Combining SiO2 NPs with biochar: a novel composite for enhanced cadmium removal from wastewater and alleviation of soil cadmium stress.

Cadmium (Cd) pollution in water and soil seriously threatens human health. Biochar and nanomaterials have high potential for solving the cadmium pollution problem due to their abundant pores and high specific surface area. Here, the preparation of the composite material SiO₂NPs@BC (SBC) using SiO₂ NPs (SN) and silkworm excrement biochar (BC) is described, along with its application in the remediation of cadmium-contaminated water and soil. Characterization experiments (SEM&EDS, BET, FTIR, XRD, and XPS) demonstrated that SiO₂NPs@BC has a high specific surface area (46.5767m²/g), a well-developed pore structure (0.608375cm³/g), and abundant surface functional groups (Si-C, Si-O, Si-O-Si), providing active sites for the adsorption of Cd. Batch adsorption experiments in water showed that the adsorption capacity of SBC is higher than that of biochar (BC) and SN, with a maximum Langmuir adsorption capacity of 141.99 mg/g. After five adsorption cycles, the removal rate of SBC was 73.04%, significantly higher than the 64.97% obtained for BC. The application of SBC not only improved the soil physicochemical properties by increasing the soil pH, the cation exchange capacity, and the soil organic matter content but also by reducing the amount of DTPA-Cd (24.6%) and the plant bioconcentration factor (28.28%) in the soil, converting Cd into more stable fractions (Red-Cd, Ox-Cd). Based on the results, SBC can effectively reduce Cd pollution.