Liquid Membrane Studies Using a Hollow Fiber Contactor

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Diglycolamide-functionalized calix[4]arene for Am(III) recovery from radioactive wastes: Liquid membrane studies using a hollow fiber contactor Seraj A Ansari, Prasanta Kumar Mohapatra, Pankaj Kandwal, and Willem Verboom Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.5b04148 • Publication Date (Web): 25 Jan 2016 Downloaded from http://pubs.acs.org on February 3, 2016

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Industrial & Engineering Chemistry Research

Diglycolamide-functionalized calix[4]arene for Am(III) recovery from radioactive wastes: Liquid membrane studies using a hollow fiber contactor Seraj A. Ansari,1 Prasanta K. Mohapatra,1,* Pankaj Kandwal2,# and Willem Verboom3 1

Radiochemistry Division, Bhabha Atomic Research Centre, Trombay, Mumbai-400085,

India; 2 Dept. of Sciences & Humanities, NIT Uttarakhand, Temporary campusPolytechnic Institute, Srinagar (Garhwal)-246174, Uttarakhand, India; 3 Laboratory of Molecular Nanofabrication, MESA+ Institute for Nanotechnology, University of Twente, P. O. Box 217, 7500 AE Enschede, The Netherlands Key words: Hollow fiber membrane, diglycolamides, Actinide partitioning, Separation *: Corresponding author: E-mail: [email protected]; #: Computations: [email protected]

ABSTRACT The transport of Am(III) from nitric acid feeds was investigated using hollow fiber supported liquid membrane (HFSLM) containing a diglycolamide-functionalized calix[4]arene (C4DGA) as the carrier extractant. The effect of feed acidity and Nd(III) concentration (used to represent Am(III)) in the feed on the permeation of Am(III) was investigated. Complete permeation of Am(III) from the source to the receiver phase was possible within 30 minutes at tracer scale. The permeation of the metal ion was unaffected with the feed HNO3 concentration in the range of 2-4 M while the presence of macro quantities of Nd(III) in the feed solution suppressed the metal ion transport rates. The permeability coefficient, mass transfer coefficient, and various diffusional parameters of the diffusing metal-ligand complex were calculated to understand Am(III) transport behavior. The transport rates of Am(III) were predicted by a mathematical model with an excellent match between the experimental and calculated data. The present study gives an opportunity to use exotic ligands for radioactive waste treatment using a very low extractant inventory using HFSLM.

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1. INTRODUCTION Diglycolamides (DGAs), a new class of diamide extractants, have been extensively studied in research related to ‘actinide partitioning’, a key step in radioactive waste management.1-5

The

DGA

ligands

such

as

TODGA

(N,N,N’,N’-tetra-n-octyl

diglycolamide) display anomalous metal ion selectivities as trivalent actinide ions are better extracted as compared to the tetravalent or hexavalent actinide ions, which is an opposite trend expected from ionic potential considerations seen with analogous extractants such as CMPO (carbamoylmethylphosphine oxide) and malonamides (extraction of trivalent metal ions are expected to be less than tetravalent and hexavalent metal ions).6 Attempts to understand this anomalous selectivities of DGA ligands with felements revealed that these ligands form reverse micelles in paraffinic solvents, such as n-dodecane, which apparently facilitate Am(III) extraction to a much greater extent than that of Pu(IV) and U(VI).7-9 In view of the reverse micelle formation tendencies of the DGA ligands, attempts were made to synthesize multiple-DGA-functionalized ligands, where DGA moieties are appended to a pivot atom to form a tripodal platform10-14 or to a calix[4]arene15 or a pillar[5]arene skeleton16 to mimic the aggregate structure of DGA. During the last couple of years, DGA-functionalized calix[4]arene (C4DGA) ligands have been extensively studied for their complexation and separation behavior towards trivalent actinides from acidic feed solutions.17-20 Solvent extraction studies indicated that these C4DGA ligands show a several orders of magnitude higher extraction of Am(III) than the bare DGA ligands such TODGA.20 The results of solvent extraction studies with these ligands are very promising and encourage their use in supported liquid membranes, with significantly lower ligand inventory. Though solvent extraction has been widely used for the separation of metal ions in the nuclear industry, generation of large volumes of spent extractants as the secondary waste, particularly for the treatment of dilute waste solutions, encourages the use of other techniques of which the waste generation is several orders of magnitude lower. One such alternative technique is the liquid membrane, which is a combination of solvent extraction and membrane diffusion, which offers promising alternative metal ion separations from dilute feed solutions.21-23 Key features of liquid membrane based separations are (i) simultaneous extraction and stripping of the solute (in this case, metal

