Comparative Study Between TBP and Dibutylalkyl Phosphonates

Published Date: 16 Jan 2018

CHAPTER 7

PHOSPHONATES AS ALTERNATIVE TO TBP FOR ACTINIDES AND FISSION PRODUCTS

Solvent extraction studies of U (VI), Th (IV), Eu (III) and Tc (VII) in dibutylalkyl phosphonates have been carried out in present study. Uptake of these metal ion and formation of metal-ligand bond is a direct consequence of phosphorus-carbon bond and to understand the influence of these changes in the bond was the main objective for the present study. Thus synthesis and solvent extraction studies of Dibutyl Propyl Phosphonate (DBPrP) and Dibutyl Pentyl Phosphonate (DBPeP) were carried and were compared with those available for Tributyl Phosphate (TBP). Thus this study will represent a comparative study between TBP and dibutylalkyl phosphonates.

7.1 Introduction

Spent nuclear fuel (SNF) is a complex system with large number of elements and there isotopes which are produced during the nuclear fission of U and Pu. These spent fuel rods containing activation products along with fission products needs to be dealt while reprocessing and waste management of SNF which is carried out at reprocessing plant.

TBP a triester of phosphoric acid is a major extractant used for nuclear fuel reprocessing that is Plutonium Uranium Extraction PUREX processes worldwide for the separation of uranium and plutonium from the dissolver solution [1]. Even though it has been a workhorse in nuclear industry since long period there are major drawbacks like its significant solubility in aqueous phase, third phase formation during macro level extraction of tetravalent actinides in nitric acid medium, low selectivity of U and Pu over Zr and Ru and presence of chemical and radiolytic degradation products of TBP viz. monobutyl and dibutyl phosphoric acid are responsible for lowering the decontamination factor (DF) [2-6]. Significant research in the scientific community using higher homologs of TBP has shown that they are more resistant to third phase formation and aqueous solubility.

Basicity of the phosphoryl oxygen and the nature of substituents attached to the P atom are key factor responsible for the extraction ability of any organophosphorus extractant. Enhancement of the basicity on the phosphoryl group may be achieved by replacement of C-O-P group directly by C-P group. Neutral organophosphorus extractants show the variation in the basicity of the phosphoryl oxygen as phosphine oxide > phosphinates > phosphonates > phosphates [7]. Studies in the past have reported that phosphonates are better extractants for the extraction of uranium and thorium as compared to that with corresponding phosphates [8, 9]. In the nuclear reprocessing industry dibutylalkyl phosphonate was found to be one of the promising candidates as a replacement for TBP. Lower D values than that of corresponding phosphinates and phosphorus oxide makes stripping easier in case of phosphonates. The main objective of this study was to focus on the potential extraction capabilities of U (VI), Th (IV), Eu (III) and Tc (VII) which are relevant from nuclear fuel cycle view point by the phosphonates DBPrP and DBPeP.

7.2 Synthesis of Dibutylalkyl Phosphonates

Phosphonates used in the present study was synthesized using Michaelis Becker reaction [10]. In this reaction equimolar amount of sodium is allowed to react with dialkylhydrogen phosphonate and dialkylsodium phosphonate thus obtained is further allowed to react with alkyl halides and final product with P-C bond is obtained.

Figure 7.1 Michaelis-Becker Reaction

Preset reaction involves nucleophilic substitution of phosphorus on alkyl halide to yield phosphonate as shown in the figure below.

Figure 7.2 Mechanism for Michaelis-Becker Reaction

The preparation of these phosphonates were carried out in a refluxation unit by drop wise addition of dibutylhydrogen phosphate over a period of 30 minutes to the reaction mixture i.e. sodium (1.15g, 0.05 mol) + hexane (100 mL). The addition of dibutylhydrogen phosphate was continued until the dissolution of sodium was complete.

After this complete reaction mixture was stirred under gentle refluxation for about 4 hours during which 1-bromoalkane was added over a period of half-an-hour. This reaction mixture was then washed with water after cooling it at room temperature after which the product was distilled using reduced pressure to get rid of impurities.

