Neodymium and Yttrium Adsorption on Citrate-Modified Cellulose: Experimental and Computational Insights

Neodymium (Nd) and yttrium (Y), two rare earth elements, play a crucial role in a wide range of technologies, and their separation is a challenging process. Adsorption-based approaches offer a sustainable and cost-efficient substitute for the most widely used solvent extraction procedure. Here, we assess the potential of cellulose citrate (CC) as an adsorbent for the removal of Y and Nd through both experimental and computational approaches. CC was successfully synthesized by reacting raw cellulose extracted from Spartium junceum (Spanish broom) with molten citric acid using a green approach that does not require any solvent. The final goal is to shed light on the mechanism of adsorption by citrate-functionalized cellulose by interpreting the adsorption measurements through kinetics and isotherm adsorption models, as well as Density functional theory (DFT) calculations and molecular mechanics (MM) simulations. Adsorption properties of the sorbent are investigated at different contact times, pH values, and metal concentrations. Cellulose citrate has proven to be a highly effective material for the adsorption of the two metals, exhibiting a slight preference for Y at low-to-medium concentrations and for Nd at higher concentrations, suggesting a different binding stoichiometry of the two cations. The adsorption process is found to be pH-dependent, with equilibrium being reached after approximately 60 min. Interestingly, a certain degree of selectivity toward Nd is observed, which becomes more pronounced at pH values below 3 and at higher metal concentrations. DFT and MM modeling confirm the experimental results and allow an adsorption mechanism to interpret the measured performance of this material.