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DOI: 10.1007/s10967-007-6888-6 Journal of Radioanalytical and Nuclear Chemistry, Vol. 275, No.3 (2008) 563–5700236–5731/USD 20.00Akadémiai Kiadó, Budapest© 2007 Akadémiai Kiadó, BudapestSpringer, DordrechtBatch and dynamic extraction of uranium(VI) from nitric acid mediumby commercial phosphinic acid resin, Tulsion CH-96K. A. Venkatesan, K. V. Shyamala, M. P. Antony, T. G. Srinivasan, P. R. Vasudeva Rao*Fuel Chemistry Division, Indira Gandhi Centre for Atomic Research, Kalpakkam 603 102, India(Received June 5, 2007)Batch and dynamic extractions of uranium(VI) in 10–3–10–2M concentrations in 3–4M nitric acid medium have been investigated using a commercially available phosphinic acid resin (Tulsion CH-96). The extraction of uranium(VI) has been studied as a function of time, batch factor (V /m ), concentrations of nitric acid and uranium(VI) ion. Dual extraction mechanism unique to phosphinic acid resin has been established for the extraction of uranium(VI). Distribution coefficient (K d ) of uranium(VI) initially decreases with increasing concentration of nitric acid, reaches a minimum value at 1.3M, followed by increases in K d . A maximum K d value of ~2000 ml/g was obtained at 5.0M nitric acid. Batch extraction data has been fitted into the linearized Langmuir adsorption isotherm. The performance of the resin under dynamic extraction conditions was assessed by following the breakthrough behavior of the system. Effect of flow rate, concentrations of nitric acid and uranium ion in the feed on the breakthrough behavior of the system was studied and the data was fitted using Thomas model.IntroductionMacroporous bifunctional phosphinic acid (MPBPA) resin is a potential candidate for the separation of uranium(VI) from nitric acid medium.1–7 It is prepared 1by Friedel-Crafts phosphination reaction on macroporous polystyrene-divinylbenzene (PS-DVB) using AlCl 3 as catalyst, which leads to the formation ofmonoaryl (primary) and diaryl (secondary) phosphinicacid ligands in the resin matrix as shown in Fig. 1. Thedual mechanism for the extraction of metal ions by the resin is well established.7–10 Ion-exchange withphosphinic acid groups or by co-ordination throughoxygen of –P=O moiety was reported for the extraction of uranium(VI) by this bifunctional resin.10Furthermore, the primary phosphinic acid is susceptible to oxidation reaction, MPBPA also behaves as redox ion-exchanger when metal ions like Ag +, Hg 2+ are extracted.7,9At present, MPBPA resin is commercially available from M/s. Thermax India Ltd., Pune, India, as Tulsion CH-96. Most of the studies reported so far on MPBPA resin involved the extraction of uranium(VI) ion present at trace levels.10–13 These studies are also carried out only on laboratory synthesized MPBPA resin.12However, the data on the extraction of uranium(VI) on commercial resin, Tulsion CH-96, are indeed required, when it is proposed for large-scale ion-exchange operation for the separation of uranium from nuclear wastes. Furthermore, nuclear wastes or dissolver solutions are composed of 3–4M nitric acid medium with varying amount of uranium present in the feed solution, ranging from 10–3 to 10–2M, also demands the investigation of extraction of uranium present at higher concentration levels on Tulsion CH-96. Thus, the present study focuses on the evaluation of commercial* E-mail: vasu@.in MPBPA resin, Tulsion CH-96, for the extraction of uranium(VI), present at 10–3 to 10–2M levels, from3–4M nitric acid medium. The distribution coefficients of uranium(VI) as a function of concentrations of nitricacid and uranium(VI) ion, time and batch factor (V /m )have been determined. Breakthrough curves at various flow rates, feed concentrations have also been evaluated. Experimental Materials All the chemicals and reagents were of analytical AR grade. Tulsion CH-96 resin was purchased from M/s.Thermax India Ltd., Pune, India. The resin (average particle size 400–500 P m) was washed extensively with 1.0M sodium hydroxide and water. It was then converted to H + form by passing sufficient quantity of 0.5M nitric acid. The resin was washed extensively withwater and acetone and dried at 343K for 2 hours. Phosphorous content in the resin was found to be 8% by