Recovery of uranium from aqueous solutions by trioctylamine impregnated polyurethane foam† Yasemin Toker, Meral Eral* and � Umran Hiçs�onmez* Ege University, Institute of Nuclear Sciences, 35100-Bornova, ÿIzmir, Turkey Polyurethane foams can be used as effective sorbents for the separation and concentration of trace amounts of metal ions. In this work, the sorption and desorption behaviour of uranium(vi) was investigated in dilute uranyl sulfate solutions on an open-cell polyurethane foam impregnated with trioctylamine (TOA) used as an extractant. The parameters affecting uranium extraction such as uranium concentration, ratio of solution volume to foam weight (ml g21), pH, concentration, retention time and temperature were studied and optimum conditions were determined. Seven different stripping reactants, (NH4)2SO4, Na2CO3, NH4Cl, NaCl, NaNO3, NaCl–H2SO4, NH4NO3–HNO3, were tested for extracting uranium from the polyurethane foam impregnated with TOA into the aqueous phase again.High stripping efficiences were obtained with 1 m NaCl–0.5 m HCl, 1 m NaCl–0.05 m H2SO4 and 0.9 m NH4NO3–0.1 m HNO3 solutions. Keywords: Uranium (separation, absorption, sorption, extraction); trioctylamine (TOA); polyurethane foam; urethane polymers The use of polyurethane foams in the separation and extraction of various inorganic and organic species was first demonstrated by Bowen1 and the field has been reviewed by Braun.2 The foams act as weak anion-exchangers with low capacity since the chemical nature of these foams contain amido and aminogroups. 1 The extraction is specific with polyurethane foams having high distribution coefficients. The apparatus required is simple and inexpensive. The hydrodynamic properties of polyurethane foams are excellent due to its quasi-spherical membrane structure.3 Because of these advantages, polyurethane foams have important and fundamental applications in analytical chemistry. It has been reported by several authors that polyurethane foams can be used as effective and inert supports for various extractants and are used for the recovery of metal ions in analytical and water treatment processes.4,5 Polyurethane foams containing different functional groups can be prepared by immobilizing various organic extractants or chelating agents.6 High molecular weight tertiary amines [(CnH2n + 1)3N] that are selective to uranium and are readily available have been used in the solvent extraction of uranium.7 The stripping of uranium from amines can be achieved easily.In our work, polyurethane foams were impregnated with trioctylamine (TOA), a tertiary amine, and the parameters effecting the efficiency of uranium extraction were investigated. Experimental Materials and methods A stock uranium solution of 0.01 m was prepared by dissolving 0.5279 g of UO2(NO3)2.6H2O (Merck, Darmstadt, Germany) in 0.1 m nitric acid. A portion of this stock solution was evaporated.After the dissolution of the residue with sulfuric acid, the solution was evaporated again to dryness. The uranyl sulfate solution of 0.01 m was prepared from this stock solution. The commercial polyurethane foam was obtained from Pinar Co. (ÿIzmir, Turkey). TOA was supplied from Merck. All other chemicals used were reagent grade. Polyurethane foam plugs, 4.5 cm in diameter and 2.2 cm long (average weight = 0.5500 ± 0.0020 g), were cut from a foam sheet. Each foam plug was squeezed in 2 m HCl in a batch extractor for 1 h, washed with distilled water until free of HCl, and again squeezed, and air-dried overnight before using.The solution of TOA in cylohexane was equilibrated with an equal volume of aqueous 0.1 m H2SO4 for 10 min. The cleaned and dried foam plugs were impregnated in a sulfate formed TOA–cyclohexane solution and then cylohexane was evaporated off in the air. The extraction was carried out by a dynamic method in an automatic squeezing system.The uranium concentration in aqueous solution was determined by the 1-(2-pyridylazo)- resorcinol (PAR) method8 (l = 510 nm, e = 3.87 3 104 l cm21 mol21) by using a Shimadzu UV/VIS 260 Recording Spectrophotometer (Shimadzu, Kyoto, Japan.) The amount of uranium extracted by the TOA was determined from the difference between the initial and final concentrations of uranium in aqueous solutions. Results and discussion Measurements of the foam extraction capacity The 0.550 ± 0.002 g of foams impregnated with and without TOA were brought into contact with uranyl sulfate solution for 1, 8 and 24 h.The concentration of UO2 2+ in the solution was determined spectrophotometrically