Recovery of Actinides and Lanthanides from High-Level Waste Using

Aug 3, 2009 - Recovery of Actinides and Lanthanides from High-Level Waste Using Hollow-Fiber Supported Liquid Membrane with TODGA as the Carrier. Sera...
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Ind. Eng. Chem. Res. 2009, 48, 8605–8612

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SEPARATIONS Recovery of Actinides and Lanthanides from High-Level Waste Using Hollow-Fiber Supported Liquid Membrane with TODGA as the Carrier Seraj A. Ansari, Prasanta K. Mohapatra, and Vijay K. Manchanda* Radiochemistry DiVision, Bhabha Atomic Research Centre, Trombay, Mumbai, 400085, India

Transport behavior of trivalent actinides/lanthanides from nitric acid solutions into distilled water was investigated using polypropylene hollow-fiber supported liquid membrane (HFSLM) technique. A mixture of 0.1 M TODGA (N,N,N′,N′-tetraoctyl diglycolamide) + 0.5 M DHOA (N,N-dihexyl octanamide) dissolved in normal paraffinic hydrocarbon (NPH) was optimized as a suitable carrier. The permeability of the transported metal species was calculated with the help of various diffusional parameters. The permeation of metal ions increased with the feed acidity; however, their transport rates were significantly affected at higher acidity (>3 M HNO3) owing to carrier-aided co-transport of nitric acid. Mass transfer coefficients were calculated on the basis of the described model, whose success was verified by excellent matching of the membrane diffusion coefficient values calculated by the given model and Wilke-Chang equation. Quantitative transport of trivalent actinides and lanthanides was achieved in 30 min from pressurized heavy water reactor simulated high-level nuclear waste. The stability of the HFSLM system was found to be satisfactory in eight successive runs. The results suggested the possible application of TODGA-HFSLM system for the recovery of actinides from high-level wastes. 1. Introduction Effective management of radioactive wastes emanating from the back end processes in the nuclear fuel cycle will ultimately improve the public acceptance of the nuclear energy program. A radioactive waste management strategy, commonly referred to as “Actinide Partitioning”, aims at bringing down the radiotoxicity of the waste drastically by removing minor actinides. Here the minor actinides (Np, Am and Cm) along with trivalent lanthanides are separated from high level radioactive waste (HLW).1-3 Several processes for actinide partitioning (viz., TRUEX, TRPO, DIDPA, and DIAMEX) utilizing the reagents such as octyl(phenyl)-N,N-diisobutyl carbamoyl methyl phosphine oxide (CMPO), trialkyl phosphine oxide (TRPO), diisodecyl phosphoric acid (DIDPA), and N,N′-dimethyl-N,N′dioctyl hexylethoxy malonamide (DMDOHEMA), respectively, have been developed at various laboratories.3,4 Though all these extractants are capable of extracting trivalent actinides such as Am3+ and Cm3+ from HLW, malonamide-based extractants have been found to be particularly promising in view of their improved back-extraction properties, complete incinerability (as these molecules are based on C, H, N, and O elements, they can be combusted to gaseous products), and the innocuous nature of their radiolytic degradation products (mainly carboxylic acids and amines that can be easily washed out).5 Recent attempts to increase the efficiency of malonamide-based extractants introduced several structural modifications including one that led to the introduction of an etherial oxygen in between the two amide groups (diglycolamide), which caused significant improvement in the extraction efficiency of trivalent actinides from HLW.6-8 Among various derivatives of diglycolamide, N,N,N′,N′-tetraoctyl diglycolamide (TODGA, Figure 1) appeared to be the most promising due to its favorable physicochemical properties.9-13 * To whom correspondence should be addressed. E-mail: vkm@ barc.gov.in. Fax: 91-22-25505151.

Though TODGA-based solvent extraction methods appeared promising, in view of the environment concerns and cost considerations, alternative techniques with low organic extractant burden need to be explored. Among such methods, liquid membrane (LM) based techniques are particularly attractive as they can overcome two major drawbacks of the solvent extraction processes, namely, third phase formation and phase disengagement, since the former technique essentially involves nondispersive mass transfer. We have investigated the transport behavior of several actinide and lanthanide ions using TODGAimpregnated flat sheet supported liquid membrane (FSSLM).14,15 These investigations revealed that the quantitative transport of Nd (as a representative of trivalent actinides and lanthanides) required >45 h from a feed solution of 1 g/L Nd at 3 M HNO3. The FSSLM transport data also indicated remarkable stability of the membrane, though the flux was quite low for actual process application. Lower transport of metal ions associated with small specific membrane surface area encountered in FSSLM could be alleviated using hollow-fiber supported liquid membrane (HFSLM). This configuration of SLM by using tubular hollow fibers (HFSLM) has the high metal transport flux as the effective surface area in such systems increased to several order of magnitudes. The HFSLM systems have advantages over flat sheet SLM because a large specific membrane area is available and also degraded membranes can be easily regenerated by passing the ligand solution through the module. Several reports on HFSLM technology for possible application in the “actinide partitioning” or “lanthanide-actinide

Figure 1. Structural formula of N,N,N′,N′-tetraoctyl diglycolamide (TODGA).

