Opportunity for a greener recovery of dysprosium(III) from secondary sources by a novel Mannich reaction-modified phosphorylated chitosan hydrogel

The depleting supply of natural sources of rare earth elements (REE) is a concern to many nations as demand for advanced technology is becoming vital for national security. In this communication, the recovery of dysprosium(III) from aqueous systems was exemplified by a modified phosphorylated chitosan (PCs/MB) prepared by the C-Mannich reaction of phosphorylated chitosan, glutaraldehyde, and 4-hydroxycoumarin in ethanolic solution. Batch adsorption studies achieved a maximum adsorption capacity (q_max) of 34 mg/g at 25 °C and pH = 5.4 for 2 h. Fourier Transform-Infrared Spectroscopy, elemental mapping, and quantitative analyses revealed ion-exchange mechanism with C6-phosphate and a synergistic complexation with the amino group between two hexose units of the chitosan chain confirming the correlation provided by the pseudo-second order kinetics (R² = 0.9996), extrapolated mean free energy of adsorption (E_ads) of 12.9 kJ/mol from the corrected Dubinin-Radushkevich isotherm, and the extrapolated enthalpy of adsorption (ΔH⁰_ads) of -42.4 kJ/mol from the linearized Van't Hoff plot. Competitive adsorption with iron(II), cerium(III), and neodymium(III) demonstrated preferential removal of dysprosium(III) and complete exclusion of iron(II), which illustrates potential application in the separation of REE from electronic wastes.