Kinetic and equilibrium study for Pd(II) removal from aqueous solution by sorption onto calcium alginate gel beads

The more and more increasing use of catalytic converters to reduce the dangerous NOx and CO emission in the atmosphere by vehicle traffic produces a corresponding increase of palladium in the environment [1]. Among the so-called “platinum group Elements” (PGE), elemental palladium seems to be the most hazardous one because it can be easily and quickly oxidized to palladium(II) when put in contact with soil. The presence of palladium oxidized form is of great concern owing to its recognized toxicity towards plants, animals and humans. Therefore, as for the classical “heavy metals”, a removal of this element from the environment is needed and a subsequent recovery for its potential re-use is welcome [1]. Among the different techniques proposed for the removal and recovery of metal ions from aqueous solutions, the sorption by materials derived from biomass is one of the most promising [2]. The biomaterials for metal ions removal are available in large quantities, inexpensive and biodegradable and usually have high yields in terms of metal ion uptake [3]. The biosorption of palladium(II) and other precious metals on different kind of raw biomaterials has been extensively studied [4], but the direct use of the polyelectrolytes contained therein, which are the main responsible for their sorption capacity [5-6], was few investigated. We report here results of a kinetic and thermodynamic study for palladium(II) removal from aqueous solutions by calcium alginate gel beads as sorbent biomaterial. The investigations were carried out in the pH range 2 to 5 at I = 0.01 mol L-1 (NaNO3 / NaCl) at different chloride concentration and at T = 25 °C. These experimental conditions were established on the basis of a speciation study on the Pd2+ - alginate system in aqueous solution, carried out to evaluate the strength of interaction between the metal ion and the alginate as sequestering agent. A parallel study on the kinetics of calcium(II) release by gel beads was also carried out in order to have more information on the sorption mechanism. Important similarities were found between the trends for the dependence on pH and chloride concentration of binding and sequestering ability (% PdAA species, pL0.5) and sorption (qe, qmax) parameters. In Figure is reported, as an example, the dependence of pL0.5 and qmax on CCl- at pH = 2 and at T = 25°C. These similar trends show a close relation between the behaviour of alginate as sequestering agent in aqueous solution and calcium alginate gel beads as sorption material towards Pd2+ ion.