10.1021/ie900265y CCC: $40.75  2009 American Chemical Society Published on Web 08/03/2009

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Table 1. Composition of PHWR-SHLW Used in the Present Studies. Acidity: 3 M HNO3 element

concentration (g/L)

element

concentration (g/L)

Na K Cr Mn Fe Ni Sr Y Zr

5.50 0.22 0.12 0.43 0.72 0.11 0.03 0.06 0.004

Mo Cs Ba La Ce Pr Nd Sm U

0.14 0.32 0.06 0.18 0.06 0.09 0.12 0.086 6.34

separation” stages are available. Kathios et al. demonstrated the utility of the membrane-based extraction modules for the extraction of Nd(III) which was used as a surrogate of trivalent actinides present in the high level waste.16 The extractants tested by them was CMPO in di-iso-propyl benzene while the strippant used was 0.01 M HNO3. Geist et al. used the hollow-fiber membrane technique for lab-scale separation of actinides from lanthanides from various solutions using 2,6-bis(5,6-dipropyl1,2,4-triazin-3-yl)pyridine (BTP) and dichlorodiphenyl dithiophosphinic acid as the extractants.17 The objective of the present work was to develop HFSLM technique for the quantitative recovery of actinides and lanthanides from HLW using TODGA as the carrier. Nd(III) has been used in the present work as a surrogate of the trivalent minor actinides (Am, Cm) and lanthanides. No attempt has been made in the present work to investigate the selectivity. The investigation is focused on the selection of the most suitable conditions for the system in order to improve the metal permeation, especially for their potential application in the large scale separation of the metal ions from HLW solution. 2. Experimental Section 2.1. Materials. N,N,N′,N′-Tetraoctyl diglycolamide (TODGA, Figure 1) was synthesized indigenously by following the procedures described elsewhere.12 The synthesized product was characterized by elemental analysis, 1H NMR, and IR spectroscopy. In the PMR spectrum, the signals were singlet at 4.39, triplets at 0.92 and 3.18, and multiplets at 1.26 and 1.56 ppm. These signals were assigned to protons of -CO-CH2-OCH2-CO-, -N-CH2-CH2-(CH2)5-CH3, -N-CH2-CH2(CH2)5-CH3, -N-CH2-(CH2)5-CH2-CH3, and -N-CH2(CH2)5-CH2-CH3, respectively. The appearance of a characteristic vibrational frequency at 1640 cm-1 in the FT-IR spectrum demonstrated the presence of carbonyl groups of diglycolamide. Elemental analysis yielded 73.1% C, 12.8% H, and 5.2% N against theoretical values of 74.5, 12.4, and 4.8%, respectively. Radiotracer 241Am was purified prior to use and its purity was ascertained by gamma spectroscopy. Nd(III) stock solutions were prepared with Nd2O3 procured from Alpha Biochem having purity of 99.9%. Methyl thymol blue indicator was procured from Fluka Chemie AG, Switzerland. All the other reagents were of AR grade and were used without further purification. Simulated high level waste (SHLW) was procured from Waste Management Division, Bhabha Atomic Research Centre, Mumbai, India. The composition of SHLW solution (Table 1) was equivalent to the generation of HLW after the reprocessing of a typical pressurized heavy-water reactor (PHWR) spent fuels of ∼6500 MWD/T (megawatt day per ton) burn up. The PHWR-HLW is expected to contain about 50-100 mg/L of minor actinides and about 0.5-1 g/L lanthanides in addition to about 6 g/L U. The acidity of SHLW was determined by acid-base titration in the presence of phenolphthaleinneutralized saturated potassium oxalate.18 Since, SHLW contains

