Development of phosphoryl-functionalized algal-PEI beads for the sorption of Nd(III) and Mo(VI) from aqueous solutions ? Application for rare earth recovery from acid leachates

Alginate-PEI beads are functionalized by phosphorylation and applied for the sorption of Nd(III) and Mo(VI). The successful grafting of phosphoryl groups (as tributyl phosphate derivative) is characterized by FTIR and XPS analysis, elemental analysis, titration (pHPZC), TGA, BET and SEM-EDX analyses. The multi-functional characteristics of the sorbent (i.e., carboxylic, hydroxyl, amine and phosphate groups) contribute in the binding of metal ions having different physicochemical behaviors. The sorption of Nd(III) is strongly increased by phosphorylation, while for Mo(VI) the enhancement is rather limited. Optimum sorption occurs at pH 3?4: maximum sorption capacity reaches up to 1.46 mmol Nd(III) g-1 and 2.09 mmol Mo(VI) g-1; sorption isotherms are fitted by the Langmuir equation. The equilibrium is reached within 30?40 min and the kinetic profiles are simulated by the pseudo-first order rate equation. The coefficients of the effective diffusivity are close to the self-diffusivity of Nd(III) and Mo(VI) in water; as a confirmation of the limited impact of resistance to intraparticle diffusion in the kinetic control. The sorbent is selective for Nd(III) over Mo(VI) and other alkali-earth or base metals (at pH close to 2.5?3). Metals can be readily desorbed using 0.2 M HCl/0.5 M CaCl2 as the eluent. The loss in sorption does not exceed 5% at the fifth cycle, while desorption remains complete. A series of treatments (including acidic leachate, cementation, precipitation, sorption and elution) is successfully applied for the recovery of rare earths from Egyptian ore; with enrichment in the oxalate precipitate of Nd(III), Gd(III), Sm(III) and Eu(III).