Carrier facilitated transport of thorium from HCl medium ... - Springer Link

J Radioanal Nucl Chem (2013) 298:707–715 DOI 10.1007/s10967-013-2576-x

Carrier facilitated transport of thorium from HCl medium using Cyanex 923 in n-dodecane containing supported liquid membrane A. K. Dinkar • Suman Kumar Singh • S. C. Tripathi • P. M. Gandhi • R. Verma A. V. R. Reddy

•

Received: 31 December 2012 / Published online: 19 June 2013 Ó Akade´miai Kiado´, Budapest, Hungary 2013

Abstract Present studies deal with supported liquid membrane (SLM) technique for the separation of thorium from hydrochloric acid (HCl) medium using Cyanex 923 as a carrier. Effects of feed acidity, strippant, and membrane pore size and membrane thickness on the transport of thorium have been studied in detail. The optimized parameters were applied for separation of thorium from a radioanalytical waste. Stability of the membrane and membrane support was investigated. Transport of thorium increased from 78.3 to about 93.7 % with increase in acidity from 0.5 to 2 M using 0.3 M Cyanex 923 in ndodecane as carrier and 2 M ammonium carbonate as stripping phase. The transport of thorium decreased above 2 M HCl. An attempt was made to model the physicochemical transport of thorium in SLM and understand the mechanism of thorium transport. Keywords Supported liquid membrane Thorium transport Cyanex 923 Radioanalytical waste Flux Permeability coefficient

Introduction The three-stage nuclear power program of India is solely based on utilization of vast amounts of naturally occurring thorium in coastal region of India [1]. Though the A. K. Dinkar S. K. Singh S. C. Tripathi P. M. Gandhi Fuel Reprocessing Division, Bhabha Atomic Research Centre, Mumbai, Maharashtra, India R. Verma (&) A. V. R. Reddy Analytical Chemistry Division, Bhabha Atomic Research Centre, Mumbai 400085, Maharashtra, India e-mail: [email protected]

extraction and separation of thorium from its natural resources have attracted attention of separation scientists, the development of suitable method for thorium separation from natural lean sources is still a challenging task [2–6]. Solvent extraction, ion exchange, extraction chromatography, and precipitation are commonly used for separation and recovery of heavy nuclear metals from various sources [7–11]. All the above processes have their own limitations and drawbacks such as solvent degradation, third phase formation, resin degradation and crud formation, etc. Liquid membrane is a promising separation technique. A membrane is a semi-permeable barrier separating two phases of different composition and is often used for separation and purification of aqueous waste streams with the objective to recover and concentrate valuable metals. Use of supported liquid membrane (SLM) in environmental applications, separation and recovery of toxic, pollutant and nuclear material from low level sources as well as for cleaning up of effluent streams has also received attention [12–14]. Consequently substantial research is being carried out worldwide on optimizing the parameters for membrane processes for the removal of toxic or valuable metal ions from its lean sources [15–19]. Studies on optimization of parameters for membrane processes for the removal of toxic or valuable metal ions from its lean sources have been reported [20–23]. The advantages of SLM based separation process are (a) high feed to strip volume ratio leading to a large enrichment factor of the transported species, (b) very low extractant (carrier) inventory, (c) no phase separation problem occurs as the organic and aqueous phases never mix, (d) negligible organic phase entrainment in the feed and strip aqueous solutions, (e) no moving parts and (f) simple to operate. Various neutral as well as acidic extractants and their synergistic combinations have been investigated as carrier in SLM for metal transport studies

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J Radioanal Nucl Chem (2013) 298:707–715

Membrane cell

[15–24]. Cyanex 923 is a mixture of four tri-alkyl phosphine oxides (R3P(O), R2R1P(O), RR12 P(O) and R13 P(O); R = octyl and R1 = hexyl group. It is a neutral organophosphorous extractant. It has the advantage of being a liquid and is completely miscible with all the commonly used diluents even at low ambient temperature and has been reported to have better extraction efficiency than trioctylphospine oxide (TOPO). Study has been carried out to investigate the transport of uranium from phosphoric acid medium across TOPO/n-dodecane-SLM [15]. A binary mixture of PC88A and Cyanex 923 in n-dodecane as carrier has been studied for uranium transport from phosphoric acid medium [17]. The detailed literature survey revealed that Cyanex 923 has not been explored for study the transport behavior of thorium from hydrochloric acid (HCl) medium. Thus a systematically planned study has been carried out to investigate the transport behavior of thorium from HCl medium using Cyanex 923 as the carrier. The effect of process controlling parameters on thorium transport has been studied in details and the optimized conditions are employed to study the transport behavior of thorium from the actual radioanalytical waste generated in HCl medium during irradiated thoria reprocessing.