Table 2. Specifications of Hollow-Fiber Membrane Contactor (Liqui-Cel X50: 2.5 × 8 Membrane Contactor) fiber material number of fibers fiber internal diameter (µm) fiber outer diameter (µm) fiber wall thickness (µm) effective pore size (µm) porosity (%) tortuosity effective fiber length (cm) effective surface area (m2)

polypropylene 9950 240 300 30 0.03 40 2.5 15 1.4

a large concentration of metal ions, they will be hydrolyzed during titration with NaOH. Therefore, the metal ions are masked by complexing with oxalate ions. 2.2. Hollow-Fiber Supported Liquid Membrane Studies. The hollow fiber module used in the present work was made up of about 10 000 hydrophobic microporous polypropylene hollow fibers enclosed in a polypropylene shell. Further details of the module are summarized in Table 2. The hollowfiber supported liquid membrane (HFSLM) was prepared by pumping ligand solution through the lumen side of module at a pressure of 20 kPa in recirculation mode. To ensure the complete soaking of the membrane pores, the ligand solution was circulated for ∼15 min when the solution started percolating from lumen side to the shell side. The excess of ligand solution was washed out completely with sufficient distilled water, prior to the introduction of the feed and strip solutions. For all the experiments, the feed solution was passed through the lumen side while strip solution was passed through the shell side of the module in recirculation mode. Both feed and receiver phases were contacted inside the hollow-fiber module in counter-current flow for the effective transport of metal ions. The flow rates of the feed and strip solutions were maintained constant at 200 mL/min with the help of gear pumps equipped with precise flow controllers. A schematic diagram of hollow fiber unit is shown in Figure 2a. The volume of feed and strip solutions were usually 500 mL each, except were stated otherwise. Distilled water was employed as strippant throughout the study. 241Am tracer, in the concentration range of 1 × 10 -6 to 3 × 10 -6 M was used in these studies. The permeability of the metal ion was obtained by estimating the feed as well as the strip samples at regular time interval. Assay of 241Am was carried out using a well-type NaI(Tl) scintillation detector interphased to a multichannel analyzer. Estimation of Nd(III) was carried out by EDTA complexometric titration using methylthymol blue as the indicator. Acid transport in the strip solution was determined by the volumetric method using saturated potassium oxalate as described above. The elemental analysis of SHLW was performed by ICP-AES technique to monitor the selectivity of separation of trivalent lanthanides over other constituents of SHLW. 2.3. Theoretical Calculations. The extraction of trivalent lanthanides (M3+) from nitric acid medium by neutral extractant, TODGA, has been reported as19 M3+(aq) + 3NO3-(aq) + 3TODGA(org) h M(NO3)3 · 3TODGA(org)

(1)

where (aq) and (org) represent the aqueous and the organic phases, respectively. The term M(NO3)3 · 3TODGA represents the extracted species in the organic phase. In the presence of

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3+

The value of Kex for lanthanide (i.e., Nd ) with TODGA in NPH was calculated from the distribution ratio (Kd) of the Nd. Considering linear concentration gradients, fast interfacial reactions, distribution coefficients of Nd between the hollowfiber membrane phase and the stripping phase is much lower than that between the feed phase and membrane phase, the final equation for permeability of metal ion could be obtained as described:20-22 ln(Ct /Co) ) -

AP φ t V φ+1

(5)

where P is the overall permeability coefficient, Ct and Co are the respective concentrations of the metal ions in the feed solution at an elapsed time t (min) and at zero time, and V is the total volume of the feed solution (mL). Here, the parameter A represents the total effective surface area of the hollow fiber (cm 2), which is calculated as follows: A ) 2πriLN∈

(6)

where, ri is the internal radius of the fiber (cm), L is the length of the fiber (cm), N is the number of fibers, and ∈ is the membrane porosity. The parameter, φ, for a module containing N number of fibers is expressed as follows: φ ) QT / (PriLNπ ∈ )

Figure 2. Schematic representation of the facilitated transport of metal ions in hollow-fiber liquid membrane: (a) Hollow- fiber module, (b) flow pattern in HFSLM, and (c) transport scheme of metal ions in liquid membrane. (L) ligand, (M-L) metal-ligand complex. 12

nitric acid in the feed, TODGA forms adducts such as TODGA · HNO3 and the equilibrium constant (KH) is given as KH

H+ + NO\z TODGA · HNO3(org) 3 + TODGA(org) y (2) We have reported12 that KH is 4.1 and can lead to significant amount of acid to be coextracted along with the extracted metal ion. Further, as only 0.1 M TODGA is used as the carrier extractant while the PHWR-SHLW (Table 1) contains as high as 0.6 g/L rare earth elements, the possibility of third phase formation is prevented by the addition of 0.5 M DHOA.12 Acid extraction into the organic phase is also due to DHOA which also forms adducts such as DHOA · HNO3. The extraction constant (Kex) for the equilibrium reaction 1 can be represented as Kex )

[M(NO3)3 · 3TODGA](org) 3 3 [M3+] · [NO3 ] · [TODGA] (org)

(3)

Similarly, the distribution coefficient (Kd) of the metal ion in equilibrium reaction 1 can be represented as Kd )

[M(NO3)3 · 3TODGA](org) [M3+]

(4)

