Efficient and Selective Adsorption of Neodymium on Expanded Vermiculite

Abstract

In this study, expanded vermiculite (EV) was applied to the adsorption of neodymium (Nd) from an aqueous solution with focus on the description of the kinetics and the mechanism of adsorption, determination of the best eluent, and selectivity. The EV was characterized before and after the adsorption process by the morphology, composition, textural properties (specific surface area, density, and porosity), and functional groups. The adsorption experiments showed that under optimal conditions, using 0.7 g of adsorbent and at pH 3.3, the efficiency achieved an enhanced adsorption percentage of 98.7%, which represents an adsorption capacity of 0.205 mmol/g. The external mass and diffusional resistance controlled the mass transfer, and ion exchange occurred mainly with magnesium cations. The proposed mechanism of adsorption was cation exchange in the interlayer region of the vermiculite and the formation of outer-sphere complexes. Calcium chloride was the most suitable eluent, regarding efficiency and low toxicity. The adsorbent preferably adsorbed Nd from a multimetal solution, achieving enhanced values of selectivity. Regarding characterization, the EV presented a well-organized structure, a lamellar morphology, and typical functional groups such as Al–O and Si–O. Thus, the EV adsorbed Nd quickly and efficiently, and the ion-exchange mechanism of adsorption allows the selective adsorption of Nd and safe desorption by the exchange with Ca2+.