Sorption of cationic complexes of palladium (II) from thiourea solutions

Abstract

This paper looks at the sorption of cationic complexes [Pd(Thio)4]2+ (where Thio stands for thiourea) on sulfo cation exchanger KU-2-8. It takes 2 days for the Pd(II) static sorption capacity to start seeing constant values; the maximum Pd(II) static sorption capacity is 2.8 mmol/g. Solutions of acids, salts, thiourea and ammonia were used as eluents for Pd(II). By the Pd(II) static desorption efficiency, the solutions can be arranged as follows: 10% Thio solution in 10% HCl (69–70%) > 6 M HCl (51–52%) > saturated KCl solution (35–36%) > 25% NH4Cl solution (6–7%)  1М H2SO4 (1–3%) 10% ammonia solution (0–1%). In the IR spectrum of the cation exchanger saturated with Pd(II) complexes, the adsorption bands of thiourea ligand appear alongside with the adsorption bands typical of the sorbent in a hydrogen form; the long-wave region contains an adsorption band at 377 cm–1, which the authors attribute to Pd – S stretching mode. A 23 cm–1 shift of the maximum of the adsorption band referring to the (ОН) stretching mode in the IR spectrum of the sorbent saturated with Pd(II) complexes into the long-wave region compared with the position of the (ОН) maximum in the spectrum of the hydrogen form sorbent is indicative of hydrogen bonding between the sulfo group of the sorbent and the amino group of the thiourea: S = O···H – N. A relationship between the sorption capacity and the acidity of the initial solution, a comparison of IR spectra of the solid phases and analysis of free thiourea contained in the mother solutions confirm that the sorption process is based on cation exchange and that all 4 Thio molecules remain in the sorption complex. Tests were conducted in which the sorbent was used for dynamic separation of Pd(II) from a standard test thiourea solution containing copper and palladium cations. It is shown that copper cations get sorbed alongside with thiourea complexes of Pd(II). Such copper cations can be partially removed from the sorbent phase by washing it with 1 M sulphuric acid solution. It was found that the thermal decomposition of the sorbent saturated with complex Pd(II) ions results in the formation of black ball-shaped granules containing 76.0–82.5% of carbon; 4.0–4.5% of sulphur; 0.3–0.4% of hydrogen; 14.0–14.5% palladium; and a trace amount of nitrogen. According to the X-ray diffraction data on the product resultant from heat treatment of the sorbent saturated with thiourea complexes of Pd(II), palladium is present in the form of metal (Fm3m) (approximately 85% of the total concentration) and its oxide PdO (P4n2). These granules can be of interest as heterogeneous carbon-containing palladium catalysts.