(7)

where QT is the total flow rate of the feed solution (mL/min). The various parameters of the hollow-fiber module employed in the present study are described in Table 2. By plotting ln(Ct/ Co) as a function of t a straight line is expected, and according to eqs 6 and 7 the P value for the given system can be obtained from the fitted slope of eq 5. Mass transfer modeling was done to calculate the mass transfer coefficients and diffusion coefficients. 3. Results and Discussion 3.1. Transport of Neodymium from HNO3 Solution. The mass transfer reaction of the metal ions through liquid membrane involves extraction at the feed-membrane interface, diffusion of metal-ligand complex inside the membrane pores, and finally, stripping of the metal ions at the membrane-strip interface. The extraction of trivalent lanthanides using TODGA as the extractant has been represented by the equilibrium reaction 1. At the aqueous feed-membrane interface, the equilibrium reaction 1 is shifted to right and the [M(NO3)3 · 3TODGA] complex then moves toward the receiving phase within the membrane as a consequence of concentration gradient. On the other hand, at the membrane-strip interface, the equilibrium reaction 1 is completely shifted to the left because of unfavorable conditions in the strippant phase. This results in dissociation of the [M(NO3)3 · 3TODGA] complex leaving behind free ligand molecules in the membrane phase. The free carrier molecules then move toward the feed solution due to negative concentration gradient to complete the cycle. The transport mechanism of metal ions through SLM is depicted in Figure 2c. As per the empirical Wilke-Chang equation,23 the bulk diffusion coefficient of the transported species (Do) is defined as Do ) 7.4 × 10-8(χ0.5M0.5T3600)/(ηVm0.6)

(8)

where M, χ, and η are the molecular weight, solvent association parameter, and the viscosity of the solvent, respectively, Vm is the molar volume of the carrier,24 and T is the temperature.

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Figure 3. Effect of DHOA concentration on the transport of Nd. Carrier, 0.1 M TODGA + DHOA in NPH; feed, 1 g/L Nd at 3.5 M HNO3 (500 mL); strippant, distilled water (500 mL); flow rate, 200 mL/min; temperature, 25 °C; fitted lines represent the calculated values of transport data.

Figure 4. Effect of TODGA concentration on the transport of Nd. Carrier, TODGA + 0.5 M DHOA in NPH; feed, 1 g/L Nd at 3.5 M HNO3 (500 mL); strippant, distilled water (500 mL); flow rate, 200 mL/min; temperature, 25 °C; fitted lines represent the calculated values of transport data.

The Wilke-Chang equation depicts that the diffusion coefficient of the forward transported metal-ligand complex [M(NO3)3 · 3TODGA] should be much lower than the diffusion coefficient of the backward transported bare ligand molecules due to smaller molar volume of the later. This fact implies that the concentration of ligand at the feed-membrane interface will always be in excess because of faster diffusion of bare ligand as compared to the bulky metal-ligand complex. It is evident from the equilibrium reaction 1 that the extraction of the metal ions, and thereby the permeation, will be governed by the concentration of carrier ligand, nitrate ions concentration in the feed solution, and the metal solute concentration. The membrane permeability of the transported species is generally expressed by KdDmem/(dτ), where Kd is the distribution coefficient of the metal ions, Dmem is the diffusion coefficient of the extracted complex in the membrane, τ is the tortuosity, and d is the thickness of the membrane.25 To increase the permeability and selectivity of the transported species, one has to use a selective ligand with satisfactory Kd value, high Dmem value, and minimum membrane thickness. TODGA dissolved in paraffinic solvent shows good Kd values for trivalent lanthanides/ actinides as reported earlier.9,19 The idea of using a thin organic layer separating two aqueous phases (as in the case of SLM) seems very attractive because the Dmem value in liquids is several times higher than in solid polymer and inorganic membranes.26 3.1.1. Effect of Carrier Concentration. Extensive solvent extraction studies on actinide partitioning have been reported from our laboratory employing 0.1 M TODGA + 0.5 M DHOA as the extractant.12,13 Here DHOA has been used as a phase modifier as 0.1 M TODGA forms a third phase at very low metal ions concentration in the organic phase (limiting organic concentration, LOC, of Nd by 0.1 M TODGA/n-dodecane at 3 M HNO3 ) 0.0065 M). DHOA has been chosen among several other phase modifiers and has been successfully employed in mixer-settler studies for the extraction of actinides and lanthanides from PHWR-SHLW.13 However, there was an interest to investigate the effect of DHOA concentration on the permeation of metal ions in a HFSLM system. In this context, the transport of Nd was investigated using 0.1 M TODGA with varying concentration of DHOA and the results are shown in Figure 3. It was observed that the transport of Nd increased significantly in the presence of DHOA. However, the effect of DHOA

