Hard–Soft Interactions in Solvent Extraction with Basic Extractants: Comparing Zinc and Cadmium Halides

Abstract

Solvent extraction is often applied to separate and purify metals on an industrial scale. Nevertheless, solvent extraction processes are challenging to develop because of the complex chemistry involved. For basic extractants, much of the chemical behavior remains poorly understood due to the conditions far from thermodynamic ideality. To elucidate the extraction mechanism, we studied the speciation and extraction of zinc(II) and cadmium(II) from chloride, bromide, and iodide media by using a basic extractant consisting of a trioctylmethylammonium cation and, respectively, a chloride, bromide, or iodide anion. These systems were specifically selected to increase the understanding of the lessstudied bromide and iodide media and to focus on the effect of hard−soft interactions on solvent extraction systems. It was observed that, in general, a metal is more efficiently extracted when its hydration in the aqueous phase is lower and its stabilization in the organic phase is higher. In the investigated systems, these conditions are obtained by forming metal complexes with a lower charge density by coordinating the right number of halide anions and by selecting a halide with a lower charge density. In the organic phase, the stability of the metal complex can be increased by forming strong metal−anion bonds and by decreasing the water content. These insights might be of interest in the development and optimization of separation schemes for metals.

Taking a deeper look into the roles of amines in atomic absorption spectrometry

“…The ion-association complexes result from electrostatic attraction between oppositely charged ions with high molecular weights 50,51…”

“…49 Neutral extractants compete with the solvent water molecules for the first solvation shell of M n + to facilitate the extraction of metal ions as a neutral complex from an aqueous phase. 50 Basic extractants are mainly amines and (quaternary) ammonium salts (R 4 N + ), 51 which powerfully allow the extraction of metals in anionic form. Nonetheless, amines are routinely used as ligands and solvents individually or combined to form mixed solvents (Table SI-2†).…”

“…The ion-association complexes result from electrostatic attraction between oppositely charged ions with high molecular weights 50,51 to form neutral extractable species. The extraction of ion pairs, mainly prevailing physical bonds, usually takes place according to the following sequences: (i) formation of the metal complex anion M n + + 4X − ⇄ MX 4 2− ; (ii) salt formation, where the amine is converted into a suitable amine salt to provide an anion: (R 3 N) o + (H + ) a + (L − ) a ⇄ (R 3 NH + L − ) o ; (iii) extraction, where metal species exchange in the form of an amine salt or polar ion pair 2(R 3 NH + X o − ) o + MX 4 2− ⇄ (2R 3 NH + MX 4 2− ) o + 2X o − , where M is a metal, X is a halogen, and R 3 NH + is a tertiary ammonium salt.…”

Identification of Cadmium Compounds in a Solution Using Graphene-Based Sensor Array

“…In addition, the biotoxicity of cadmium compounds differs depending on the type of compound [ 1 , 8 ]. Cadmium compounds are present in solution as compounds instead of as ions owing to their low ionization [ 9 , 10 , 11 , 12 , 13 ]. Therefore, the identification of cadmium compounds in water is important.…”