Sorption of Pd(II) ion by calcium alginate gel beads at different chloride concentrations and pH. A kinetic and equilibrium study

A kinetic and thermodynamic study on the sorption ability of calcium alginate gel beads towards Pd(II) ion was made in aqueous solution at different pH and chloride concentrations. The experimental conditions adopted in the sorption experiments were established on the basis of a speciation study on the Pd2+-AA system in which all the side reactions, such as the hydrolysis of Pd(II), the protonation of alginate and the interactions of the ligand and the metal ion with ions of ionic medium were taken into account. In the pH range 2–5 and at chloride concentration in the range 0 ⩽ CCl-/mmol L−1 ⩽ 10 the gel beads showed good sorption ability towards Pd(II) ion as confirmed by kinetic and thermodynamic data calculated by a pseudo second order equation and using Langmuir and Freundlich models. The comparison between qe values related to the kinetics of Pd(II) sorption and Ca(II) release in solution by gel beads indicated that the ion exchange is one of the main sorption mechanisms of the sorbent material. Similarities were found between the trends of sequestering (% PdAA species, pL0.5) and the sorption (qe, qmax) parameters as function of pH and chloride concentration. It confirms a close relation between the behaviour of alginate as sequestering agent for Pd(II) ion in aqueous solution and of calcium alginate gel beads as sorbent material for the removal of the metal ion.