spectrophotometry using amidol as coloring agent.1Uranyl nitrate was purchased from E. Merck. Effect of [HNO 3]All the experiments were carried out at 298 K. Typically, the experiments involved a batch factor of 20 ml/g, that is equilibration of 0.5 g of the resin with 10 ml of the test solution. Extraction of uranium(VI) as a function of nitric acid concentration was studied by equilibrating the ion-exchanger with a solution containing 4.2.10–3M uranium present in desired concentration of nitric acid. After ten hours of equilibration, an aliquot was taken from the supernatant.K. A. V ENKATESAN et al.: B ATCH AND DYNAMIC EXTRACTION OF URANIUM (VI) FROM NITRIC ACID564The concentration of uranium present in the aliquot before and after equilibration was measured either by spectrophotometric procedure 14 using Arsenazo III as coloring agent at O max of 655 nm or by D AVIES andG RAY 15 method. The distribution coefficient [K d , ml/g of U(VI)] and the percentage of uranium extracted by the resin were calculated using Eqs (1) and (2), respectively:¸¹·¨©§ m V K d fin fin ini U][U][U][ (1)%Extraction = 100U][U][1ini fin u ¸¸¹·¨¨©§ (2)where V and m are the volume of the solution and massof the sorbent taken for equilibration, respectively.Kinetics of the extraction The rate of extraction of uranium(VI) by TulsionCH-96 resin was studied by batch equilibration method.The experiments involved equilibration of the resin withthe solution containing uranium(VI) (4.2.10–3 or4.2.10–2M) diluted in desired concentration of nitricacid. At various intervals, equilibration was stopped andthe concentration of uranium present in the solution wasdetermined as described above.Langmuir adsorption isothermLoading of uranium(VI) on Tulsion CH-96 resin was studied by equilibrating 0.5 g of the resin with 10 ml of the solution containing the desired concentration of uranium in 3.0M or 4.0M nitric acid. The concentration of uranium(VI) in the solution was varied from 4.2.10–4to 2.1.10–1M. The concentration of uranium in resin phase was measured and plotted against theconcentration of the aqueous phase to obtain the Langmuir isotherm. Breakthrough experiments: The performance of the resin under dynamic condition was assessed by column breakthrough experiments. In these experiments 10 g(=24 ml bed volume) of the resin was immersed inwater and loaded into a glass column of radius 0.5 cm.The resin bed was washed with 250 ml of nitric acid of concentration equivalent to that of feed. Theexperimental feed solution containing 4.2.10–3M (or4.2.10–2M) uranium present in desired concentration ofnitric acid was passed at desired flow rate (0.5 ml/min to2 ml/min). The effluent was collected at variousintervals of time and analyzed for uranium. The ratio ofthe concentration of uranium in the effluent (C ) to thatof feed (C 0) was plotted against the volume of thesolution passed through the bed to obtain thebreakthrough curve. Breakthrough was followed up toC /C 0=0.8 and then the column was washed with 4.0M sodium nitrate solution. Elution of uranium from Tulsion CH-96 resin was carried out using 0.5Mammonium carbonate solution.Fig. 1. Preparation scheme of phosphinic acid resinK. A. V ENKATESAN et al.: B ATCH AND DYNAMIC EXTRACTION OF URANIUM (VI) FROM NITRIC ACID565Results and discussionEffect of [HNO 3]Distribution coefficient [K d of U(VI)] of uranium(VI) as a function of concentration of nitric acid on Tulsion CH-96 resin is shown in Fig. 2. K d of U(VI) initially decreases from 395 ml/g at 0.1M nitric acid, reaches a minimum value at 1.3M. Further increase in the concentration of nitric acid, increases the K d value of U(VI), reaching a maximum of 1988 ml/g at 5.0M nitric acid followed by a decrease. Similar trend was also reported for the diffusion coefficients of uranium(VI) in MPBPA resin.10K d of U(VI) for laboratory synthesized MPBPA resin 13 also reported to decrease from 3055 ml/g at 0.1M nitric acid and reach a minimum value between 1–2M in nitric acid followed by continuous increase in U(VI) K d values. Higher U(VI) K d value observed in this case could be due to the employment of trace levels of U(VI) for extraction. This unusual trend, not observed for conventional ion-exchangers, indicates that different types of mechanisms are responsible for the