Adsorption mechanism of neodymium onto a South American ion‐adsorption clay and its associated minerals (goethite and silicon dioxide)

Abstract Ion‐adsorption clays, where rare earth elements (REEs) are adsorbed on surfaces due to weathering, elution, and adsorption processes, have been the primary sources of REEs, especially in China. Recently, South American ion‐adsorption clays, containing significant amounts of associated minerals such as goethite, silicon dioxide, and monazite, have gained attention for their complex REE occurrence and surface adsorption mechanisms. This study examines the performance and mechanisms of Nd 3+ adsorption by South American ion‐adsorption clay. Adsorption kinetics and isotherms are investigated for the clay and its associated minerals (goethite and silicon dioxide) to clarify the REE adsorption mechanism. Additionally, attenuated total reflectance–Fourier transform infrared spectroscopy (ATR‐FTIR) is used to analyze the samples before and after adsorption, and zeta potential measurements are conducted to determine the point of zero charge (pH pzc ). Results indicate that ion‐adsorption clay carries a negative charge and surface hydroxyl groups, leading to both physisorption and chemisorption of Nd 3+ , with an activation energy of 6.0 kJ/mol. The negative surface charge is attributed to kaolinite, while hydroxyl groups are provided by both clay and associated minerals. Nd 3+ adsorption on goethite and silicon dioxide is homogeneous monolayer chemisorption, driven by surface hydroxyl groups. Silicon dioxide exhibits a lower activation energy (23.6 kJ/mol) compared with goethite (39.6 kJ/mol), likely due to its smaller pH pzc and larger negative zeta potential at pH 6. However, goethite showed a higher adsorption capacity due to its more abundant surface hydroxyl groups.