with sodium salicylate.9 After it was equilibrated, the sorbed amount of UO2 2+ (UO2 2+ mmol g21 foam) was calculated by measuring decreasing UO2 2+ concentration in the solution. Table 1 shows that the extraction capacity of uranium on TOA impregnated foams is higher than that on foam without TOA.Therefore, it is assumed that uranium desorbs if time is extended to 24 h and so extraction efficiency decreases. Parameters affecting uranium extraction Effect of uranyl concentration The effect of uranyl concentration was tested by using standard solutions of varying initial concentrations from 5.0 to 150.0 † Presented at The Sixth Nordic Symposium on Trace Elements in Human Health and Disease, Roskilde, Denmark, June 29–July 3, 1997.Table 1 Extraction capacity of polyurethane foam for uranium Foam (0.5 g) Time/h Capacity/ mmol g21 %U extracted Impregnated with 10% TOA (10 ml) 1 0.8762 97.0 Without impregnation 1 0.3907 43.2 Impregnated with 10% TOA (10 ml) 8 0.9011 99.8 Impregnated with 10% TOA (10 ml) 24 0.8050 89.0 Without impregnation 24 0.3050 33.8 Analyst, January 1998, Vol. 123 (51–53) 51mg l21 to investigate the extraction behaviour of uranium by TOA-impregnated foam. The extraction of uranium increases with increasing uranyl concentration, because the capacity of impregnated foam is high, as is shown in Fig. 1. Since the purpose of these experiments is to recover uranium from dilute aqueous solutions, uranium solutions of concentrations 25 mg l21 were used. Effect of the ratio of solution volume to foam weight Fig. 2 shows the effect on uranium extraction of the ratio of solution volume (V, ml) to the mass of dried foam (m, g). Foam samples of 0.1 g were used and volumes of solutions were varied from 50 to 1000 ml.As is shown in Fig. 2, the efficiency of extraction remains constant between 500–4000 (V:m), but decreases with increasing V:m after 4000. The V:m ratio of 500 was chosen in subsequent experiments. Therefore the recovery of uranium from large volumes of solution should be possible by using small amounts of impregnated foam. Effect of pH The pH of uranium solution was adjusted to a desired value by addition of H2SO4 or NH4OH solution.The effect of pH on the extraction of uranium by TOA-impregnated foam is shown in Fig. 3. The extraction efficiency was investigated by varying the initial pH of solutions from 1 to 7. It was observed that there is no significant change between pH 2–5. During the experiments, precipitation formation and turbidity were observed in solutions where the pH was !4, because the uranyl ions hydrolyze at this pH. In addition, the amines convert free salts up to pH 4.10 A pH of 2 was determined to be the optimum value.In comparison, � Olmez and Eral have reported that the extraction efficiency of uranium by tributylphosphate (TBP)-impregnated foam is low at acidic solutions but is high in neutral solutions.11 Effect of TOA concentration In this set of experiments, the amine concentration in cylohexane was varied from 5 to 20 vol%. The amines formed as sulfate salt were prepared by equilibrating the amine and 0.1 m H2SO4 in the volume ratio of 1 : 1.Foam samples of 0.1 g were impregnated with 2 ml of these solutions. The extraction efficiency of uranium was investigated by using different amounts of TOA. The results are shown in Fig. 4. The highest efficiency was obtained by using 10 vol% TOA, and efficiency again decreased at 15 vol% and 20 vol% TOA. In comparison, the extraction efficiency of uranium by using 30 vol% of regnated foam is 72%.11 However, the extraction efficiency is 82% in the experiments using 5 vol% TOA-impregnated foam.Effect of time The time dependence of uranium extraction was examined between 15–480 min. The experimental results are given in Fig. 5. Solvent extraction is an equilibrium process and shaking time is one of the important factors influencing the extraction of metals. The efficiency that is 83% for 15 min, reaches 90% for 120 min. But, 1 h is suitable for extraction from the economic point of view. At 1 h, high extraction efficiency has been reported using TBP-impregnated foam at room temperature.Fig. 1 Effect of uranium concentration on uranium extraction (A, mg and B, %). Conditions were: 0.55 g polyurethane foam, pH 2, V, = 50 ml, 10 ml of 10% (v/v) TOA, 1 h room temperature. Fig. 2 Effect of the ratio of solution volume to foam weight on the extraction of uranium (A, mg and B, %). Conditions were: 0.1 g polyurethane foam, pH = 2, [U] = 25 ppm, 2 ml of 10% (v/v) TOA, 1 h, room temperature. Fig. 3 Effect of pH on the extraction of uranium (A, mg and B, %) onto polyurethane foam impregnated with 10% TOA–cyclohexane.Conditions were: 0.1 g polyurethane foam, 50 ml of 25 ppm U, 1 h, room temperature. Fig. 4 Effect of TOA concentration on uranium extraction (A, mg and B, %). Conditions were: 0.1 g polyurethane foam, 50 ml of 25 ppm U, 1 h, room temperature. Fig. 5 Effect of shaking time on the rate of uranium extraction (A, mg uranium extracted, and B % uranium extracted). Conditions were: 0.1 g polyurethane foam, 50 ml of 25 ppm U, pH = 2, 2 ml of 10% (v/v) TOA, room temperature. 