Immobilization of Cyphos IL-101 in biopolymer capsules for the synthesis of Pd sorbents

Cyphos® IL 101, a tetraalkyl-phosphonium chloride salt, (ionic liquid, IL) was immobilized in capsules prepared by ionotropic gelation in calcium chloride solutions. These resins were tested for Pd recovery from HCl solutions. The sorption performance is slightly controlled by the concentration of HCl, in the range 0-2 M, with maximum sorption capacities close to 130-145 mg Pd g−1 resin (i.e. 1.2-1.4 mmol Pd g−1 resin or 0.96-1.07 mmol Pd mmol−1 Cyphos IL-101). Palladium, essentially present in the form PdCl42- is probably bound to the resin through anion exchange with protonated phosphonium groups (ion pair formation). Decreasing the amount of IL immobilized in the resin improves the rational use of the IL (higher molar ratio Pd/IL). The presence of other anions, for example nitrate, or other chloro-metal complexes, ZnCl42- decreases Pd binding, while other metals that do not form anionic complexes under the selected experimental conditions (for example Cu or Ni) do not interfere with Pd uptake. The pseudo-second-order rate equation was successfully used to model the kinetic profiles. Kinetics are mainly controlled by intraparticle diffusion resistance (low effect of agitation speed on the kinetic profiles). Crank's equation was used to model intraparticle diffusion, giving intraparticle diffusivities close to 0.7-1.5 × 10−12 m2 min−1 for dry material and up to 5 × 10−12 m2 min−1 for wet resin. Drying the resin has a significant impact on mass transfer: uptake kinetics are improved by using raw resins. Desorption of the metal loaded resins can be performed using thiourea (in 0.1 M HCl solution) or HNO3 (5 M). The recycling and re-use of the resin was successfully demonstrated over three cycles; at the third desorption step a partial degradation of the resin was observed.