Details of glass SLM transport cell used in this studies has been described elsewhere [24]. Single-stage SLM measurements were carried out with two compartment permeation cell in which a source aqueous solution (15 mL) was separated from the aqueous receiving solution (15 mL) by a membrane with an effective membrane area of 9.62 9 10-4 m2. The source and receiving solutions were mechanically stirred using magnetic stirrer to avoid concentration polarization at the membrane interface and in the bulk solution. Membrane permeability was determined by monitoring the thorium concentration spectrophotometrically primarily in the receiving phase, as a function of time (Table 1). Membrane supports Flat-sheet PTFE hydrophobic microporous polymeric membrane was used in this study. The porosity of the membrane was about 64 % and the average pore diameter and thickness were 0.45 and 80 lm, respectively. Pore filling of membrane with the carrier solution was accomplished by immersing the membrane in the organic phase for at least 24 h before the use. The pores were immediately and quantitatively filled with the carrier solution by capillary action. This type of SLM polymeric support eliminates the transport of water through the membrane and is free from osmotic effects.

Materials Reagents

Analysis

Cyanex 923, a product of American Cyanamid Company, USA, has been used after appropriate dilution in suitable diluents. Accurately weighed amounts of Th(Cl)46H2O (BDH) were dissolved in HCl for preparing standard feed solution. 2 M ammonium carbonate (SD Fine Chemical Ltd, Mumbai) solution was used as the stripping agent in the receiver compartment. Poly-tetrafluoroethylene (PTFE) membrane support from Millipore (India) Pvt. Ltd Mumbai was used in this study. All the reagents used were of analytical grade.

Thorium in aqueous/organic phase was measured by spectrophotometrically at 545 nm, kmax [25]. Colour in the solutions was developed using aqueous and organic thoron as chromogenic reagents using Shimadzu UV–Visible recording Spectrophotometer (UV-240) from Shimadzu Kyota, Japan. Thorium was also determined by inductively coupled plasma-atomic emission spectrometer after appropriate dilution. All batch experiments were carried

Table 1 Permeability and flux of thorium across Cyanex 923 liquid membrane as a function of feed acidity Feed acidity (M)

Thorium (feed) (mol L-1)

Transport of thorium (%)

P 9 106 (m s-1)

Flux 9 106 (mol m-2 s-1)

0.5

0.41 9 10-3

78 ± 2.54

1.01 ± 0.03

2.12 ± 0.07

1.0

0.28 9 10-3

85 ± 3.2

1.25 ± 0.04

2.62 ± 0.08

2.0

0.15 9 10

-3

94 ± 3.82

2.02 ± 0.06

4.24 ± 0.1

3.0

0.14 9 10-3

90 ± 3.64

1.67 ± 0.05

3.50 ± 0.1

4.0

0.51 9 10-3

74 ± 2.54

0.87 ± 0.03

1.83 ± 0.06

6.0

0.6 9 10-3

70 ± 2.62

0.77 ± 0.02

1.62 ± 0.05

-3

Experimental conditions: initial feed concentration, 2.1 9 10 stripping solution, 2 M ammonium carbonate

123

mol L

-1

of thorium; carrier concentration, 0.3 M Cyanex 923 in n-dodecane;


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out at room temperature (25 ± 1 °C). The material balance was within ±5 %. Solvent extraction studies Thorium solution (15 mL), prepared in 2 M HCl, was taken in a separating funnel. To this 15 mL of 0.3 M Cyanex 923 in n-dodecane was added. The mixture was equilibrated for 30 min and then allowed to settle. The organic and aqueous phases were collected separately in beakers. The thorium loaded composite organic phase was used to study the back-extraction behaviour of thorium using commonly available strippants like ammonium carbonate, tartaric acid, HCl, sulphuric acid, oxalic acid, etc. Back extraction of thorium was calculated by using standard formulae.

