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

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. 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). 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. 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.