Adsorption of Cd by the Different Soil Solid–Phase Components Kaolinite, Mica, and Goethite

Pollution caused by the heavy metal cadmium (Cd) in smelting site soil has become an environmental problem that cannot be ignored. The process of Cd ion adsorption, migration, and diffusion in soil is influenced by various factors, including soil solid–phase composition and soil environment conditions. In order to investigate the adsorption behavior of Cd in smelting site soil, this study collected uncontaminated soil samples from the vicinity of a smelter, with kaolinite, mica, and goethite identified as abundant components of the soil solid phase. We conducted batch adsorption experiments, zeta-potential analysis, scanning electron microscopy (SEM), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS) to investigate the mechanism of adsorption of Cd ions by kaolinite, mica, and goethite. Our results showed that the pH of the solution significantly influenced the adsorption of Cd ions by these minerals, with increased adsorption observed as the pH increased. Furthermore, the adsorption of Cd ions on kaolinite, mica, and goethite exhibited the best fit with the Freundlich model. SEM and XRD analyses indicated that the crystal structures of the minerals remained unchanged after Cd ion adsorption, suggesting that the Cd ions were primarily bound to the mineral surfaces. XPS measurements of the binding energy changes in the O1s and Cd3d orbitals before and after adsorption were compared with standard binding energy spectra. This comparison confirmed that heavy metal Cd primarily existed on mineral surfaces in the form of outer-sphere and inner-sphere complexes. This study examined the impact of pH on the adsorption of Cd by three types of mineral; fit the data using the isotherm adsorption model equation; and analyzed the mechanisms of binding Cd on different mineral surfaces through characterization techniques. The study's findings should contribute to a better understanding of Cd adsorption mechanisms and provide valuable insights into the behavior of cadmium in soil at smelting sites.