Cellulose and chitosan derivatives for enhanced sorption of erbium(III)

What is it about?

The chemical modifications of cellulose and chitosan (cross-linked with glutaraldehyde) allow increasing Er(III) sorption properties. Thiol groups increase metal sorption on cellulose by 50%, though the best results were obtained with the poly(aminocarboxymethylation) of chitosan: sorption capacities increase from 40 to 50 mg Er(III) g−1 for GLA-Chit to 117–144 mg Er(III) g−1 for PCM-Chit. Sorption capacity increases with pH due to the progressive deprotonation of reactive groups (amine functions, thiols groups and carboxylic groups) (depending on the sorbent). The saturation plateau of sorption isotherms is consistent with the Langmuir model, though the Freundlich equation showed very comparable determinations coefficients. The sorption capacities also increase with temperature: sorption is endothermic spontaneous (even increased by the poly (aminocarboxymethylation) of the biopolymer). The main difference between the different sorbents is identified in terms of entropy change: negative for Cell, Thio-Cell and GLA-Chit while positive values are obtained for poly(aminocarboxymethylated) chitosan materials, magnetic and non-magnetic composites showed similar behavior. The kinetic profiles are best fitted by the pseudo-first-order rate equation although the pseudo-second-order rate equation also fitted well experimental data. The incorporation of magnetic core in the composite sorbent does not improve (a) the uptake performance: actually the maximum sorption capacity decreased at the pro-rata of the proportion of inert magnetic core in the material, or (b) the uptake rate: the equilibrium is reached within the first 4–5 h of contact. All the sorbents can be efficiently desorbed using acidic thiourea solutions (0.5 M) and the sorption and desorption performances decrease at the fifth cycle by less than 9% and 6% for sorption and desorption, respectively. Poly(aminocarboxymethylated) chitosan appears the most promising sorbent for the recovery of Er(III) from weakly acid solutions (pH 5).

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Why is it important?

Cellulose (Cell) was chemically modified by grafting thiourea (Thio-Cell) and glutaraldehyde cross-linked chitosan (GLA-Chit) was functionalized by poly(aminocarboxymethylation) (PCM-Chit); his material was also prepared as a composite material that incorporates a magnetic core (Magn PCM-Chit). The sorption properties of these materials have been compared to non-modified cellulose and GLA-Chit for Er(III) uptake. Sorption increases with progressive deprotonation of reactive groups, such as R OH, R SH, amine, and carboxylic acid groups. The chemical modification significantly increases sorption performance and more specifically the poly(aminocarboxmethylation): sorption capacities increase up to 117–145 mg Er(III) g−1. Sorption capacities also increase with temperature: the sorption is endothermic and spontaneous. The spontaneity of the reaction significantly increases with chemical modification of chitosan-based sorbent. The entropy of the system is negative for GLA-Chit, Cell, and Thio-Cell and positive for PCM-Chit materials. Acidic solutions of thiourea efficiently desorb Er(III) and allow the recycling of the sorbents for a minimum of 5 sorption/desorption cycles. FT-IR spectrometry, XRD, TGA, elemental analyses and SEM observations have been used for characterizing the materials. (The magnetic properties of Magn PCM-Chit were also characterized by VSM.

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