concentration was insignificant on the transport of Nd in the concentration range of 0.25-0.75 M. Insignificant change in the transport of Nd beyond 0.25 M DHOA confirmed its role as a phase modifier only. It appears that the addition of DHOA suppresses the possibility of aggregate formation of TODGA in the pores of membrane (as reported earlier27), thereby enhancing the transport rate. The transport of Nd was also investigated with varying concentration of TODGA along with fixed concentration of DHOA (0.5 M) and the results are shown in Figure 4. As expected, the transport of Nd increased with the TODGA concentration, which can be explained on the basis of equilibrium reaction 1 where the formation of metal ligand complex is favored at higher ligand concentration. Though higher transport of Nd could be obtained with increased ligand concentration, it also enhanced the transport of acid. On the basis of our experience in counter-current extraction studies involving SHLW13 and the observations made in the present study, a mixture of 0.1 M TODGA + 0.5 M DHOA/NPH was selected as the optimum carrier concentration for all the subsequent studies to evaluate different parameters. 3.1.2. Effect of Feed Acidity. The effect of feed acidity on the transport of Nd was investigated from a feed solution containing 1 g/L Nd and the results are shown in Figure 5. The results revealed an enhanced permeation rate of Nd with the feed acidity up to 3.5 M HNO3. With increased nitric acid concentration, the nitrate ions concentration is also increased and, therefore, equilibrium reaction 1 is favored due to a salting out effect. Further increase in the feed acidity suppressed the metal permeation rate due to co-transport of nitric acid in the receiver phase. Investigation of strip-phase acidity confirmed the higher permeation of nitric acid with increased feed acidity. As already discussed, this co-transport is partly ascribed to the extraction of TODGA · HNO3 and DHOA · HNO3 adducts into the organic phase which diffuse through the membrane phase and get stripped at the strip phase. At 3.5 M of feed acidity, the strip-phase acidity reached to 0.7 M after 60 min, which increased to 0.95 M when feed acidity increased to 6 M HNO3. Transport of such a high acid into the strip phase suppresses the dissociation of the metal-ligand complex at the membraneaqueous strip interface, thereby reducing the metal transport. The P values calculated from the fitted slope of Figure 5 are tabulated in Table 3. It was interesting to observe that quantitative transport of Nd could be achieved in 45 min of operation

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Figure 5. Influence of feed acidity on the permeation of Nd by HFSLM. Carrier, 0.1 M TODGA + 0.5 M DHOA in NPH; feed, 1 g/L Nd (500 mL); strippant, distilled water (500 mL); flow rate, 200 mL/min; temperature, 25 °C. Table 3. Permeability Coefficient (P) of Nd by HFSLM System at Different Feed Aciditya [HNO3] (M) 0.5 1.0 2.0 3.5 6.0

P (cm/min) -4

2.97 × 10 9.97 × 10-4 2.35 × 10-3 4.32 × 10-3 1.79 × 10-3

% transport in 45 min 3.5 33.6 87.9 99.9 82.6

a Carrier, 0.1 M TODGA + 0.5 M DHOA/NPH; feed, 1 g/L Nd (500 mL); receiver, distilled water (500 mL); flow rate, 200 mL/min; temperature, 25°C.

from feed solution of 1 g/L Nd at 3.5 M HNO3. It should be noted that under identical conditions of feed and strip solutions in flat sheet SLM, more than 45 h were required for quantitative transport of Nd when the feed and strip solutions were 20 mL. Higher transport of Nd in the case of HFSLM is very obvious due to large surface area and favorable hydrodynamic condition of the system. 3.1.3. Effect of Solute Concentration. The transport of Nd was investigated at various feed solute (Nd) concentrations at 3.5 M HNO3. The results revealed that quantitative transport of Nd was possible in 99.9%, 91.1%, 40.9%, and 14.8% for 0.1, 1, 3, and 6 g/L Nd at 3.5 M HNO3, respectively. As shown in Figure 6, the rate of permeation decreased with increased Nd concentration in feed solution suggesting the saturation effect. The P values calculated from the fitted slope of Figure 6 for 0.1, 1, 3, and 6 g/L Nd were 8.80 × 10-3, 4.32 × 10-3, 9.78 × 10-4, and 2.57 × 10-4 cm/min, respectively. Lower P values at higher feed solute concentration was attributed to the metal loading effect in the liquid membrane. The carrier loading effect was further investigated by measuring the permeation of Nd at different feed-to-strip volume ratio at fixed metal ion concentration (1 g/L) in the feed solution. The experimental results were obtained while working with 0.5-6.5 L of the feed solution containing 1 g/L Nd at 3.5 M HNO3 and fixed volume of receiver phase (0.5 L distilled water). The results revealed saturation in the Nd transport at a feedto-strip ratio of 2 and 4 with 98% and 85% transport,