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ions), (ii) transport rates are controlled by the diffusion of the solute, and (iii) low secondary waste generation. Out of the liquid membranes, supported liquid membranes are particularly attractive and there are numerous studies, involving FSSLM (flat sheet supported liquid membrane) and HFSLM (hollow fiber supported liquid membrane), reported for the treatment of nuclear waste.21,22 Recently, we have demonstrated the feasibility of using the C4DGA ligand in FSSLM for the separation of Am(III) from nitric acidic feeds.24 The study was performed at 20 mL scale with a 3.14 cm2 membrane surface area and the results were highly encouraging. However, the small surface area in FSSLM does not allow any practical application of the method. In this context, use of the HFSLM technique, with significantly large surface area, will be a more efficient way for the recovery of Am(III) from radioactive waste solutions which could result in large throughput values. In the present paper, therefore, the separation behavior of Am(III) was explored using a C4DGA-based HFSLM. The effect of feed acidity and Nd(III) concentration on the Am(III) transport was studied. The permeability coefficient, mass transfer coefficient, and various diffusional parameters of the metal-ligand complex were calculated to understand the permeation behavior of Am(III). A mathematical model developed to predict Am(III) permeation rates was found to conform well to the experimental data. C8H17 N

C8H17 C8H17

O O O

N

O

C8H17 C8H17 C8H17 N C8H17 O N N C8H17 O O O O O O O O N N N

O

O

O

Figure 1. Structural formula of the diglycolamide-functionalized calix[4]arene (C4DGA)

2. EXPERIMENTAL SECTION 2.1. Materials. C4DGA (Figure 1) was synthesized by acylation of tetrakis(aminopropoxy)calix[4]arene with propionyl chloride, with subsequent reduction of the 3 - Environment ACS Paragon -Plus


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product with LiAlH4. Subsequently, the N-alkylated tetrakis(aminopropoxy)calix[4]arene was reacted with p-nitrophenol activated diglycolamide (DGA) in the presence of triethylamine to give the desired product, C4DGA. The detailed synthesis procedure has been described elsewhere.15 Characterized of the C4DGA was done by 1H-NMR, HR-MS as well as IR spectroscopy. The radiotracer

241

Am (from laboratory stock solution), was

freshly purified before use and its radiochemical purity was ascertained by gamma ray spectroscopy using an HPGe detector (Note: Extreme precautions must be taken while handling

241

Am because of the high radiation hazards from alpha radioactivity). Nd2O3

(99.99%, Alpha Biochem) was dissolved in nitric acid to yield the stock solution of Nd(III). Suprapur nitric acid (Merck) and MilliQ water (Millipore) were used to prepare the nitric acid feed solutions which were subsequently standardized by acid-base titrations using phenolphthalein indicator (Fluka). Standardization of the Nd(III) stock solution was done by complexometric titration using EDTA (complexing agent) and methyl thymol blue (indicator).25 All the other reagents were of AR grade and were used as received from the suppliers. Table 1 Specifications of hollow fiber membrane contactor (LiquiCel® mini module G543) used in the present work. Parameter

Specification

Fiber material

Polypropylene

Number of fibers

2200

Fiber internal diameter (µm)

240

Fiber outer diameter (µm)

300

Fiber wall thickness (µm)

30

Effective pore size (µm)

0.03

Porosity (%)

40

Tortuosity

2.5

Effective fiber length (cm)

11.8

Effective surface area (m2)

0.18


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Figure 2. Schematic diagram of the hollow fiber contactor set-up used in the present study. The aqueous feed passed through the tube side and the aqueous strip phase passed through the shell side of the module. 2.2. Hollow fiber supported liquid membrane studies. A LiquiCel® hollow fiber membrane module (Alting, France) whose specifications are given in Table 1 was used in the present study. The HFSLM was made by passing the extractant (C4DGA) solution through the lumen side of the module. A pressure of about 20 kPa was applied at the other end of the module to enable the C4DGA solution to percolate from the lumen side to the shell side. After soaking the hollow fiber membrane, the C4DGA solution sticking to the hollow fiber surface was washed out completely by passing distilled water from both the shell side and the tube side. A schematic diagram of the hollow fiber membrane set-up is shown in Figure 2. This technique of preparing HFSLM leads to liquid membrane was found to be acceptable for subsequent Am(III) transport studies as the %transport data were reproducible within an error limit of 5%. The stability of HFSLM membranes containing neutral ligands dissolved in paraffinic solvent such as n-dodecane, a case similar to the present work, were found to be excellent over several weeks of continuous operation.21 The HFSLM experiments were performed by passing the feed and the strip solutions through the tube and the shell sides, respectively in counter-current direction. Gear pumps with very precise flow controllers (Cole Parmer) were used to control the flow rates of the feed and strip solutions at 200 mL/min. The volume of feed and strip solutions was 300 mL each. Samples (100-200 µL) from the feed and the receiver compartments were taken out at regular intervals and assayed for subsequent used to calculate the Am(III) permeability.

