Studies on Platinum(II) and Palladium(II) binding to PADA in SDS micellar medium: a kinetic method for metal ions separation and recovery

Selective separation and recovery of metal-ions is a subject of importance both from an industrial and an environmental point of view. In particular, the increasing prices of the platinum group metals has promoted growing interest in their recovery. The replacement of the traditional organic phase by pseudo-phases built up with surfactants seems to offer an interesting and promising alternative to classical solvent extraction. Separation of the micellar pseudo-phase from the aqueous medium can be then achieved by means of ultrafiltration techniques. In this communication we present the results of a study devoted to the selective separation of platinum(II) (in the PtCl42- from) from palladium(II) (in the PdCl42- form) using the ligand PADA (pyridine-2-azo-p-dimethylaniline) in a micellar SDS (sodium dodecyl sulphate) pseudo-phase. The separation is based on the much higher rate of Pd(II) binding to PADA, so that this metal is transported on the micelle while Pt(II) stays in the aqueous phase. First of all the kinetic features of formation of the Pt(II)/PADA and Pd(II)/PADA complexes were analysed. It was actually found that the latter complex do form faster with respect to the former. Therefore, the Pd(II)/PADA complex can be retained by the SDS micelles grace to hydrophobic interactions, whereas PtCl42- is electrostatically repulsed by the negatively charged micelles. Ultrafiltration experiments show that the LM-MEUF (ligand modified – micellar enhanced ultrafiltration) approach is able indeed to separate Pt(II) from Pd(II) with promising results. Stripping data are also collected that provide information on the optimisation of Pd(II) recovery from the micellar pseudo-phase by using different acids or salts.