Synergistic extraction of Th(IV) by 2-hydroxy-1-naphthaldehyde thiosemicarbazone and neutral donors

Abstract

The extraction behaviour of Th(IV) from aqueous nitric acid medium employing 2-hydroxy-1-naphthaldehyde thiosemicarbazone has been studied in the presence of various donors, like trioctyl phosphine oxide (TOPO), calix[3]OH[3]OMe[6]arene, trioctyl amine (TOA), dimethyl sulphoxide (DMSO) in ethylacetate solvent. The constants (log k_ex) for the binary complex in organic phase [Th(A)(NO₃)₃], where A is the ligand, was found to be 3.99, which was by far the largest amongst the corresponding values known for the other thiosemicarbazones. The overall equilibrium constants (log K) for the ternary species [Th(A)TOPO(NO₃)₃], [Th(A)TOA(NO₃)₃], [Th(A)Calix[3]OH[3]OMe[6]arene(NO₃)₃], [Th(A)DMSO(NO₃)₃] were estimated to be 8.287, 8.862, 8.415, 6.921 respectively. The trend in equilibrium constants were in accordance with the substitution of the donor. The extraction of Th⁴⁺ by the ligand–donor combination was maximum at pH=1 and extraction decreases with increase in pH. It has been found that the extent of extraction of Th⁴⁺ in the organic phase as the binary as well as ternary complex [Th(A)(NO₃)₃] and [Th(A)(NO₃)₃S], where S is the donor, increases with increase in the concentration of the ligand. Similar trend is obtained in the extraction by donors in absence of ligand. In case of ternary extraction, using different donors, amines are found to perform best compared to the other donors. The trend is as follows: TOA > calix[3]OH[3]OMe[6]arene > TOPO > DMSO. In addition, the effect of different diluents on extraction was also studied and the observed trend was methyl salicylate > ethyl acetate > methyl isobutyl ketone > ethyl benzoate.