241

Am was assayed radiometrically by



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counting the 60 Mev gamma rays using a well type NaI(Tl) scintillation detector (Para Electronics) coupled to a multi-channel analyzer (ECIL, India). The concentration of Am(III) in the feed solution was at tracer level, which is about 10-7 mol/L. The Nd(III) in the feed was used as surrogate of Am(III) as handling of mg level of Am(III) under our experimental set-up was not possible. In view of comparable chemistry of the trivalent actinide and lanthanides, the transport of

241

Am was taken as same as that of Nd. An

identical (within experimental error) extraction behaviour of Nd(III) and Am(III) was confirmed independently by measuring their distribution coefficient values in solvent extraction. Co-transport of nitric acid was studied by the estimation of the acid in the receiver compartment by standard acid-base titrations using phenolphthalein indicator. The transport data were reproducible within ±3%. 3. RESULTS AND DISCUSSION 3.1. Metal-ligand complexation equilibria. The equilibrium reaction between Am(III) and the C4DGA ligand, taking place at the feed-membrane and the membrane-receiver interfaces, is described as follows:24 Am3+(aq) + 3NO3-(aq) + xE (mem) \===\ Am(NO3)3⋅xE (mem)

(1)

where, the term E stands for the extractant, C4DGA while the species with the subscripts ‘(aq)’ and ‘(mem)’ indicate those present in the aqueous and the membrane phases, respectively. The species, Am(NO3)3⋅xE, represents the extracted complex in the membrane phase (or organic phase supported inside the pores of the membrane support). The term ‘x’ is the number of C4DGA molecules present in the metal bearing complex, which has been confirmed to be 1 in the earlier solvent extraction studies.24 The twophase extraction constant (Kex) for Eq. (1) is given as:

K ex =

Am( NO3 )3 ⋅ E(mem) [ Am3+ ](aq) ⋅ [ NO3− ]3(aq) ⋅ [ E ](mem)

(2)

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


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Kd =

Am( NO3 )3 ⋅ E(mem) [ Am3+ ](aq)

(3)

The Kex value of Am(III) with C4DGA was calculated from the measured Kd values under the given experimental conditions. The transport of Am(III) across the liquid membrane can be best explained with the help of the extraction equilibrium given by Eq. (1). By proper adjustment of the feed condition one can ensure the effective transfer of the metal bearing complex, Am(NO3)3⋅E, to the membrane phase. The complex (Am(NO3)3⋅E) is subsequently diffused inside the membrane to the membrane-receiver interface as a consequence of the difference in the concentrations of the complex. Similarly, the receiver phase condition shifts the equilibrium (Eq. (1)) in the reverse direction ensuring the back extraction of the metal ion. This results in the dissociation of the Am(NO3)3⋅E complex and release of the ligand (C4DGA), for subsequent transportation of the metal ion in the membrane phase, and Am3+ and an equivalent number of nitrate ions in the receiver phase. The free ligand molecules then move towards the feed-membrane interface by diffusion due to the negative concentration gradient to complete the transport cycle.26 3.2. Permeation of Am(III) from nitric acid. The permeation of Am(III) at tracer concentrations (ca. 10-7 M) was investigated from a feed solution containing 3 M HNO3 to the receiver phase containing distilled water. As shown in Figure 3, near quantitative Am(III) transport was possible in < 20 min at 300 mL feed scale. It is worth mentioning that under identical feed and receiver compositions in studies involving FSSLM (20 mL feed and receiver phase volumes), >4 h were needed for near quantitative Am(III) transport.24 The higher transport efficiency in the present case is obviously due to the significantly large effective surface area (1800 cm2) provided by the hollow fiber contactor as compared to the FSSLM (3.14 cm2). In order to assess the effect of the concentration of the metal ion in the feed (containing 241Am radiotracer) on the transport efficiency, macro concentrations of Nd (used as a representative of trivalent lanthanides and a surrogate of Am) were added to the feed. It should be noted that high level waste may contain 0.5 – 2 g/L total lanthanides.1,2 In view of very similar chemistry of Am(III) and Nd(III), their transport efficiencies are expected to be same. However, in the 7 - Environment ACS Paragon -Plus