extraction of uranium(VI) from nitric acid medium. Uranyl ion (UO 22+) in nitric acid medium forms a series of complexes with nitrate ion as shown in Eqs (3) to (6):UO 22+ + NO 3– [UO 2(NO 3)]+(3) UO 22+ + 2 NO 3– [UO 2(NO 3)2] (4)UO 22+ + 3 NO 3– [UO 2(NO 3)3]–(5) UO 22+ + 4 NO 3– [UO 2(NO 3)4]2– (6) In 0.1M nitric acid medium uranium existsessentially as hydrated UO 22+. Increase in theconcentration of nitric acid favors the complexation ofuranyl ion with nitrate leading to neutral and anioniccomplexes. Phosphinic acid being a weak acidic ion-exchanger,1,4 the initial decrease in the distributioncoefficient with the increase in the concentration ofnitric acid, from 0.1M to 1.5M, could be attributed to the ion-exchange of uranyl ion with H + of phosphinic acid as shown in Eq. (7):2>P(=O)OH + UO 22+ (>P(=O)O)2UO 2 + 2 H + (7) Increase in the concentration of nitric acid from 0.1M, shifts the equilibrium reaction shown in Eq. (7) towards left and thus the K d of U(VI) values decrease. However, when the concentration of nitric acid is higher than 1.5M, the formation of neutral uranyl nitrate species are favored. Since these neutral species are also extracted by phosphinic acid resin through >P=O co-ordination, as shown in Eq. (8), the distribution coefficient of uranium increases further and reaches a maximum at 5.0M nitric acid. Further increase in nitric acid concentration favors anionic complexes of uranium [Eqs (5) and (6)], which are inextractable by the resin leading to the decrease in K d of U(VI) values:>P=O + UO 2(NO 3)2 >P=O---(UO 2(NO 3)2) (8) Extraction rate and effect of batch factorThe rate of extraction of uranium(VI) by Tulsion CH-96 resin at different initial concentrations of uranium and nitric acid is shown in Fig. 3. Rapid extraction is observed in the initial stages of equilibration followed by the establishment of equilibrium occurring within eight hours when the initial concentration of uranium is 4.2.10–3M. When theconcentration of uranium in the feed is increased to4.2.10–2M, the equilibrium is not established even after10 hours. The amount of uranium loaded in the resinincreases with the increase in the initial concentration ofuranium and nitric acid concentration. The effect ofbatch factor, namely the ratio of volume of the solutionto mass of the resin taken for equilibration, on thedistribution coefficient of uranium is shown in Table 1.Table 1. Variation of distribution coefficient of uranium(VI) with batch factor (V /m )3.0M HNO 34.0M HNO 3Batch factor, ml/g U(VI) K d , ml/g Extraction, % U(VI) K d , ml/g Extraction, % U(VI) = 4.2.10–3M 2040100200642506189109979465351390100325018199967147U(VI) = 4.2.10–2M20401002005.45.14.64.021124.42.19.08.87.96.8311893T =298 K, equilibration time = 10 hours.K. A. V ENKATESAN et al.: B ATCH AND DYNAMIC EXTRACTION OF URANIUM (VI) FROM NITRIC ACID566Fig. 2.Variation in the distribution coefficient of uranium(VI) with the concentration of nitric acid on Tulsion CH-96Fig. 3.Rate of extraction of uranium(VI) by Tulsion CH-96Fig. 4. Extraction isotherms of uranium(VI) on Tulsion CH-96K. A. V ENKATESAN et al.: B ATCH AND DYNAMIC EXTRACTION OF URANIUM (VI) FROM NITRIC ACID567Fig. 5. Langmuir plots for the extraction of uranium(VI) by Tulsion CH-96It is observed that both the U(VI) K d values andaccordingly the percentage of uranium(VI) extracted by the resin decrease with the increase in the value of batch factor. Increase in the initial concentration of uranium lowers the distribution coefficient and also reduces the difference in the K d of U(VI) values obtained at 20 ml/g and 200 ml/g.Langmuir adsorption isothermThe extraction isotherm of uranium on Tulsion CH-96 resin is shown in Fig. 4. Loading of uranium in the resin increases with increasing the aqueous phase concentrations of uranium(VI) ion and nitric acid. Figure 5 shows the linearized Langmuir adsorption isotherm for the extraction of uranium by Tulsion CH-96 at different nitric acid concentrations. The linearized Langmuir equation governing the amount of metal ion extracted (C s ) and its concentration in solution (C f ) is given by: b C b K C C f L s f 1 (9) where C f is the equilibrium concentration of uranium(mg/ml), C s is the amount of uranium extracted byTulsion CH-96 (mg/g), K L is the Langmuir