52 Analyst, January 1998, Vol. 123Effect of temperature The effect of temperature (0–45 °C) on the extraction efficiency of uranium is shown in Fig. 6. It shows that the extraction efficiency is 83% between 0–4 °C and is 88% at room temperature. Also, the efficiency decreased by 4% at higher temperatures. For this reason, room temperature (23 °C) was chosen as the appropriate extraction temperature.Optimum conditions The optimum extraction conditions were found to be: U concentration = 25 ppm; pH of solution = 2; V:m ratio (ml g21) = 500; TOA concentration = 10% (v/v) TOA– cyclohexane; time = 1 h, and temperature = 23 °C. At these optimum conditions, the recovery of uranium was found to be 90.0 ± 3.0%. Stripping uranium from foam Solutions of (NH4)2SO4, Na2CO3, NH4Cl–HCl, NaNO3, NaCl– H2SO4, NH4NO3–HNO3 were evaluated for the extraction of uranium from the polyurethane foam impregnated with TOA into the aqueous phase again.Stripping experiments were carried out by squeezing the foam samples in two stages for one hour. The stripping results are presented in Table 2. The nitrate ions in a mixture of ammonium nitrate and nitric acid are easily displaced by uranyl sulfate ions bonded to amine during the extraction stage. An efficiency of 90% was obtained by using both NaCl–H2SO4 and NH4NO3–HNO3 solutions and are therefore suitable for the stripping of uranium into the aqueous phase.Conclusions The results of the present work indicate that TOA-impregnated polyurethane foam can be used for the recovery of uranium from dilute aqueous solutions. The fact that extraction is high at pH = 2 indicates the need to use acidic solutions, such as waste acidic leach solutions. The polyurethane foam acts as a cheap and readily available support material for TOA. It has been found that the use of TOA is more economical than the use of TBP when unit price, percentage of solvent and extraction efficiency were evaluated together.The stripping of uranium is easy using 1 m NaCl, 1 m NaCl–0.05 m H2SO4 or 0.9 m NH4NO3–0.1 m HNO3. This method can also be used for concentrated uranium solutions because the TOA-impregnated foam has a high extraction capacity. On the other hand, the effects of some interfering impurities need to be studied in order to apply this method to real solutions. References 1 Bowen, H.J. M., J. Chem. Soc. A, 1970, 1082. 2 Braun, T., Fresenius’ Z. Anal. Chem., 1983, 314, 592. 3 Huang, T. C., Chen, D.-H., and Huang, S.-D., J. Chem. Eng. Jpn, 1993, 26, 361. 4 Shakir, K., Beheir, G., and Aziz, M., J. Radioanal. Nucl. Chem., 1992, `61, 371. 5 Gesser, H. D., and Ahmed, S., J. Radioanal. Nucl. Chem., 1990, 140, 396. 6 Mizuike, A., Enrichment Techniques for Inorganic Trace Analysis, Springer-Verlag, New York, 1983, pp. 90–91. 7 Ritchey, G. M., and Ashbrook, A. W., Solvent Extraction Principles and Applications to Process Metallurgy, Part II, Elsevier, Amsterdam, 1979. 8 Korkish, J., Modern Methods for the Separation of Rare Metal Ions, Pergamon Press, Oxford, 1974, p. 574. 9 Kabay, N., and Egava, H., Sep. Sci. Technol., 1993, 28, 1985. 10 Merritt, R. C., The Extractive Metallurgy of Uranium, 1971, Colorado School of Mines Research Institute, USAEC, Johnson, Boulder, Co, 1971, p. 195. 11 � Olmez, S., and Eral, M., J. Biol. Trace Element Res., 1994, 43–45, 731. Paper 7/04882C Received July 8, 1997 Accepted October 28, 1997 Fig. 6 Effect of temperature on uranium extraction (A, mg and B, %). Conditions were: 0.1 g polyurethane foam, 50 ml of 25 ppm U, pH = 2, 2 ml of 10% (v/v) TOA, 15 min. Table 2 Stripping yields by using different stripping reagents Stripping reagent pH of stripping reagent Amounts of U on foam/mg Total stripping (in two stages) U(%) 1.5 m (NH4)2SO4 3.4 1141.8 6.0 0.6 m Na2CO3 8.1 1102.0 47.3 1.5 m NH4Cl 3.5 1138.5 63.5 1 m NaCl–0.5 m HCl 2.0 1020.2 88.7 1 m NaNO3 7.1 1061.3 68.6 1 m NaCl–0.05 m H2SO4 1.5 1015.1 90.2 0.9 m NH4NO3–0.1 m HNO3 1.6 1105.9 90.1 Analyst, January 1998, Vol. 123