Effect of pH to the retention activities of lanthanum, neodymium and gadolinium oxides on anion and cation exchange resin

Rare earth elements possess very similar chemical properties, which makes their selective retention and separation a challenging task. Optimization of mobile phase parameters in resins for REE retention, especially in terms of pH, remains underexplored, which limits the large-scale application of IEC for REE recovery. This study investigates the influence of pH on the retention activities of rare earth elements (REEs), specifically lanthanum (La), neodymium (Nd), and gadolinium (Gd) oxides, using batch ion exchange experiment on two groups of ion exchange resins, i.e. anion resins Extrepure PM 405 and PM 406, and cation resins Bio Rad AG 50W-X4 and Lewatit Monoplus S108 H. Retention efficiencies were evaluated at three different pH levels: 2, 7 and 12. At acidic pH 2, anion resins showed no retention of REEs, while cation resins had high retention efficiencies with 97.78% and 91.64% for Bio Rad AG 50W-X4 and Lewatit Monoplus S108 H, respectively. At neutral pH 7, anion resins exhibited optimal retention rates above 99%. However, at basic pH 12, retention efficiency decreased across all resins, with Lewatit Monoplus S108 H dropping notably to 77.63%. This reduction is attributed to the formation of hydroxide complexes of REEs, which hinder their interaction with the cation exchange resins. These findings highlight the critical role of pH in optimizing the extraction processes of REEs, providing insights for enhancing the efficiency of resource recovery in industrial applications.