Selective recovery of palladium and rhodium by combined extraction and photocatalytic reduction

Abstract

• The extraction efficiency of Pd(II) was higher than 99% for DDTC-ILs system. • The extraction mechanisms of DDTC-ILs with Pd(II) was elucidated. • Pure Pd or Rh nanoparticles can be obtained by the extraction-photocatalytic process. • PGMs recovery with photocatalytic reduction minimizes environmental impact. • DFT calculations were used to optimize extraction system. In this study, four novel functionalized ionic liquids ([C 4 mim][DDTC], [C 5 mim][DDTC], [C 6 mim][DDTC], and [BMPip][DDTC]) were synthesized and used for the selective extraction of Pd(II) from an HCl medium containing Pt(IV), Pd(II), and Rh(III). The extraction mechanism was investigated using Job’s plots, UV–Vis, SXRD, and MS analysis. A mechanistic investigation revealed the formation of hydrophobic extraction complexes Pd(DDTC) 2 and PdClDDTC via coordination between [DDTC] − and Pd(II). In addition, the solubility and polarity of the solutes were investigated using DFT calculations to determine the optimal extractant and diluent. From the results of the theoretical analysis, [C 5 mim][DDTC]/CHCl 3 was selected as the optimum system for the selective extraction of Pd(II). The effects that factors have on Pd(II) extraction, including temperature, [C 5 mim][DDTC] amount, HCl concentration, and O/A ratio, were investigated and optimized using single-factor and response surface experiments. Under the optimized conditions, Pd(II) extraction efficiencies were higher than 99%, and the separation factors (β Pd(II)/Pt(IV) and β Pd(II)/Rh(III) ) were greater than 10 4 . After extraction, Pd and Rh were directly recovered from the organic and aqueous phases via photocatalytic reduction, respectively. TEM, XPS, and PXRD results revealed that the resulting nanoparticles were Rh(0) or Pd(0). The recovery of Pd and Rh exceeded 95%. Overall, a simple hydrometallurgical process with low reagent consumption was developed to yield nanometer-scale Pd(0) and Rh(0) from simulated secondary resource leaching solutions.