Adsorption performance of functionalized chitosan–silica hybrid materials toward rare earths
Chitosan–silica hybrid adsorbents were prepared and functionalized with ethylenediaminetetraacetic acid (EDTA) and diethylenetriaminepentaacetic acid (DTPA). The method consisted of sol–gel hybridization of chitosan and silica, followed by the addition of anhydrides to graft EDTA- and DTPA-ligands on the amine groups of the chitosan moieties in the hybrid particles. The resulting adsorbents were characterized by a range of analytical techniques: FTIR, BET, SEM, TGA, ICP and CHN. Coordination of Eu(III) to immobilized EDTA- and DTPA-groups was investigated by luminescence spectroscopy. The adsorption performance of the chitosan–silica adsorbents was investigated for Nd(III) as a function of the contact time, the pH of the aqueous feed and the adsorbent mass. Stripping and reusability studies were performed for both EDTA-chitosan–silica and DTPA-chitosan–silica. Differences in affinity amongst the rare-earth ions were investigated for DTPA-chitosan–silica in mono-component solutions of five rare earths (La, Nd, Eu, Dy and Lu). The order of affinity was in agreement with the trend in stability constants for the respective rare-earth ions with non-immobilized DTPA (bearing five available carboxylic acid groups). Multi-element mixtures were used to determine the selectivity of the adsorption process. Special attention was paid to separation of Nd and Dy, since these elements are relevant to the recovery of rare earths from End-of-Life permanent magnets.