Speciation and separation of platinum(iv) polynuclear complexes in concentrated nitric acid solutions

Understanding the solution chemistry of Pt(iv) is crucial for the hydrometallurgy of precious metals. To gain such an understanding, the speciation and separation of Pt(iv) complexes in concentrated HNO₃ solutions were investigated via Pt L_III edge X-ray absorption fine structure (XAFS) spectroscopy. The XAFS results for concentrated HNO₃ solutions of Na₂Pt(OH)₆ revealed the dominant presence of Pt polynuclear complexes, wherein the formation of Pt(iv) polynuclear complexes depended on the metal concentration and the Na₂Pt(OH)₆ dissolution temperature. The dominant species present in a heated nitrate solution of 0.90 g-Pt L^-1 and a non-heated nitrate solution of 3.2 g-Pt L^-1 were dinuclear Pt(iv) complexes, whereas those in a heated solution of 3.0 g-Pt L^-1 were predominantly larger polynuclear complexes, such as, tetra- and hexa-nuclear complexes. The presence of larger Pt(iv) complexes was confirmed via XAFS spectroscopy, wherein the adsorption of Pt(iv) ions from a 10 M HNO₃ solution by a chelating resin functionalised with iminodiacetic acid and a strongly basic anion-exchange resin bearing trimethyl ammonium nitrate was examined. The adsorption of 50 mg L^-1 of Pt(iv) by the two resins was tested using aqueous solutions diluted from heated HNO₃ solutions with varying metal concentrations, and also from a non-heated solution. We found that Pt(iv) complexes from heating solutions containing high Pt(iv) concentrations displayed high adsorption percentages. In addition, the selective adsorption of Pt(iv) over Pd(ii), Ag(i), Cu(ii), Ni(ii), and Fe(iii) from a 10 M HNO₃ solution was achieved using a strongly basic anion-exchange resin.