Selective adsorption of Pd(II) over Ag(I) in nitric acid solutions using nitrogen-donor-type adsorbents

Abstract

The HNO3 leaching process is used in recycling Pd metal from spent products that primarily contain Ag, and most Pd residues are separated from solutions containing Ag(I). However, a small amount of Pd(II) remains in these Ag(I) solutions. Therefore, the separation of Pd(II) and Ag(I) in HNO3 solutions is essential for promoting efficient Pd recycling. In this study, the separation of Pd(II) and Ag(I) in HNO3 solutions was investigated using four N-donor-type adsorbents bearing amine (R-Amine), iminodiacetic acid (R-IDA), pyridine (R-Py), and bis-picolylamine (R-BPA) functional groups. R-IDA and R-Py demonstrated efficient separation of Pd(II) (82 %–99 %) from Ag(I), Cu(II), Ni(II), and Fe(III) (<3%) over a range of HNO3 concentrations (0.5–7 M for R-IDA and 0.3–7 M for R-Py). R-Amine showed similar selectivity but lower Pd(II) adsorption (<65 %). By contrast, R-BPA adsorbed Pd(II), Ag(I), and Cu(II) efficiently and non-selectively (>90 %; 0.3–5 M HNO3). Structural analyses of the adsorbed metal ions using Fourier-transform infrared and extended X-ray absorption fine structure spectroscopies revealed the separation mechanisms of the N-donor-type adsorbents. Pd(II) adsorption on R-IDA, R-Py, and R-BPA occurred via Pd(II) coordination with their respective functional groups, whereas that on R-Amine occurred via anion exchange of NO3− with [Pd(NO3)4]2−. The coordinative adsorption mechanisms resulted in higher Pd(II) adsorption of R-IDA, R-Py, and R-BPA. HCl (5.0 M) eluents desorbed 83 % of Pd(II) from R-IDA, whereas thiourea eluents (0.1 M) desorbed 95 % of Pd(II) from R-Py. R-Py was the most effective Pd(II) adsorbent based on adsorption selectivity and desorption efficiency.