Fig. 1 Effect of stripping agents on back extraction of thorium

Influence of feed acidity on permeation of thorium Transport studies Transport studies were carried out using a two compartment glass shell (15 mL volume each) and PTFE microporous membrane support (pore size: 0.45 lm; porosity: 64 %). Cyanex 923, a mixture of four tri-alkyl phosphine oxides, was used as liquid membrane carrier solution. Carrier (Cyanex 923 in n-dodecane) loaded flat-sheet PTFE membrane support was interspersed between the feed and strip compartments and both the compartments were clamped mechanically to prevent leakage. The feed compartment contained 2.1 9 10-3 mol L-1 of thorium in 2 M HCl and 2 M (NH4)2CO3 was used as the strippant in the other compartment. To avoid concentration polarization, feed and strip solutions were stirred mechanically (500–600 rpm) using a magnetic stirrer. Transport behavior was investigated by measuring concentration of thorium in the feed solution at different time intervals under different experimental conditions.

To study the effect of acidity on transport behavior of thorium, feed acidity was varied from 0.5 to 6 M of HCl keeping the carrier concentration as 0.3 M of Cyanex 923 and 2 M ammonium carbonate as receiving phase. Thorium transport increased with increase in the feed acidity and decreased above 2 M HCl. About 78.3, 84.7 and 93.7 % thorium was transported in 420 min at 0.5, 1 and 2 M HCl medium, respectively whereas it decreased to 89.8, 73.8 and 69.9 % at 3.0, 4.0 and 6.0 M of HCl, respectively (Fig. 2). At lower acidity, probably this may be due to increase in adduct formation between neutral thorium chloride and Cyanex 923 at membrane–feed interface which diffuses to receiving phase due to concentration gradient. At higher acidity, decrease in thorium transport is probably due to co-transport of HCl along with thorium chloride across Cyanex 923 SLM. It is in agreement with the work reported earlier in case of multi-tracers transport in SLM by TBP [26].

Results and discussion Solvent extraction studies Percentage stripping of thorium by various strippant is shown in Fig. 1. Order of the stripping efficiency (%) of thorium by various reagent was 2 M ammonium carbonate [ 1 M tartaric acid [ 10 M HCl [ 10 M sulphuric acid [ 1 M oxalic acid [ 0.1 M sodium salt of ethylenediaminetetra-acetic acid [ demineralised water. Thorium was quantitatively stripped by 2 M (NH4)2CO3 in 30 min by a single batch contact. Therefore, 2 M ammonium carbonate solution was selected as a receiving phase for liquid membrane permeation studies.

Fig. 2 Transport of thorium as a function of feed acidity

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The permeability coefficient and average flux were 1.01 9 10-6 m s-1 and 2.12 9 10-6 mol m-2 s-1, respectively for 0.5 M HCl and 2.02 9 10-6 m s-1 and 4.24 9 10-6 mol m-2 s-1, respectively for 2 M of HCl, 0.77 9 10-6 m s-1 and 1.62 9 10-6 mol m-2 s-1 at 6 M of HCl, respectively. The flux of a cation/transport of thorium varies with Cl- ion, thorium- and carrier concentrations in different ways. The % transport of thorium increases with increase in its concentration in the feed- and carrier concentrations in the membrane phase where it decreases with increase in Cl- concentration. Thus this is partly in agreement with the expected trend [21]. JM ¼ AT=g½Cl naq ½Carriernorg CM;feed ;

ð1Þ

where A is the area of membrane (m2), T is the absolute temperature (K), g is the viscosity (cp), and CM,feed is the concentration of metal in the feed (mol L-1). Influence of thorium concentration in the feed The metal ion concentration in the feed solutions plays an important role in its transport through SLM. The effect of metal concentration in the aqueous feed solution on the transport behavior of thorium was investigated in a separate test run. Thorium concentration in process streams wastes and radioanalytical waste of the laboratory was in the range of (1.29–6.46) 9 10-3 mol L-1. In view of this, studies were carried out in this concentration range. Figure 4 shows permeation as a function of thorium concentration (1.07–6.46) 9 10-3 mol L-1 in 2 M HCl medium using 0.3 M Cyanex 923 in n-dodecane as the carrier. It is observed that about 97 and 24 % of thorium was transported from a feed containing 1.07 9 10-3 and 6.46 9 10-3 mol L-1 of thorium concentration, respectively (Fig. 3).