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Figure 6. Effect of feed metal ion concentration on the permeation of Nd by HFSLM. Carrier, 0.1 M TODGA + 0.5 M DHOA in NPH; feed, 3.5 M HNO3 (500 mL); strippant, distilled water (500 mL); flow rate, 200 mL/ min; temperature, 25 °C.

respectively. At this stage, the strip-phase acidity was 1 and 1.5 M HNO3, respectively. The presence of nitric acid in the strip phase will suppress the dissociation of the metal ligand complex at the membrane-strip interface, and therefore, will suppress the transport rate. It was observed that when the stripphase acidity was 2 M HNO3. The preconcentration factor (PF) was obtained as, PF ) Cst/Cfo, where Cst is the concentration of Nd in the strip solution at an elapsed time t and Cfo is the Nd concentration in the feed solution at zero time. After 60 min, the PF values were 1.0, 1.58, 2.56, and 2.20 for a volume ratio of 1, 2, 4, and 13, respectively. Though the PF value for the volume ratio of 4 was the highest, quantitative transport was not possible owing to the accumulation of nitric acid into the receiver phase. Nevertheless, quantitative transport may be achieved by replacing the strip solution with fresh one after a suitable time interval or neutralizing the strip-phase acidity. 3.2. Transport of Actinides and Lanthanides from SHLW. The main objective of the present work was the recovery of actinides and lanthanides from nuclear waste solution (HLW). In this context, the transport of Am(III) was studied using simulated HLW. The composition of a typical PHWR-HLW solution is given in Table 1. It is worth mentioning here that the actinide partitioning reagent, such as TODGA, cannot differentiate between trivalent lanthanides and actinides because of their similar chemical properties. Therefore, in the actinide partitioning step all the lanthanides and actinides are recovered together and later they are separated by different methods. Since, HLW contains lanthanides at several times higher concentration (∼50 times) than the minor actinides (Am, Cm, and Np), it was of interest to investigate the effect of rare-earth metal concentration on the transport of Am(III). Though macroconcentration of Nd was used, Am has been used at much lower concentration (typically in the range 1 × 10-6 to 3 × 10-6 M). By taking Nd as the representative element of lanthanides, the transport of Am(III) was investigated in the presence of 1 g/L Nd and the results are shown in Figure 7. It was evident that the quantitative transport of Am(III) was achievable in 45 min of operation under the given experimental conditions. The transport was also predicted theoretically from the permeability coefficient data

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P × 103 (cm/min)

% transport in 10 min

% transport in 20 min

0.0 5 23 29 47 53 71 77 95 101

9.07 9.30 10.3 9.58 9.10 10.6 9.30 9.10 10.8 9.04

83.1 84.2 81.9 80.6 83.3 84.5 86.5 81.0 87.1 80.4

99.5 100.2 100.3 102.2 100.6 100.5 99.8 98.9 101.4 101.7

a Carrier, 0.1 M TODGA + 0.5 M DHOA in NPH; feed, 3.5 M HNO3 spiked with 241Am tracer (500 mL); receiver, distilled water (500 mL); flow rate, 200 mL/min.

Figure 7. Transport of Am(III) by TODGA-HFSLM. Carrier, 0.1 M TODGA + 0.5 M DHOA in NPH; feed, 1 g/L Nd at 3.5 M HNO3 or PHWR-SHLW spiked with 241Am tracer (500 mL); receiver, distilled water (500 mL); flow rate, 200 mL/min; temperature, 25 °C; Fitted lines represent the calculated values of transport data.

