Studies on supercritical fluid extraction of uranium and thorium from liquid and solid matrix

Supercritical fluid extraction (SFE) is being widely used in pharmaceutical and food industry. Because of its simplicity, ease of operation and more importantly the reduction in the analytical waste generation, this technique is being viewed as a potential application technique in nuclear industry also. CO2 is employed as supercritical fluid (SCF) as it is easily recyclable, non-toxic, chemically inert, radiochemically stable and inexpensive. Radioanalytical chemistry section (Radiochemistry and Isotope group) has recently procured a supercritical fluid extraction/chromatography system. The present report describes the work carried out on the system. Detailed study on uranium and thorium extraction from highly acidic medium and tissue paper matrix has been carried out. Direct dissolution and extraction of uranium compounds employing SCF has been carried out. CO2 was employed as supercritical fluid along with very small amount of Tri n-butyl phosphate (TBP) and Tri n-octyl phosphine oxide (TOPO) as co-solvents. The effect of various operating parameters like CO2 flow rate, co-solvent percentage, temperature and pressure on extraction was investigated and parameters for maximum extraction were optimized. For comparison, the modes of extraction viz. static and dynamic and modes of complexation viz. in-situ and online were studied. Uranium extraction of ∼98% has been achieved from nitric acid medium employing TBP as co-solvent in 30 minutes extraction time, whereas with TOPO ∼99% uranium extraction could be achieved. Uranium from tissue paper matrix could be extracted upto the extent of 98% with TOPO as co-solvent whereas with TBP extraction of (66.83± 9.80)% was achievable. Direct dissolution of UO2, U3O8, U metal, U-Al alloy solids into SCF CO2 was carried out employing TBP-HNO3 complex and SFE of uranium was performed using TBP as co-solvent. UO2 and U3O8 solids could be dissolved within 20 minutes and extraction of ∼98% was achieved. For U metal and U-Al alloy, complete dissolution required 30 and 40 minutes respectively and uranium was extractable upto the extent of ∼83%. For thorium extraction from acidic solution, extraction of 81.87% was obtained with TBP, while with TOPO thorium extraction of ∼99% was achievable. From tissue paper matrix, thorium could be extracted upto 25% only with TBP, which improved to 66% when TOPO was employed as co-solvent. (author)