Polybenzimidazole resin based new chelating agents. Palladium(II) and platinum(IV) sorption on resin with immobilized dithiooxamide

Abstract

Microporous polybenzimidazole (PBI) of 250–500 μm bead size has been epoxidized and subsequently reacted with dithiooxamide in the presence of a phase-transfer catalyst to obtain a sorbent with immobilized dithiooxamide, EPBI(DTOX). The sorption of Pd(II) and Pt(IV) in HCl solutions has been measured on both PBI and EPBI(DTOX) resins, employing the sorbate species singly, in mixtures, and in the presence of a number of base metal ions, which include Cu(II), Ni(II), Zn(II), Co(II), Fe(II) and Fe(III). The saturation sorption capacities (mg/g dry resin) of PBI for Pd(II) and Pt(IV) in 0.1 N HCl are 276 and 288, respectively, the corresponding values for EPBI(DTOX) being 157 and 168. The sorption capacities decrease with the acid strength of the medium, the change, however, being relatively more pronounced for PBI, so much so that in concentrated (> 1 N) acids the sorption of EPBI(DTOX) is higher than that of PBI. In mixtures of Pd(II) and Pt(IV) with 10-fold molar excess of Pt(IV), PBI and EPBI(DTOX) show, respectively, 80% and 85% selectivity for Pd(II). While EPBI(DTOX) exhibits high selectivity (> 85% and > 80%, respectively) for Pd(II) and Pt(IV) in the presence of 100-fold molar concentration of any of the above base metal ions, on PBI, however, the Pd(II) and Pt(IV) selectivities are reduced to 50% and 20%, respectively, by 100-fold molar concentration of Fe(II). The neutral chelating ligand thiourea causes rapid stripping of Pd(II) and Pt(IV) from PBI and EPBI(DTOX) resins. The sorbed metal ions on PBI, being bound as complex anions by ionic forces, are more easily stripped by strong acids than the sorbed metal ions which are bound to EPBI(DTOX) by chelation. Having a relatively lower binding constant, Pt(IV) is however more easily stripped than Pd(II), thus permitting a selective stripping of Pt(IV) on PBI. On the other hand, a selective stripping of Pd(II) can be achieved with a strong solution of thiocyanate, which prevents stripping of Pt(IV) while allowing that of Pd(II). Thus, separation of Pd(II) and Pt(IV) in mildly acidic solutions in the absence of Fe(II) can be conveniently achieved by sorption on PBI followed by stripping with a strong solution of thiocyanate. EPBI(DTOX), on the other hand, is more useful for separation in concentrated acid solutions and in the presence of large concentrations of base metal ions including Fe(II).