Sequential Separation of U(VI) and Th(IV) by a Cyanex-921-Based Amberlite XAD-4 Chelating Resin

INTRODUCTION Among heavy metals, considering their radioactivity and toxicity, uranium and thorium are undoubtedly the most dangerous to the environment. The strategic issue of chemists is the exploration and mining, besides the processing, of ore materials that contain nuclear elements such as uranium and thorium, which represents the cornerstone of (industrial) nuclear tech-nology [1-3]. Uranium and thorium find extensive application as nuclear fuel in power, and their main sources are pitch-blende, monazite sand and sea water. Both metal ions are known to cause acute toxicological effects in mammals, and their compounds are potential occupational carcinogens [2]. Their recovery is essential to minimize their discharge into the envi-ronment from the point of view of safety and economy. Their selective extraction, simultaneously in the presence of each other and closely associated metal ions, has drawn much attention from chemists the world over because of their importance in energy-related applications [1]. Thus, there remains a need for pre-organized complexing agents that discriminate U(VI) and Th(IV) from associated metal ions present in great excess in solid or aqueous media [4]. The solvent extraction technique has been serving this purpose for many decades. Solid-phase extraction (SPE) has been increasingly used for the pre-concentration /separation of trace and ultra-trace amounts of inorganic and organic species from complex matrices, as seen from recent reviews [5,6]. Chelating resins have been frequently used as SPEs as they provide good stability, a high sorption capacity for metal ions and good flexibility in working conditions. Impregnated resins, which are designed by physi-cally loading the successful organic extractants on a solid inert support material, have also been found to be effective for this process [7]. Chelating resins that are employed include Amberlite XAD series resins, 4-vinyl pyri-dine-divinylbenzene/acrylonitrile-divinylbenzene copolymers, Amberlite IRA-400, Biorad AGMP-1, silica-based C18 support, Amberlyst A-26, Dowex-2 and Merrifield chloromethylated resins. Amberlite XAD series resins have shown promise for de-signing chelating resins. There are two methodologies frequently adopted for designing such chelate-functionalized Amber-lite XAD resins. The first involves the physical sorption of ligands onto a matrix. The other is based on covalent coupling of a ligand with a polymer backbone through a spacer arm, generally a –N=N– or –CH2 group [8]. The latter strategy renders rugged systems, free from ligand leaching problems, but their sorption capacities are low. One way of achieving high sorption capacity is by the use of ligands of small size which can extensively functionalize an appropriate crosslinked polymer. Several previous studies have been reported for the XAD series resins in which an inert support is impregnated or functionalized with a selective organic extractant, e.g., bicine [9], quinoline-8-ol [10] o-phenylene dioxydiacetic acid [11], tiron [12], Oc-ta-O-methoxy resorcin [2] arene [13], Chrome Azurol S [14], succinic acid [15], o-aminobenzoic acid [16], N,N-dibutyl-N’-benzoylthiourea [17], Pyrogallol [18], alizarin red-s [19] and Octa-O-methoxy resorcin arene [2], to produce a solid sorbent for the isolation of U(VI) and/or Th(IV) from various analytical matrices.