Organophosphonate ligands in solvent extraction: insights from experiment and theory

Tributyl Phosphate (TBP) has been the vital solvent in nuclear industry since last seven decades. It has dominated the front and back end of the nuclear fuel cycle. TBP is a member of the organophosphorus family of extractants. The other members of this family are phosphonates, phosphinates and phosphine oxides. The data on these classes of extractants are scarce in the literature. Pioneering work on the potential of these organophosphorus extractants for various applications was carried out by Siddall, Mason and Griffin. The phosphoryl oxygen in the organophosphorus compounds is responsible for their complexing ability with various ions. Thus the extraction ability of the organophosphorus extractant depends on the basicity of the phosphoryl oxygen and the nature of the substituents attached to phosphorus atom. The basicity of phosphoryl group can be enhanced by replacing the C-O-P group in the extractant by a C-P group. The basicity on the phosphoryl oxygen in the neutral organophosphorus extractant series is found to increase in the order phosphates < phosphonates < phosphinates < phosphine oxides. Among the organophosphorous compounds, phosphonates have shown greater promise towards actinide extraction. The present talk discusses two members of phosphonate family viz. H-phosphonates and dialkylalkyl phosphonates. The method of synthesising these compounds followed by characterisation, evaluation of physical properties, solvent extraction behaviour towards actinides will be presented. This will be followed by theoretical studies carried out on these extractants and their metal complexes. (author)