7.3 Mechanism of Extraction in Dibutylalkyl Phosphonates

Uptake of metal ions from the aqueous phase using dibutylalkyl phosphonates is by formation of neutral complex formation. Solvation of metal ion takes place by nitrate ion which is the aqueous phase used in the present studies. Then the solvation of these neutral metal nitrate species takes place with the help of dibutylalkyl phosphonate which gets extracted be the organic phase.

M^X+_aq + X NO₃^- + nDBAP_org M(NO₃)_X.nDBAP_org

Following equation gives the equilibrium constant for the above reaction

K_eq = [M(NO₃)_X.nDBAP]_org / [M^X+_aq] [NO₃^-]^X[DBAP_org]ⁿ

Distribution ratio (D) is the ratio of activity of metal ion in organic phase to that in the aqueous phase at equilibrium, which can be rearranged and represented in the following way.

D = K_eq [NO₃^-] ^X[DBAP_org]ⁿ

Distribution ratio depends on the concentration of nitrate ions and concentration of extractant. There is always a rise in the D value as the nitrate ion concentration increases while the fall at higher acidity indicates the extraction of nitric acid.

7.4 Solvent Extraction Studies

Extraction of U (VI), Th (IV), Eu (III) and Tc (VII) with were carried out in a plastic tube with preequliberated organic phase that comprised of 1.1 M DBPrP and DBPeP in n-dodecane. 2 mL of preequliberated extractant was agitated with 2 mL of nitric acid in a shaking incubator at 25 ⁰C for 1 hour. After the equilibration the two phases were allowed to separate and were analysed for the metal ion content using suitable technique.

7.4.1 Extraction studies of nitric acid

Around 2 mL of various concentrations of nitric acid (0.1-6M) were taken in an equilibration tube and equilibrated with 1.1 M DBPrP/DBPeP, n-dodecane at room temperature for an hour. The nitric acid concentration in both the phases was determined by acid-base titration. Figure below depicts the uptake of nitric acid in DBPrP and DBPeP compared with the available literature values of TBP. As observed from the plot it is clear that D values in case of phosphonates are higher as compared with that of TBP which is the direct consequence of the higher basicity of the phosphonates. Prasanna et al. have reported that the changes in alkyl group structure do not have significant affect on extraction of nitric acid [11].

7.4.2 Extraction Studies of U (VI)

After the equilibration the two phases were separated and analysed for U (VI) content spectrophotometrically using Arsenazo-III as chromogenic agent [12]. Organic phase concentration was estimated by subtracting concentration of U (VI) in equilibrated aqueous phase from the initial feed concentration. Below figure shows the comparative data for the uptake of U (VI) in TBP, DBPeP and DBPrP in the complete nitric acid range (0.1-6 M). There was a constant increase in the uptake of U (VI) metal ion with the increase in nitric acid concentration. Also the observed increase in the uptake of U (VI) as TBP < DBPrP < DBPeP is due to the increasing basicity on phosphoryl oxygen leading to the formation of stronger complex bonds [13]. Thus the increase in the extraction behaviour of these neutral extractants suggests that the change in the alkyl group attached to the P atom contributes significantly enhancing the extraction capabilities of the newly synthesized phosphonates.

Figure 7.3 Variation of D_U(VI) as the function of acid concentration for Dibutylalkyl phosphonates at 25 ⁰C

7.4.3 Extraction Studies of Th (IV)

Figure 7.4 depicts the variation of extraction behavior of Th (IV) by 1.1 M of TBP, DBPrP and DBPeP extraction in n-dodecane under identical conditions. As expected there is a constant rise in the D values as the concentration of nitric acid goes on increasing. Also higher analogs of neutral organophosphorus extractant shows the higher uptake which is again a direct conciquence of the increased bascicity on phophoryl oxygen the highest uptake of Th (IV) is

Figure 7.4 Variation of D_Th(IV) as the function of acid concentration for Dibutylalkyl phosphonates at 25 ⁰C

7.4.4 Extraction Studies of Europium (III)

Figure 7.5 Variation of D_Eu(III) as the function of acid concentration for Dibutylalkyl phosphonates at 25 ⁰C

7.4.5 Extraction Studies of Technetium (VII)

Figure 7.3 Variation of D_Tc(VII) as the function of acid concentration for Dibutylalkyl phosphonates at 25 ⁰C

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