Fig. 3 Transport of thorium of carrier concentration

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Fig. 4 Effect of thorium concentration on its transport across Cyanex 923 in n-dodecane liquid membrane

Effect of carrier concentration on thorium transport Composition of the organic solution has a marked effect on the permeation of a metal ion from the feed to receiving phase. When transport across a membrane occurs via a carrier, as in facilitated transport, the transport is generally expected to increase with increasing carrier concentration. The effect of Cyanex 923 in n-dodecane concentration on the transport of thorium was studied in a separate run. The studies were made over a concentration range of 0.03– 0.45 M Cyanex 923 in n-dodecane. Generally an increase in the carrier concentration produces an increase in cation flux leading to an increase in thorium transport. The percentage transport of thorium increased from 59.9 % (0.03 M of Cyanex 923 in n-dodecane) to 95.1 % (0.45 M of Cyanex 923 in n-dodecane). The permeation coefficients and average flux are 1.68 9 10-6 m s-1, and 3.51 9 10-6 mol m-2 s-1, respectively for 0.3 M Cyanex 923 in n-dodecane (Table 2). Similar results are reported earlier in case of strontium transport by crown ether using SLM [27, 28]. It is also obvious that the transport of thorium in such a system should be a function of both the distribution coefficient and diffusion coefficient because the transfer of metal ions through the membrane may be considered diffusive in nature. The difference in permeability among the experiments with varying feed acidity, carrier and metal ion concentration can be understood by considering the probable expression for the rate of formation of the expected diffusing species at the feed interface:


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Table 2 Permeability and flux of thorium across Cyanex 923 liquid membrane as a function of carrier concentration Cyanex 923 in n-dodecane (M)

Feed concentration, final (mol L-1)

Transport of thorium (%)

P 9 106 (m s-1)

Flux 9 106 (mol m-2 s-1)

0.03

0.84 9 10-3

60 ± 1.5

0.57 ± 0.025

1.19 ± 0.045

0.06

-3

0.58 9 10

72 ± 2.04

0.79 ± 0.035

1.67 ± 0.064

0.15

0.38 9 10-3

82 ± 3.1

1.05 ± 0.04

0.3

0.14 9 10-3

93 ± 4.26

1.68 ± 0.062

3.51 ± 0.13

0.45

0.11 9 10-3

95 ± 1.98

1.82 ± 0.073

3.82 ± 0.016

Experimental conditions: initial feed concentration, 2.1 9 10 ammonium carbonate

-3

d½Th(Cl)4 2ðCyanex 923Þ=dt ¼ k ½Cyanex 9232 ½Cl 4 Th4þ ;

mol L

-1

2.2 ± 0.089

of thorium; feed acidity, 2 M of HCl; stripping solution, 2 M

ð2Þ

where k* is rate constant for the formation of the Th– [Cyanex 923] complex. This equation is in agreement with the fact based on the stoichiometry of thorium extraction [20, 21]. Rate of diffusion of metal species will thus depend on Cl-, carrier and Th metal ion concentration in the feed. Though there are many accumulative factors responsible for thorium transport, the concentration of thorium in the feed is one of the major factors influencing its transport. The percentage transport of thorium increases with respect to the increase in its concentration in the feed solution. From the results, it is clear that thorium could be quantitatively transported from a feed containing thorium in HCl medium.

Fig. 5 Percentage permeation of thorium from 2.0 M HCl at various thickness of membrane

Effect of membrane thickness on transport of metal ions In the membrane based transport phenomenon, membrane thickness dictates the resistance in the process. Thinner is the membrane, lower is the membrane resistance and also better is the contact between the two phases. The permeability of metal ions is influenced by effective diffusion path length in the membrane phase. During the present work, the thickness of the membrane was increased by immersing several membrane sheets separately into the carrier solution and pressing them together. Diffusion resistance of the membrane increases linearly with membrane thickness. The permeate flux is inversely proportional to the membrane thickness [22]. Figure 5 shows that as the thickness of the membrane increases, permeation (%) decreases. About 93.7 % permeation of thorium was observed in 420 min for a membrane thickness of 80 lm whereas about 78 and 64.1 % transport was achieved in 420 min for membrane thickness of 160 and 240 lm, respectively. Plot of thorium permeability coefficient (P) versus reciprocal of the membrane thickness is linear (Fig. 6) where thickness is in meter. The diffusion coefficient (Do) can be calculated using the following formula [20].

