NEW 8-HYDROXYQUINOLINE DERIVATIVE EXTRACTANTS FOR PLATINUM GROUP METALS SEPARATION PART 4: KINETICS OF Pd(II) EXTRACTION AND STRIPPING

ABSTRACT

The extraction and stripping kinetics of Pd(II) by chloroform solutions of 7-substituted-8-hydroxyquinoline derivatives, 8-HQ, (labelled TN 1911, an unsaturated and sterically-hindered derivative and TN 2336, a saturated and sterically-free derivative) from aqueous chloride media are reported. The extraction process is shown to go through the fast formation of the ion-pair PdCl₄ ²⁻(H₂Q⁺)₂ (where H₂Q⁺ represents the protonated extractant) and the slow intra molecular transformation of the latter to an inner sphere (chelate) complex. As the slow intramolecular conversion reaction (the postulated rate-limiting step) apparently occurs away from the interface, the extraction reaction was found to be independent of the interfacial area and the agitation speed. The order of the extraction reaction, in the HCl concentration range 0.20-2.00M was found to be with respect to [PdCl₄ ²⁻], 0 with respect to [H⁺], 2 with respect to [HQ]_org and - 2 with respect to [CI⁻] for both derivatives. The respective orders for the stripping reaction were 1 for [PdQ₂]^org, 2 for [H⁺] and 2 for [CI⁻] for both extractants again. Pd(II) extraction and stripping rate constants, k_ext, and k_strip were found to be 0.36±0.05 s⁻¹ and 6.8±0.3 (×10⁻⁶) s⁻¹ for TN 1911 and 0.6±0.1 s⁻¹ and 8.9±0.3 (× 10⁻⁶) s⁻¹ for TN 2336. It was also observed that the presence of Pt(IV) in the organic phase slows down (although to a modest degree) the extraction rate of Pd(II). It is demonstrated finally that an exchange reaction takes place between Pd(II) and Pt(IV) when the two metals are co-extracted from a mixed metals solution. Thus Pt(IV) appears to be initially extracted very rapidly as an ion-pair and then is crowded out by the slow loading of Pd(II) as chelate in the organic phase.

Additional information

Notes on contributors

George P. Demopoulos

To whom correspondence should be addressed