Remediation of cadmium-contaminated soil using Fe3O4@SiO2-NH2: removal performance and long-term stability

• Magnetic adsorbent Fe 3 O 4 @SiO 2 -NH 2 (TEOS:Fe 3 O 4 =11:2) is rationally designed with a protective SiO 2 shell and -NH 2 groups, offering a high specific surface area (76.08 m 2 /g), mesoporous structure, and strong magnetic responsiveness for soil Cd remediation. • The adsorbent Fe 3 O 4 @SiO 2 -NH 2 (11:2) achieved a total Cd removal efficiency of 34.18%, primarily targeting the acid-extractable fraction of Cd. • It demonstrates outstanding long-term stability, retaining its structural integrity, functional groups, and magnetic recoverability (>95%) even after 120 days in soil, making it a sustainable and practical solution for Cd-contaminated agricultural land. To address secondary pollution risks and material recovery challenges in Cd-contaminated soil remediation, this study synthesized a series of Fe 3 O 4 @SiO 2 -NH 2 magnetic adsorbents with different mass ratios of tetraethyl orthosilicate (TEOS) to Fe 3 O 4 . Systematic investigations were conducted on their structural characteristics, soil Cd remediation performance, long-term stability, and adsorption mechanism. The adsorbents were characterized using multiple techniques including Fourier Transform Infrared Spectroscopy (FTIR), Vibrating Sample Magnetometry (VSM), Brunauer-Emmett-Teller (BET) analysis, Thermogravimetric Analysis (TGA), Scanning Electron Microscopy (SEM), X-ray Photoelectron Spectroscopy (XPS), and Energy Dispersive Spectroscopy (EDS). Batch experiments were carried out to examine the effects of adsorbent dosage, solid-liquid ratio, and remediation time on cadmium removal rate, Cd speciation transformation, and adsorbent recovery rate. The results showed that Fe 3 O 4 @SiO 2 -NH 2 (11:2) (with a TEOS:Fe 3 O 4 mass ratio of 11:2) exhibited the optimal remediation performance due to its more abundant porous structure and sufficient -NH 2 active sites. Under the optimal conditions (2.0% adsorbent dosage, 1:1 water-soil ratio, 30 days), the cadmium removal rate in soil reached 34.18%, and the proportion of acid-extractable Cd was significantly reduced. Furthermore, after 120 days in the soil, the functional groups and crystal structure of the adsorbent Fe 3 O 4 @SiO 2 -NH 2 (11:2) remained stable, and the recovery rate was still exceeded 95%. Mechanism analysis confirmed that the adsorption of Cd 2+ by the adsorbent is mainly achieved through the coordination interaction of surface -NH 2 groups. The magnetic adsorbents prepared in this study provide a new technical solution and theoretical support for the efficient and green remediation of Cd-contaminated soil.