High selectivity and extractability of palladium from chloride leach liquors of an automotive catalyst residue by azothiacalix[4]arene derivative

Abstract Pd(II) is a noble metal in the platinum group metals (PGMs) and has been used extensively in automobile catalysts. However, its resources are strictly limited; thus, the recovery of Pd(II) from wastes or secondary sources currently has great importance. A new ligand, namely, 11,17,23-tetrakis-[( p -methoxyphenyl)azo]-25,26,27,28-tetrakis[(dimethyl thiocarbamoyl)oxy]thiacalix[4]arene (MADTTCA), was investigated for the extraction of Pd(II) from a hydrochloric acid medium using chloroform as a diluent. Extraction data indicate that the extraction efficiency depends on the concentration of HCl, from 0.1–5.0 M. The equilibrium extraction time of MADTTCA was achieved with a short contact time (15 min), which made MADTTCA a suitable candidate for industrial applications. Notably, the maximum uptake capacity of MADTTCA for Pd(II) was found to be 417 mg/L using 1.0 mM MADTTCA. On the basis of the obtained data, the stoichiometric ratio between Pd(II) and MADTTCA was found to be 1:1. MADTTCA was shown to be highly effective at extracting Pd(II) from simulated mixed metal and automotive catalyst residue (ACR) solutions composed of six metal ions (PGMs), such as Pd(II), Pt(IV), Rh(II), Al(III), La(III) and Ce(III). More than 99.9% of the extracted Pd(II) was stripped with 0.1 M thiourea in a 1.0 M HCl solution. The title extractant was able to be recycled and reused at least five times without a significant loss of its extraction features. All of the obtained results revealed that MADTTCA can be used for the selective recovery of Pd(II) from ACR solutions.