P¼

Du Do do s

ð3Þ

;

where Du is the distribution coefficient, Do is membrane diffusion coefficient, do is the membrane thickness and s is the tortuosity factor. The linearity of the curve clearly indicates that the observed transport phenomenon is diffusion controlled. The variation of permeability coefficient of anionic complex of thorium as a function of membrane thickness would clearly demonstrate whether transport of an element of interest across the SLM is controlled by diffusion or by kinetics at membrane–aqueous interfaces. As per Fig. 5 and Eq. (3), transport of thorium (%) versus membrane thickness clearly indicate that the transport of thorium is diffusion controlled. During the process of diffusion of anionic complex of thorium, the path length resists the process of diffusion and it is in directly proportional to membrane thickness. Effect of membrane pore size on transport of metal ions Solvent is held within the pores of PTFE membrane by capillary action. The pore structure plays a major role in

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Fig. 7 Permeation of thorium as a function of pore size of support Fig. 6 Permeation coefficient as a function of inverse membrane thickness

the stability of the composite liquid membrane. The effect of pore structure on stability of liquid membrane is in accordance with the Laplace equation [Pc=(2c/r) cosh] which correlates the minimum transmembrane pressure (Pc) required to displace the impregnated phase out of the pore with radius (r), solvent water interfacial tension (c) and the contact angle (h). It is known that the higher membrane porosity results in higher metal ion permeation and flux. The diffusion limited flux through a composite liquid membrane is influenced by porosity (u) and tortuosity factor (b) of the support. The tortuosity factor can be reduced substantially with a larger-pore size of membrane, leading to higher diffusion constants. The tortuosity b is related to tortuous factor (s) (average pore length/membrane thickness) by a correction for difference in pore diameter (a) according to s = ab2 [29]. Results obtained for thorium transport with varying pore size of membranes are shown in Fig. 7. The percentage permeation of thorium was 80.96 and 93.7 % for 0.22 and 0.45 lm pore size, respectively. It was observed that the percentage permeation increased as the pore size of the supports increased and it is in full agreement with the facts established earlier [16, 17]. Stability of liquid membrane The stability of liquid membrane is one of the major limitations in the membrane based separation techniques. The PTFE support material was dipped in 0.3 M Cyanex 923 in n-dodecane for 24 h before the use. The stability of the 0.3 M Cyanex 923 liquid membrane was studied. Transport was studied over a number of continuous experimental cycles where each experimental cycle was of 7 h duration. During each cycle, feed and strip were replaced with fresh

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Fig. 8 Stability of liquid membrane for number of cycles of operations

solutions. The results presented in Fig. 8 show that there was rapid decrease in transport of thorium and decreases to 30 % in the third cycle. The average permeation of thorium was more than 94 % and decreased to about 32 % in the third cycle. This suggests that the liquid membrane of 0.3 M Cyanex 923 is quite unstable. The decrease in permeability of thorium is due to degradation of liquid membrane [29, 30]. Degradation of liquid membrane may be due to solubility of carrier and solvent in the feed and strip solutions, wetting of the support by aqueous phase, blockage of the support pores and osmotic pressure gradient (Table 3). The chemical resistance of PTFE membrane support material against 0.3 M Cyanex 923 in n-dodecane was tested over a period of 80 h by running it for periodical


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Table 3 The caption to the figures and experimental conditions for each figures Figure numbers

Figure captions

Experimental condition

1

Effect of stripping agents on back extraction of Th

Concentration of Th in organic phase: 2.02 9 10-3 mol L-1 Extractants: 0.3 M Cyanex 923 in n-dodecane Contact time: 30 min Phase ratio: 1

2

Transport of Th as a function of feed acidity

Feed acidity: 0.5–6 M of HCl Initial feed concentration of Th: 2.1 9 10-3 mol L-1 Carrier concentration: 0.3 M Cyanex 923 in n-dodecane Strippant: 2 M ammonium carbonate Pore size: 0.45 lm Pore thickness: 80 lm

