The effect of the nature of the сhalcogen atom on the extraction and adsorption characteristics of chalcogen-containing oligomers based on chlorex

Abstract

The extraction and adsorption characteristics of chalcogen-containing oligomers derived from chlorex and elemental chalcogens in the hydrazine hydrate–base systems were studied. Sulphur- and selenium-containing oligomers synthesised in hydrazine hydrate–monoethanolamine systems are soluble in organic solvents. The as-obtained solutions are capable of extracting the following ions from water solutions: Hg2+, Cd2+, Zn2+, Pb2+, Cu2+ and Ni2+. Oligomers synthesised in the hydrazine hydrate–water–KOH system (sulphur- and selenium-containing oligomers) and a tellurium-containing oligomer obtained in the hydrazine hydrate–KOH system were used as adsorbents. It was assumed that the extraction and adsorption of metal ions by chalcogen-containing oligomers is carried out through the formation of complexes in which the chalcogen atoms act as ligands. The effect of the nature of chalcogen atoms on the possibility of forming complexes, i.e. on the extraction and adsorption properties of the oligomers used, was considered using the theory of hard and soft acids and bases (HSAB). According to this theory, the cations Hg2+ and Cd2+ belong to soft acids, while the cations Zn2+, Pb2+, Cu2+ and Ni2+ belong to acids of intermediate hardness. Chalcogen atoms are classified as soft bases. The conducted experiment showed that Hg2+ and Cd2+ ions are easily extracted and adsorbed by all the oligomers used, while the intermediate-hardness cations most easily interact with the sulphur-containing oligomers. The formation of oligomer–metal complexes was confirmed by analysing IR and 77Se NMR spectra recorded for sulphur- and selenium-containing oligomers before and after the extraction. The recorded IR spectra contained obvious changes in the nature of the absorption bands due to the stretching vibrations of the C–S and C–Se bonds. An additional signal of de-screened selenium nuclei was observed in the 77Se NMR spectrum. An insignificant change in the nature of the IR spectra in the region of stretching C–O vibrations indicated a weak participation of oxygen in the formation of complexes. It can be assumed that the role of oxygen in maintaining the high rates of extraction and adsorption is associated with an increase in the flexibility of macromolecules, which ensures a more favourable geometry for complex formation.