Cadmium sorption on γ-Al₂O₃ and goethite in the presence of silicate: insights from XRD and FTIR studies

Purpose: The application of silicon could effectively alleviate Cd contamination in the soil. However, the interaction between silicon and Cd in soils is not well understood. Our research aims to investigate the mechanism of the impact of silicate on the sorption of Cd on metal oxides in soils. Materials and methods: Sodium silicate and Cd(II) co-sorption on γ-Al₂O₃ and goethite at pH 5.5 and 7.5 was studied using a combination of batch sorption experiments, X-ray diffraction (XRD), and Fourier transform infrared spectroscopy (FTIR). Results and discussion: Silicate enhanced the sorption of Cd(II) onto γ-Al₂O₃ at pH 5.5 and 7.5. However, the sorption of Cd(II) on goethite at pH 5.5 and 7.5 was only promoted when the silicate concentration reached 1 mmol L⁻¹. Cadmium(II) enhanced silicate sorption on γ-Al₂O₃ at pH 5.5 and on goethite at pH 7.5, while it decreased silicate sorption on γ-Al₂O₃ at pH 7.5 and had no significant effect on goethite at pH 5.5. At pH 5.5, Cd(II) reacted with silicate to generate Cd₂SiO₄, and Cd-Si-Al ternary complexes were formed on γ-Al₂O₃. At pH 7.5, silicate promoted the formation of Cd(OH)₂ on γ-Al₂O₃, which ultimately transformed into CdCO₃. When silicate concentration reached 1 mmol L⁻¹, more silicate complexes were formed through the Fe-O-Si bound on goethite at both pH 5.5 and 7.5 to promote the adsorption of Cd(II) owing to its electronegative functional group on the surface of the complex. Conclusions: The type of clay minerals, pH, and the presence of silicate all significantly affected Cd sorption. Furthermore, the sorption mechanism of Cd on γ-Al₂O₃ and goethite in the presence of silicate has been elucidated. The above findings improve our understanding of the mechanism for immobilizing Cd in soils using silicon-containing materials.