adsorptionconstant (ml/mg) related to the affinity of the resin towards the metal ion, and b is the apparent extractioncapacity for uranium of Tulsion CH-96 under the studied conditions. The experimental capacity (b , mg/g) and K L are obtained from the slope and intercepts of the straight line are also shown in Fig. 5. The experimental capacity, b , increases from 55 mg/g to 70 mg/g when the concentration of nitric acid is increased from 3.0M to 4.0M. Higher magnitude of Langmuir constant (K L )obtained for the extraction of uranium(VI) by Tulsion CH-96 indicates the stronger affinity towards uranium at 4.0M nitric acid. Breakthrough studiesThe performance of Tulsion CH-96 resin under dynamic loading conditions can be evaluated by the breakthrough curve.16 Figure 6 shows the breakthrough (BT) curves for the extraction of uranium(VI) from3.0M nitric acid solution by Tulsion CH-96 resin. It isseen that the beginning of a breakthrough (or breakthrough edge) is a function of flow rate, and it decreases with increasing flow rate. 1% breakthrough occurs after passing 110 ml of the feed solution [4.6 bed volume (BV)] when passed at a flow rate of 2 ml/min and it is shifted to 600 ml (25 BV) when passed at a flow rate of 0.5 ml/min. Various simple models have been developed to describe the dynamic behavior of extraction of metal ions in a fixed bed column.17–21 TheT HOMAS model 17 is one of the most generally used fordescribing column performance, especially when the extraction follows Langmuir adsorption model. TheT HOMAS equation for describing a breakthrough curve isrepresented by:)/((0011Q V C Bm K T e C C (10)where C and C 0 are the concentration of uranium (mg/l) in the effluent and in the feed respectively; K T is the T HOMAS rate constant (ml/min .mg); B (mg/g) is the maximum loading capacity of uranium under the specified conditions; m (=10 g) is the mass of the resin taken in the column; V (l) is the throughput volume andQ is the flow rate in ml/min. Breakthrough data obtainedK. A. V ENKATESAN et al.: B ATCH AND DYNAMIC EXTRACTION OF URANIUM (VI) FROM NITRIC ACID568in this case were fitted using Eq. (10) by non-linear regression and the coefficients (K T and B ) obtained from regression were tabulated in Table 2. It is seen that at constant uranium concentration (4.2.10–3M), the increase of flow rate decreases the breakthrough capacity (BTC) of the resin under column conditions. It decreases from 73.7 to 36.4 mg/g when the flow rate is increased from 0.5 to 2 ml/min. Similarly, the effect of flow rate on the breakthrough behavior of the system from 4.0M nitric acid feed is shown in Fig. 7. Breakthrough edge occurs early with the increase in the flow rate of the feed as expected. Comparing Figs 6 and7, it is seen that the breakthrough edge also increases with the increase in the concentration of nitric acid. It is observed that 1% breakthrough occurs after passing 300 ml (12.5 BV) of 3.0M nitric acid feed whereas the same breakthrough occurs after passing 590 ml (24.6 BV) of 4.0M nitric acid feed under similar flow rates (1 ml/min). It is important to note to Table 2, that the BTC at 4.0M nitric acid is nearly two times higher than that observed for the extraction from 3.0M nitric acid, which could be due to the higher distributioncoefficient of uranium(VI) in 4.0M nitric acid.Fig. 6. Breakthrough profiles at various flow rates. Dynamic extraction of uranium(VI)by Tulsion CH-96 from 3.0M nitric acid mediumFig. 7. Breakthrough profiles at various flow rates. Dynamic extraction of uranium(VI)by Tulsion CH-96 from 4.0M nitric acid mediumK. A. V ENKATESAN et al.: B ATCH AND DYNAMIC EXTRACTION OF URANIUM (VI) FROM NITRIC ACID569Effect of initial concentration of uranium(VI) in 3.0M nitric acid feed solution on the breakthrough behavior of the system is shown in Fig. 8. Early breakthrough is observed with increasing of concentration. Beginning of 1% breakthrough is observed after passing 130 ml (5.4 BV) when theconcentration of uranium is 4.2.10–2M and is shifted to600 ml (25 BV) when the concentration of uranium is ten times lower. However, the capacity values obtained from the T HOMAS model, as tabulated in Table 2, indicate that the BTC increases with the increase in the concentration of the feed. It is noted that when the concentration