0%, respectively, compared to the control. Furthermore, FeRBG enhanced root system architecture, increasing total root length, surface area, mean diameter, and tip number. Biochar amendment reshaped soil bacterial communities, with core taxa including Pirellulaceae, Nitrosomonadaceae , Sphingomonadaceae , and Comamonadaceae . Redundancy and correlation analyses revealed that soil Sb/As availability and Fe content were key environmental factors regulating bacterial community succession. Structural equation modeling revealed that FeRBG enhanced metalloid immobilization through Fe–O–Sb/As complexation, thus reducing grain accumulation and increasing rice yield. These findings provide a competitive functionalized biochar strategy for the sustainable remediation of Sb/As co-contaminated paddy soils and for improving rice cultivation.