which was inline with the experimental data. This observation suggested the possible use of the present HFSLM system for the recovery of actinides and lanthanides from PHWR-HLW. With this encouraging result, the transport of Am(III) using tracer was investigated from simulated PHWR-HLW whose composition is given in Table 1. It should be noted that PHWRHLW also contained ∼6 g/L of U, which will cause unnecessary loading of the carrier in the membrane. It is, therefore, recommended that HLW should undergo sufficient contact with 30% TBP (tributyl phosphate) to remove uranium quantitatively without affecting the concentration of trivalent lanthanides/ actinides. At this stage, Zr may also be removed at least partially. The present study was, therefore, carried out with SHLW devoid of any uranium and zirconium. As shown in Figure 7, quantitative transport of Am(III) was achieved in 30 min under PHWRSHLW conditions. Fast transport of Am(III) from PHWRSHLW (as compared to 1 g/L Nd feed) was ascribed to lower concentration of lanthanides (∼0.6 g/L) and salting out effect due to presence of a large concentration of metal nitrate salts. 3.3. Lifetime of the Liquid Membrane. The long-term stability of the HFSLM is important from the standpoint of its industrial application. For such stability test, the module was impregnated once with the carrier solution (0.1 M TODGA + 0.5 M DHOA) and the system was run continuously for several days. The permeation of Am(III) was measured at different time intervals for monitoring the stability of the impregnated membrane. From the data reported in Table 4, it can be observed that the Am(III) transport was consistent within the error limit even after 100 h of continuous operation. Pearson, et al.28 have given the probable causes of membrane instability as (a) loss of extractant by solubility in adjacent aqueous solution, (b) progressive wetabilty of the support pores induced by lowering of the organic-water interfacial tension which results from the surface active nature of the carrier molecules, and (c) the differential pressures existing between the inside and outside of the HFSLM caused by pumping of the solutions. Danesi, et al.22,29 reported that the loss of carrier from the pores is more significant when carrier molecules are very strong surfactants such as alkylaryl sulfonic acids and long chain quaternary ammonium salts. On the other hand, no significant loss was observed when weaker surface active carriers such as CMPO, TOPO, and long chain amines were used. They reported that the HFSLM system was quite stable for 60 days of continuous

operation with polypropylene hollow-fiber SLM containing tridodecylamine in n-dodecane as the carrier, when outside and inside hydraulic pressures were balanced. In the present work, TODGA being a weaker surface active carrier, we expect a similar stability of HFSLM though our study was restricted to only several successive cycles of continuous operation. Present observations indicate that HFSLM offers a promising alternative approach for actinide partitioning using TODGA as the carrier. The basic phenomena which occur at supported liquid membrane separation processes are sufficiently well understood to permit the scale-up of laboratory units to commercial scale. However, additional effort is needed to ensure the radiation stability of the hollow-fiber modules before applying the system to the actual HLW. 3.4. Mass Transfer Modeling. A liquid membrane composed of carrier solution (extractant) immobilized in the pores of a hydrophobic microporous support, selectively complexes one of the components present in the feed solution. The SLM separates the aqueous feed and strip solution, and the species of interest are selectively accumulated in the strip phase. The permeation of the species is due to a chemical potential gradient (the driving force of the process) existing on two sides of the SLM and proceeds mainly via three steps, that is, (1) complexation reaction at feed-membrane interface, (2) diffusion of the complex through the membrane phase, and (3) decomplexation reaction at membrane-strip interface. The mathematical model of a hollow-fiber supported liquid membrane for the separation of Nd using permeability coefficient (P) depends upon three mass transfer resistances as the number of steps in the transport mechanism is three.25,30 One of them is the resistance when the liquid is flowing through the hollow-fiber lumen. The second resistance is the diffusion of the Nd-TODGA complex across the liquid membrane which is immobilized in the porous wall of the fiber. The third resistance is due to the strip solution and organic interface at the outside of the fiber (shell side). These three resistances are related to the reciprocal of the overall permeability coefficient (1/P) by the following equation:25 ri ri 1 1 ) + + P kf rlmPm rokr

(9)

where ri, ro, and rlm are the internal radius, outer radius, and log-mean radius of the hollow-fiber tube, respectively. Pm is the membrane permeability; kf and kr are the aqueous feed and receiver phases mass transfer coefficient in tube and shell side, respectively. The membrane permeability (Pm) is related to the metal ion distribution coefficient (Kd) by the following equation:

Ind. Eng. Chem. Res., Vol. 48, No. 18, 2009

Pm ) K d k m )

3 3 Kex[NO3 ] [TODGA] km

8611

(10)

where km is the membrane mass transfer coefficient. Therefore, eq 9 becomes ri ri 1 1 ) + + 3 3 P kf rokr rlmkmKex[NO3 ] [TODGA]

(11)

Assuming that the stripping reaction is instantaneous at the membrane-strip interface, the contribution of the outer aqueous interface corresponding to the shell-side aqueous phase resistance is neglected. Therefore, eq 11 can be reduced as follows: ri 1 1 ) + 3 3 P kf rlmkmKex[NO3 ] [TODGA]

(12)

