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 LIII 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.

Sorption mechanisms of chromate with coprecipitated ferrihydrite in aqueous solution.

Hexavalent chromium (Cr(VI)) attracted researchers' interest for its toxicity, natural availability and removal difficulty. Nevertheless, its sorption mechanism is not clearly understood yet. In this work, we elucidated the sorption mechanism of the co-precipitation of chromates with ferrihydrite through quantitative analysis. The influence of Cr/Fe molar ratio on sorption was investigated by zeta potential measurements, X-ray diffraction (XRD) and X-ray adsorption fine-structure analysis (XAFS). Coprecipitation at pH 5 showed almost twice the sorption density of adsorption at pH 5. In co-precipitation, a shift of the XRD peak due to inner-sphere sorption of chromate was observed at Cr/Fe molar ratio 0.5. For adsorption, the same peak shift was confirmed at Cr/Fe molar ratio of 1. Zeta potential at pH 5 suggested that the sorption mechanism changed at Cr/Fe molar ratio 0.25 for coprecipitation and at Cr/Fe molar ratio of 1 for adsorption. Fitting of Cr and Fe K-edge extended X-ray adsorption fine-structure suggested that ferrihydrite immobilized Cr(VI) via outer sphere surface complexation for lower Cr/Fe ratios and via inner-sphere surface complexation for higher molar ratios. At higher molar ratios, bidentate binuclear CrFe bonds were well established, thus resulting in the expansion of the ferrihydrite structure.