Joris Roosen

Adsorption and chromatographic separation of rare earths with EDTA- and DTPA-functionalized chitosan biopolymers

Chitosan, which is derived from chitin by deacetylation, is one of the most promising biopolymers for adsorption of metal ions from diluted waste streams. By functionalization of chitosan with ethylenediaminetetraacetic acid (EDTA) or diethylenetriaminepentaacetic acid (DTPA) groups, it is possible to obtain a material that is much less soluble in acidic aqueous solutions than native chitosan. The coordinating EDTA and DTPA ligands are very efficient for binding of rare-earth (lanthanide) ions. The functionalization was achieved by reaction of chitosan with EDTA bisanhydride or DTPA bisanhydride. The binding of lanthanide ions to functionalized chitosan was investigated by FTIR (binding of Nd3+) and luminescence spectroscopy (binding of Eu3+). Comparison of the luminescence decay times of the europium(III) coordinated chitosan complexes swollen in water and in heavy water allowed determination of the hydration number of the coordinated Eu3+ ion. Batch adsorption tests for the uptake of neodymium(III) from aqueous nitrate solutions were performed for EDTA-chitosan and DTPA-chitosan. Different experimental parameters such as the adsorption kinetics, loading capacity and pH of the aqueous feed were investigated. The modified chitosan materials are much more effective for adsorption of rare earths than unmodified chitosan. It was shown that adjustment of the pH of the aqueous feed solution allows achieving selectivity for adsorption of rare-earth ions for mixtures containing two different ions. After stripping of the metal content, the modified chitosans could be reused for new adsorption experiments. Medium pressure liquid chromatography (MPLC) with DTPA-chitosan/silica as the stationary phase and a dilute nitric acid solution as eluent was used for the separation of the following mixtures of rare-earth ions: Nd3+/Ho3+, Pr3+/Nd3+ and Pr3+/Nd3+/Ho3+. The experiments show that separation of the rare-earth ions is feasible with DTPA-chitosan/silica, without the need for using solutions of chelating agents as eluents.

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.

Recovery of scandium from leachates of Greek bauxite residue by adsorption on functionalized chitosan–silica hybrid materials

Bauxite residue (red mud) is a waste residue that results from the production of alumina by the Bayer process. Since it has no large-scale industrial application, it is stockpiled in large reservoirs. Nevertheless, it should be considered as a valuable secondary resource as it contains relatively large concentrations of critical metals like the rare earths, scandium being the most important one. In this work, we investigated the recovery of scandium from real leachates of Greek bauxite residue. In the separation of scandium from the other elements, the biggest challenge arose from the chemical similarities between scandium(III) and iron(III). This hampers high selectivity for scandium, especially because iron, as one of the major elements in bauxite residue, is present in much higher concentrations than scandium. In order to achieve selectivity for scandium, chitosan–silica particles were functionalized with the chelating ligands diethylenetriamine pentaacetic acid (DTPA) and ethyleneglycol tetraacetic acid (EGTA). Both organic ligands were chosen because of the high stability constants between scandium(III) and the corresponding functional groups. The adsorption kinetics and the influence of pH on hydrolysis and adsorption were investigated batchwise from single-element solutions of scandium(III) and iron(III). In binary solutions of scandium(III) and iron(III), it was observed that only EGTA-functionalized chitosan–silica appeared to be highly selective for scandium(III) over iron(III). EGTA–chitosan–silica shows a much higher selectivity over state-of-the-art adsorbents for the separation of scandium(III) from iron(III). The latter material was therefore used as a resin material for column chromatography in order to effectively separate scandium from bauxite residue. Full separation was achieved by eluting the column with HNO3 solution at pH 0.50; at this pH all other elements had already eluted.