Two chitosan hydrogels (prepared by NaOH neutralization and by polyphosphate ionotropic gelation) have been tested in the dry state for Pd(II) and Pt(IV) sorption at pH 2. Similar sorption isotherms with maximum sorption capacities close to 190 mg Pd g −1 and 235 mg Pt g −1 were achieved. The sorption mechanism involves electrostatic attraction of the chloro-anionic species onto protonated amine groups; the drastic decrease of sorption capacity with the addition of chloride ions supports this hypothesis. SEM-EDAX analysis suggests that sorption proceeds, in kinetic terms, through a shrinking core mechanism. Metal ions can diffuse throughout all the sorbent volume. The main differences between the sorbents are revealed by kinetics. The hydrogels prepared by ionotropic gelation in polyphosphate (C-PPh) allows reaching equilibrium much faster than the hydrogels prepared by the neutralization process (C-NaOH). While for C-PPh sorbent the chemical reaction rate seems to control sorption profiles, in the case of C-NaOH a combination of mechanisms including intraparticle diffusion resistance controls uptake kinetics. Metal desorption from loaded sorbents is possible using thiourea alone or in association with HCl solutions. The recycling of the sorbents is possible but for a limited number of cycles.
Mercury sorption on chitosan was investigated in batch and continuous systems. Chitosan sorption properties were determined through sorption isotherms. Langmuir and Freundlich equations were used for the modeling of isotherms at pH 5. In batch systems, maximum sorption capacities reached 550 mg Hg/g. Sorption kinetics have been studied as a function of sorbent particle size and stirring rate. Dynamic removal of mercury was tested in a fixed bed reactor investigating the following parameters: particle size, column size, flow velocity and metal ion concentration. Clark and Adams-Bohart models were evaluated for the simulation of breakthrough curves. This study shows that chitosan is an effective sorbent for the treatment and recovery of mercury from dilute effluents at near neutral pH.
Chitosan (an aminopolysaccharide), cross-linked with glutaraldehyde, is very efficient for Pd(II) and Pt(IV) recovery from acidic solutions (pH 2). Due to poorly porous properties, it is necessary to condition the biopolymer as hydrogels. The uncontrolled drying of hydrogels depreciates the diffusion properties of original materials. A different method (to the acidic dissolving/alkaline neutralization procedure) has been used: after chitosan dissolving in acetic acid solution, the viscous solution was dropped into an ionotropic gelling solution (polyphosphate and glutaraldehyde to form stable gel beads with improved porosity. The restriction of diffusion properties due to drying step was less drastic with the modified procedure: a few hours were sufficient for reaching equilibrium while more than two days were required for standard material
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