From eq 12, the mass transfer rate controlling step can be predicted. When liquid membrane is an organic extractant with low distribution coefficient of the metal ions, the contribution of the aqueous feed mass transfer coefficient (kf) to P is negligible and the supported liquid membrane resistance may control the mass transport rate. Similarly, if the organic extractant in the liquid membrane has high distribution coefficient of the metal ions, kf will have a significant role in the determination of the rate controlling step. 3.4.1. Calculation of Mass Transfer Coefficients and Diffusion Coefficient. The aqueous feed-phase mass transfer coefficient in tube (kf) can be obtained from the intercept of the linear fits of eq 12 by plotting 1/P vs 1/[TODGA].3 Similarly, the value of membrane mass transfer coefficient (km) was obtained from the slope of the same plot (i.e., slope ) ri/ (rlmkm)), as the radius of the hollow fiber is known. Here, the shell-side mass transfer coefficient (kr) is not included in the eq 12 as the decomplexation reaction at the membrane-strip interface is assumed to be instantaneous, and the term has been ignored. Figure 8 shows the plot of 1/P vs 1/ Kex[NO3-]3[TODGA]3(org). Experimental values show a good fitting of eq 12 with a regression coefficient R2 of 0.996. The values of kf and km were calculated as 1.01 × 10-4 and 1.95 × 10-5 cm/ s, respectively. The value of kf was much higher as compared to km, suggesting that the membrane mass transfer is the rate controlling step. The membrane diffusion coefficient (Dmem) for the Nd-TODGA complex in the liquid membrane phase is related to the solid support properties (thickness, porosity, and tortuosity) by the following equation:25 km ) Dmem /tmτ

(13)

where tm is the fiber wall thickness and τ is the tortuosity factor. The Dmem for the present system was calculated to be 1.46 × 10-7 cm2/s. The Dmem value was also calculated using the Wilke-Chang equation (eq 8) to verify the present model. The Dmem calculated by Wilke-Chang equation was 2.27 × 10-7 cm2/s, which gave excellent agreement with the value calculated using eq 13 and hence validating the transport model. 4. Conclusions Transport behavior of trivalent actinides and lanthanides from nitric acid solutions was investigated using hollow-fiber-based supported liquid membrane. A mixture of 0.1 M TODGA + 0.5 M DHOA dissolved in NPH was optimized as a suitable carrier for maximum transport of trivalent actinides and lanthanides. Mass transfer coefficients were calculated on the basis of the described model, whose success was verified by excellent

Figure 8. Plot of 1/P vs 1/ Kex[NO3-]3[TODGA]3(org) for the calculation of mass transfer coefficients.

matching of the membrane diffusion coefficient values calculated by the model and the Wilke-Chang equation. The permeability of lanthanides increased with the feed acidity. Quantitative transport of lanthanides and actinides was achieved within 45 min from a solution containing 1 g/L Nd at 3.5 M HNO3 as well as from PHWR-SHLW. The stability of the HFSLM was found to be satisfactory in eight successive runs. The results suggested the possible application of a TODGA-HFSLM system for the recovery of minor actinides from nuclear wastes. The separation of lanthanides(III) from actinides(III) (which is needed from the transmutation point of view of the actinides) was also successfully achieved by hollow-fiber supported liquid membrane using a different carrier.31 The work is in progress to couple the two HFSLM systems (one for total lanthanide/ actinide separation and other for lanthanide/actinide separation) to get pure fraction of lanthanides and trivalent actinides separately. Acknowledgment The authors thank Dr. C.P. Kaushik of Waste management Division, BARC, for providing PHWR-SHLW employed in the present work. Nomenclature CMPO ) carbamoyl methyl phosphine oxide DHOA ) di-n-hexyl octanamide DIAMEX ) diamide extraction DIDPA ) diisodecyl phosphoric acid DMDOHEMA ) N,N′-dimethyl-N,N′-dioctyl hexylethoxy malonamide HFSLM ) hollow-fiber supported liquid membrane HLW ) high level waste MWD/T ) megawatt day per tonne PHWR ) pressurized heavy water reactor SHLW ) simulated high level waste TODGA ) tetraoctyl diglycolamide TRPO ) trialkyl phosphine oxide TRUEX ) trans uranium element extraction Symbols A ) internal surface area of membrane Co ) initial metal ion concentration (at t ) 0) Ct ) concentration of metal ion in aqueous feed at time t Dmem ) membrane diffusion coefficient

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Kd ) distribution coefficient of the metal ions kf ) aqueous feed mass transfer coefficient km ) membrane mass transfer coefficient kr ) receiver phase mass transfer coefficient L ) length of the fiber N ) number of fibers P ) overall permeability coefficient Pm ) membrane permeability QT ) flow rate of the feed solution ri ) internal radius of the hollow-fiber tube rlm ) hollow-fiber log mean radius ro ) outer radius of the hollow-fiber tube tm ) wall thickness hollow-fiber tube V ) time average aqueous feed volume Vm ) molar volume of the carrier Greek Symbols ∈ ) porosity of the membrane τ ) tortuosity χ ) diluent association parameter η ) viscosity

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ReceiVed for reView May 9, 2008 ReVised manuscript receiVed July 14, 2009 Accepted July 16, 2009 IE900265Y