3

Concentration of Th in feed: 2.1 9 10-3 mol L-1 Carrier concentration: 0.03–0.45 M Cyanex 923 in n-dodecane

Transport of thorium of carrier concentration

Contact time: 30 min Phase ratio: 1 Feed acidity: 2 M of HCl 4

Effect of Th concentration on its transport across 0.3 M Cyanex 923/n-dodecane liquid membrane

Concentration of Th in feed: 0.00107–0.0065 mol L-1 Carrier concentration: 0.3 M Cyanex 923 in n-dodecane Contact time: 30 min Phase ratio: 1 Feed acidity: 2 M of HCl

5

Percentage permeation of thorium from 2M HCl as a function of membrane thickness

Initial feed concentration: 2.1 9 10-3 mol L-1 of Th Carrier concentration: 0.3 M Cyanex 923 in n-dodecane Pore size: 0.45 lm Strippant: 2 M ammonium carbonate Thickness of membrane: 80, 160 and 240 lm

6

Permeation coefficient as a function of inverse membrane thickness

Initial feed concentration: 2.1 9 10-3 mol L-1 of Th Carrier concentration: 0.3 M Cyanex 923 in n-dodecane Strippant: 2 M ammonium carbonate Thickness of membrane: 80, 160 and 240 lm

7

Permeation of thorium as a function of pore size of membrane

Initial feed concentration: 2.1 9 10-3 mol L-1 of Th Carrier concentration: 0.3 M Cyanex 923 in n-dodecane Strippant: 2 M ammonium carbonate

8

Stability of liquid membrane for number of cycles of operations

Feed acidity: 2 M of HCl Initial feed concentration: 2.1 9 10-3 mol L-1 of Th Carrier concentration: 0.3 M Cyanex 923 in n-dodecane Strippant: 2 M ammonium carbonate

9

Stability of liquid membrane support for number of days of operations

Feed acidity: 2 M of HCl Initial feed concentration: 2.1 9 10-3 mol L-1 of Th Carrier concentration: 0.3 M Cyanex 923 in n-dodecane Strippant: 2 M ammonium carbonate

experiments. Each set of experiments were run for 7 h with the same membrane. Between two experiments, the feed and receiving compartment of the cells were filled with demineralised water. The fresh thorium standard solution

and strippant were used in every transport experiment. The results presented in Fig. 9 indicate that the membrane is quite unstable and need more study on improving the stability of Cyanex liquid membrane.

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than 90 % of thorium was separated from actual radioanalytical waste generated during analysis of iron by spectrophotometric method in the THOREX process. The optimized system seems to be promising for separation and purification of valuable nuclear materials from radioanalytical wastes. Cyanex 923 is a promising carrier for thorium transport from HCl medium in liquid membrane technology subject to improvement on its stability. Acknowledgments Authors are grateful to Dr. P. K. Wattal, Dir, NRG Bhabha Atomic Research Centre, Trombay, Mumbai for his keen interest in this work.

References

Fig. 9 Stability of liquid membrane support for number of days of operations

Application During the reprocessing of irradiated thoria at Uranium Thorium Separation Facility of the centre, iron contamination was observed in the thorium nitrate product solution. The iron contamination in thorium nitrate solutions probably may be due to the use of low concentration of hydrofluoric acid (HF) in the initial phase of thoria dissolution. To evaluate the level of iron contamination in the product solution, samples were drawn from the various stages of the THOREX process. Iron in different samples were analyzed by spectrophotometric method using 1,10 phenanthroline hydrate as chromogenic reagent and significant quantities of radio analytical waste in HCl medium was generated. The optimized process conditions were employed to study the transport behavior of thorium from analytical waste generated during iron analysis in the THOREX process. About 90 % of thorium was transported from an actual radioanalytical waste containing a complex matrix of anionic and cationic species like sodium, 4,667 ppm, acetate ion (0.2 M), iron (19 ppm) 1,10 phenanthroline hydrate (0.025 %) and hydroxyl amine hydrochloride (0.5 M).

Conclusion The results on transport behavior of thorium from HCl medium with respect to feed acidity reveals that acidity of 2 M of HCl is most suitable for quantitative transport of thorium. Among the various stripping reagents tested, 2 M (NH4)2CO3 was found to be most effective for quantitative back extraction of thorium in a single batch contact of 30 min. Using optimized experimental conditions, more

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