of uranium in the feed increases from 4.2.10–3M to 2.1.10–2M, BTC increases from 73.7 mg/g to 215 mg/g. Further increase in the concentration to 4.2.10–2M does not result in any appreciable increase of BTC, indicating the saturation of the column capacity under the studied dynamic conditions. Effect of columndiameter on the breakthrough behavior of the system is shown in Fig. 9. Breakthrough edge decreases with increase in the diameter of the column taken for the column study.The column utilization can be defined as the ratio of the amount of uranium extracted by a gram of Tulsion CH-96 resin under the given column condition to the theoretical capacity (i.e., 595 mg/g for 8% P). It is seen from Table 2 that the column utilization increases with the increase in the concentrations of nitric acid and uranium ion in the feed and decreases with the increase of flow rate. Tulsion CH-96 can be effectively utilized up to 35% of theoretical capacity at higher uranium ion concentrations in the feed solution. The loaded column was washed with 3.0M ammonium nitrate solution and eluted using 0.5M ammonium carbonate (~250 ml) bythe procedure described elsewhere.12Table 2. Breakthrough results obtained from the T HOMAS model[HNO 3],M Flow rate, ml/min [U(VI)], M K T ,u 102, ml/min .mgColumn capacity,(B ) mg/g F 2u 103Column utilization, % 3 0.5 1.02.0 4.2.10–34.2.10–34.2.10–30.971.501.8673.746.436.40.622.923.0512.27.56.13 0.5 2.1.10–24.2.10–20.160.14215.9211.2 3.722.0036.535.54 0.5 1.02.0 4.2.10–34.2.10–34.2.10–30.250.561.78133107652.542.353.9222.318.011.0Fig. 8. Breakthrough curves for the extraction of uranium by Tulsion CH-96at various initial concentrations of uranium in the feedK. A. V ENKATESAN et al.: B ATCH AND DYNAMIC EXTRACTION OF URANIUM (VI) FROM NITRIC ACID570Fig. 9. Effect of column diameter on the breakthrough curvesfor the extraction of uranium(VI) by Tulsion CH-96ConclusionsThe distribution coefficient of uranium(VI) on a commercially available phosphinic acid resin, Tulsion CH-96, decreases from 396 ml/g at 0.1M nitric acid to a minimum value of 150 ml/g at 1.3M is attributed to the ion-exchange of uranyl ion with protons of phosphinic acid. Increase in K d of U(VI) above 1.5M nitric acid is due to co-ordination of >P=O moiety present in Tulsion CH-96 with neutral uranyl nitrate species. Rapid extraction is observed in the initial stages of equilibration followed by the establishment of equilibrium occurring within eight hours. Higher magnitude of K L obtained from Langmuir adsorption model indicates favorable extraction of uranium(VI) from 4.0M nitric acid medium by the resin. Breakthrough edge decreases with increasing flow rate, concentration of uranium in the feed and lowering of nitric acid concentration. Breakthrough capacity and column utilization factor increases with the increase in concentrations of nitric acid and uranium ion in the feed and decrease with the increase in the flow rate.Thus, the commercial Tulsion CH-96 is a very promising resin for the extraction of uranium from nitric acid medium representing nuclear wastes. Both -OH and P=O sites present in the resin can act as extracting groups. However, the resin needs to be improved from the point of view of enhancing the rate of extraction especially at higher concentration of uranium in the feed.*The authors wish to thank Mr. K. S URESH , M.Sc. Student, PSG College of Arts and Science, Coimbatore – 641 014, India for assistance.References1. S. D. A LEXANDRATOS , M. A. S TRAND , D. R. Q UILLEN ,A. J. W ALDER , Macromolecules, 18 (1985) 829.2. R. A. B EAUVAIS , S. D. A LEXANDRATOS , React. Funct. Polym., 36(1998) 113.3. A. M. E L -N AGGAR , A. S. E MARAT , S. G. A BD A LLA , Polym.Degr. Stab., 58 (1997) 97.4. A. W. 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H ELFFERICH , Ion Exchange, McG raw-Hill Book Co., NewYork, 1962.17. H. C. T HOMAS , J. Am. Chem. Soc., 44 (1964) 1664. 18. Z. A KSU , F. G ONEN , Process Biochem., 39 (2004) 599.19. T. M ATHAIALAGAN , T. V IRARAGHAVAN , J. Hazard. Mater., B94(2002) 291.20. R UEY -S HIN J UANG , S U -H SIA L IN , K UNG -H SUEN T SAO , J. ColloidInterface. Sci., 269 (2004) 46. 21. M. V. S IVAIAH , K. A. V ENKATESAN , P. S ASIDHAR ,R. M. K RISHNA , G. S. M URTHY , J. Nucl. Radiochem. Sci., 5 (2004) 7.。