Poly(styrenesulfonic acid)-impregnated alginate capsule for the selective sorption of Pd(II) from a Pt(IV)-Pd(II) binary solution

Poly(styrenesulfonic acid)-impregnated alginate capsule (PSSA-AC) was prepared using a simple fabrication process, and used for selective separation of Pd(II) and Pt(IV) from their mixture. Evaluation of the pH effect revealed that PSSA-AC had good Pd(II) selectivity especially when the pH was between 3 and 5 at which neutral species Pd(OH)₂ are present. Experiments on metal penetration through the Ca²⁺-alginate film showed that anionic species hardly penetrate through the alginate film (acting as an ionic barrier). The selective sorption mechanism is proposed as the following steps: (1) selective penetration of the neutral Pd(OH)₂ through the ionic barrier (Ca²⁺-alginate shell) and then (2) chelation reaction of the neutral Pd(OH)₂ with the −SO₃⁻ groups of PSSA in the core. The maximum Pd(II) uptake was 291.19 ± 17.48 mg/g, which was about 32 times higher than that of Pt(IV). The results of the sorption/desorption test indicated that the PSSA-AC has good reusability potential. Even through one cycle of sorption/desorption, Pd(II) and Pt(IV) were successfully separated from their mixture with significantly high purities of 98.65% Pd(II) and 98.71% Pt(IV). This study reports for the first time the feasibility and potential of ionic barrier-based sorbents as selective separation of precious metals which have different speciations.