Gamma and alpha radiolysis of aqueous solutions of diethylenetriaminepentaacetic acid

J. inorg, nucl. Chem., 1972, Vol. 34, pp. 1417-1425.

GAMMA AND SOLUTIONS

Pergamon Press.

Printed in Great Britain

ALPHA RADIOLYSIS OF AQUEOUS OF DIETHYLENETRIAMINEPENTAACETIC ACID

N E D E. BIBLER Savannah River Laboratory, E. 1. du Pont de Nemours and Co., Aiken, South Carolina 29801

(Received 11 May 1971 ; in revised form 9 August 1971) A b s t r a c t - T h e radiolytic destruction of diethylenetriaminepentaacetic acid (DTPA), an eluting agent for the purification of ~44Cm by cation exchange chromatography, was studied in H2SO4 and HNO~ solutions. In 0.4M H2SO4, DTPA is effectively destroyed by both H atoms and OH radicals. For 3' radiation, G ( - D T P A ) = GH+GoH; and for a radiation, G ( - D T P A ) = GH+GoH+Gno~. At doses > 10~1 eV/g in solutions less than 4M acid, lanthanide and actinide oxalates precipitate. In - 4M HNO:~ solutions containing > 1 g/l. 244Cm eluted from cation exchange columns, the DTPA must be completely destroyed by a-radiolysis before the 244Cm can be concentrated by precipitation with IM NaOH. At high pH values and in the presence of metal cations that are complexed by DTPA, G(-DTPA) is lowered significantly. Competition studies with various solutes established that OH radicals destroy DTPA effectively ( k = 2 × I & M -~ sec-l). Failure of excess Br- ions to lower G(-DTPA) suggests that DTPA can react by a charge transfer mechanism in addition to H atom abstraction. INTRODUCTION

DIETHYLENETRIAMINEPENTAACETICacid (DTPA) is a polyaminocarboxylic acid that forms stable l : 1 complex ions with transition [1], lanthanide [2], and actinide [3] metal cations. This reagent has been used as an eluting agent to separate the lanthanide and actinide elements by cation exchange chromatography[4, 5]. Recently, approximately 2 kg of 244Cm[6] was separated by pressurized cation exchange chromatography. In D T P A solutions containing the z44Cm eluted from the ion exchange columns, precipitates formed after - 12 hr as a result of the alpha radiolysis of the D T P A [5]. Because this precipitate formation complicates further purification and recycle steps, the radiolysis both by a-particles and y-rays of aqueous solutions of D T P A was studied. EXPERIMENTAL DTPA from the J. T. Baker Chemical Co. was used without further purification. Titration with a standard solution of Sm(NOa)3 showed that the reagent was > 99 per cent pure. All other chemicals were research grade and used without further purification. Solutions were prepared in triply distilled water. Both gamma- and alpha-ray irradiations were performed. A Gammacell 220* was used for the I. 2. 3. 4. 5. 6.

E.J. Durham and D. P. Ryskiewich, J. Am. chem. Soy. 80, 4812 (1958). T. Moeller and R. C. Thompson, J. inorg, nucl. Chem. 24, 499 (1962). R. D. Baybarz, J. inorg, nucl. Chem. 27, 1831 (1965). E.J. Wheelwright, F. P. Roberts and L. A. Bray, U S A E C Report BNWL-SA-1492 (1968). W. H. Hale and J. T. Lowe, lnorg, nuel. Chem. Letters 5,363 (1969). J. T. Lowe, W. H. Hale and D. F. Hallman, Ind. Eng. Chem., Process Develop. 10(1), 136 (1971). *Atomic Energy of Canada, Ltd., Ottawa, Canada. 1417