presence of macro concentrations of Nd in the feed, it was expected that the percent metal ion transport would be slower due to loading of the metal ions by the carrier ligand present in the membrane phase. The results indicated quantitative transport of Am(III) (Figure 3a) in 5 h when the feed solution contained 0.05 – 0.1 g/L Nd. On the other hand, the permeation of Am(III) was significantly lower even after 5 h of operation when the feed Nd concentration increased to 0.5 g/L. The Am(III) transport in 5 h for different feed solute concentrations was: >99.9%, 99.5%, 97.2%, 84.6%, 67.2%, and 31.1% for 0.05 g/L, 0.1 g/L, 0.2 g/L, 0.3 g/L, 0.4 g/L, and 0.5 g/L Nd, respectively. As shown in Figure 3b, the slope of the curve decreased with increased Nd concentration in the feed solution indicating a decrease in the permeation rate. 4.0

(a) 100

(b)

3.5

Am-Tracer 0.05 g/L Nd 0.1 g/L Nd 0.2 g/L Nd 0.3 g/L Nd 0.4 g/L Nd 0.5 g/L Nd

60

40

Am(III)-Tracer 0.05 g/L Nd 0.1 g/L Nd 0.2 g/L Nd 0.3 g/L Nd 0.4 g/L Nd 0.5 g/L Nd

3.0 2.5

- ln(Ct / Co)

80

%Transport

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2.0 1.5 1.0

20

0.5 0.0

0

-0.5

0

50

100

150

200

250

300

0

50

100

150

200

250

300

Time (min)

Time (min)

Figure 3. Effect of Nd concentration on the transport of Am(III) by C4DGA-HFSLM; Carrier: 1 mmol/L C4DGA in 5% isodecanol/n-dodecane; Feed: 3 M HNO3 (300 mL); Strippant: Distilled water (300 mL); Flow rate: 200 mL/min. (a) Transport profile of Am(III) with increasing Nd concentration in the feed, (b) Plot of ln(Ct/Co) vs time to calculate the P values.

3.2.1. Calculation of permeability coefficient. In HFSLM, the permeation of metal ions from the feed side to the receiver side takes place in the following three steps: (i) extraction of the metal ions by the ligand present in the liquid membrane, (ii) diffusion of the extracted complex from the feed side of the SLM to its receiver side, and (iii) backextraction of the metal ion at the membrane-receiver interface. Considering (i) linear concentration gradients of the diffusing species inside the membrane, (ii) fast interfacial


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reactions at the membrane-feed or membrane-receiver interfaces, and (iii) back extraction at the receiver-membrane interface is quite fast, the final equation for the permeability of a metal ion can be obtained as described:27-29

C   A  φ  − ln  t  = P ⋅   ⋅   ⋅t φ C V + 1      o

(4)

where, P and V are the overall permeability coefficient and feed volume (mL) while Ct and Co represent Nd concentrations in the feed solution after the time ‘t’ (min) and at the start of the experiment, respectively. The parameter ‘A’ in Eq.(4) stands for the total effective surface area of the hollow fiber (cm2), given by the following equation:

A = 2π ri LN ε

(5)

where, the internal radius and length of the hollow fiber capillary are noted as ri and L (both in cm), respectively while ε is the standard notation for the porosity. The parameter ‘φ’ in Eq. (4) is expressed as:

φ=

Q T P ri LNπε

(6)

where, N is the number of fibers and QT is the volumetric flow rate of the feed solution (mL/min). Specifications of various parameters of the hollow fiber contactor used are given in Table 1. Linear graphs are expected from Eq. (4) when one plots ln(Ct /Co) vs t. The P value for the given system can be obtained from the slope using Eqs. (5) and (6). The P values calculated from the fitted slope of Figure 3b are listed in Table 2. The lower P values at higher feed Nd concentrations are due to loading effect in the liquid membrane similar to that seen in case of solvent extraction studies, which is reflected in the decreased Kd values of Am(III) under loading conditions. It is important to note that the co-transport of nitric acid was very low,

Liquid Membrane Studies Using a Hollow Fiber Contactor

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