Polyamine functionalised chelation ion exchange resins for uranium extraction

Ion exchange (IX) techniques are ubiquitous in industrial processes throughout the world, being used in areas such as desalination, mineral extractions and the nuclear fuel cycle. In the nuclear fuel cycle IX has been implemented at the front end for the extraction of uranium from pregnant leach liquors, chemical control of the coolant water within reactors and also at the back end in spent fuel reprocessing and waste treatment. Though there is a major competitor to this technology in solvent extraction (SX), IX has always been a major workhorse technology in the nuclear industry. This is due to IX not having some of the drawbacks of SX such as the lower ability for preconcentration, potential for solvent loss, phase disengagement in multiple contact stages, third phase formation and the generation of large volumes of liquid aqueous and organic waste streams. Chelation is a thermodynamic effect inferring an enhanced stability for a multidentate ligand and metal complex when compared with a complex containing the same metal ion and a group of similar monodentate ligands. Chelation IX resins have also been shown to resist uptake suppression due to the ionic strength of uptake media. This effect can aid in potentially producing selective extractants which could be tailored to specific metals in a range of aqueous feeds. N-donor functional groups (ethylenediamine (EDA), diethylenetriamine (DETA) and pentaethylenehexamine (PEHA)) have been grafted onto the Merrifield resin (chloromethylated polystyrene). Soft N-donor ligands are well known to bind to uranium, with N-donor functional groups being commonly used at the front end of the nuclear fuel cycle for uranium extraction from pregnant leach liquors. These IX resins are classed as weak base resins, which are commonly found to be effective when sulphuric acid leach conditions have been used. There are reports of like resins being used in the literature, however, a comprehensive analysis of uptake (loading isotherms, pH dependence) and direct comparison between the homologous functional groups has not been published. The three synthesised resins and a commercial resin, Purolite S985, have been assessed for their uptake behaviour towards uranium (as UO₂SO₄) in sulphuric acid media. Purolite S985 is also a polyamine resin, which was designed for the selective removal of heavy metals. It has been shown to extract platinum, palladium, rhodium, nickel, copper and zinc under varying conditions, but its uptake towards uranium is relatively unknown. Uranium loading isotherms have been performed at constant pH, as well as studies to explore the effect of pH and SO₄^2- concentration. Isotherm models have been derived using data from extended x-ray absorbance fine structure (EXAFS) data and compared with well known isotherm models such as Langmuir and Dubinin-Radushkevich. Elution methods will also be investigated.