Uptake the rare earth elements Nd, Ce, and La by a commercial diatomite: kinetics, equilibrium, thermodynamic and adsorption mechanism

This study addresses the application and evaluation of diatomite as a low-cost, natural, abundant, and highly efficient adsorbent of Nd³⁺, Ce³⁺, and La³⁺ rare earth elements (REEs). The results revealed that the diatomite was porous and cylindrical, with a specific surface area of 150 m²/g. The diatomite displayed a point of zero charge equal to 1.8, maintaining its surface negatively charged at pH values higher than 1.8, which was beneficial for cationic REEs uptake. At pH 1, all three REEs had their lowest uptake values; at pHs 2–6, the REEs removal efficiency practically did not change. The uptaken percentage increased in the order of La³⁺ (50%), Ce³⁺ (77%), and Nd³⁺ (92%), influenced by the electronegativity of the ions. The Avrami-fractional order (AFO) and Liu isotherm model better fit the kinetic and equilibrium process (q_max = La³⁺ (22.8 mg g⁻¹), Ce³⁺ (56.2 mg g⁻¹), and Nd³⁺ (100.9 mg g⁻¹)). Since the diatomite is highly porous, pore filling/pore-surface diffusion was the main mechanism for REEs adsorption, as well as ion exchange and chelation. The reusability tests revealed that the diatomite could be reused 8 times without losing performance. This research shows that diatomite is an efficient strategy for designing effective, low-cost, sustainable adsorbents to recover REEs from synthetic and real effluents.