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N E D E. B I B L E R

y-ray irradiations. Source intensity was determined with the Fricke dosimeter[7] and the dose to the irradiated solutions was calculated using the ratio of electron densities of the two respective solutions. The dose rate in the solutions was 1.3 x 104 rad/min. Solutions were irradiated at 25--- I°C. For a-ray irradiations, an aliquot of a 244Cm solution was dissolved directly into the sample solution. The dose rate in this solution was calculated from the amount of z44Cm present (determined by alpha counting). For the pure D T P A solutions (no complexing metal ions presen0, the amount of complexing agent remaining after each irradiation was determined by titrating an aliquot of the solution with a standard Sm(NO3)3 solution (0.006M) with Xylenol Orange as an indicator[8]. The method was accurate to 10-~M DTPA. This titration is not specific for DTPA, and one or more of the radiolysis products may interfere by complexing the Sm 3+. However, as will be shown, mechanistic considerations suggest that this is probably not the case. When solutions of the Sm-DTPA complex were irradiated, the amount of D T P A decomposed was calculated by determining the amount of Sm 3+ that could be titrated with standard E D T A solution[8]. In some experiments, the solutions were degassed before irradiation and the amount of radiolytically produced gases that were noncondensible at - 196°C and at - 78°C was determined. These products were identified by mass spectrometry. The pH of all the solutions was adjusted with either H2SO4 or N a O H and measured with a Beckman Expandomatic SS-2 pH meter.

RESULTS AND DISCUSSION

Precipitate formation Gamma radiolysis of a solution of Sm-DTPA that simulated the pH and concentration of z44Cm product solutions from the cation exchange columns (pH = - 2 , [Sin 3+] = 0.03M, and [DTPA] = 0.05M) produced a precipitate after 1.6 × 107 fads. Elemental and infrared analyses showed that this precipitate was impure Smz(CO3)3. At larger doses, the pH of this solution increased to a maximum value of approximately 9 at a dose of 1.3 × 109 tad. At this dose, the solution was dark brown, opaque, and contained black solids. Titration with E D T A indicated that approximately 28 per cent of the Sm 3+ was still complexed. When the acidity of the solution before irradiation was > 0.5M, impure S m z ( C 2 0 4 ) a precipitated at a dose of 3 × 10r rad. Within the range of 0.5-3M HNO3, decreasing amounts of Smz(C204)3 precipitated with increasingly larger doses. Also, precipitates in solutions > 1M HNO3 redissolved with larger doses. When the acidity was > 3M precipitates failed to form at any dose. For this reason, process solutions containing 244Cm from the ion exchange columns were adjusted to 4M HNO3 immediately after collection. These solutions then remained clear and colorless indefinitely.

Removal of DTPA To prepare the C m - D T P A process solution for further purification, D T P A was removed by precipitating curium with N a O H . However, D T P A effectively competes with the hydroxide ions when their concentrations are < 4M. For example, in test solutions only 10 per cent of the Sm present could be precipitated from a 0.03M Sm-DTPA solution using 0.6M N a O H . Radiolysis of this solution destroys the D T P A , and Sm 3÷ can be completely precipitated with dilute base. Figure 1 shows the effect of gamma radiolysis on this process. When the solution 7. H. Fricke and E. J. Hart, in: Radiation Dosimetry (Edited by F. H. Attix and W. C. Roesch), p. 167, Academic Press, New York (1966). 8. J. Ktirbl and R. Pribil, Chemist-Analyst 45, 102 (1956).

Gamma and alpha radiolysis 1

I

I

I

I

1419 I

i

IOO

o~ .

75

~D

ig 50~ E 25

03

oL o

I

4

I

8 12 Irrodiation Time, hr

16

20

Fig. 1. Effect of 6°Co y-Radiation on the Precipitation of Sm'~+from a 0.03M Sm-DTPA Solution (Initially 4M HNOJ Using 0.6M NaOH. contains C m - D T P A , a-radiolysis destroys the D T P A ; and in those solutions with > 1 g/l. 244Cm, the C m could be completely precipitated with IM N a O H after approximately 1 2 hr after elution from the columns. T h e time dependence of the alpha-induced decomposition of the D T P A is shown in Fig. 2 for a dilute solution of z44Cm in 4M HNO3. T h e results o b e y e d first-order kinetics described by Equation (1), where t is the irradiation time. [DTPA]o and [DTPA]t are the concentrations of D T P A (M) initially and at time t, respectively. T h e curium concentration is given in g/l. [ D T P A ] t = [ DTPA]oe-(°'°°162)(244Cm)t/[DTPAIo

(1)

The effect of the radiolytically produced oxalate ions on the precipitation process I'0

00.8 I--

i

1

0.6

a_ 0.4 0.2i

O-

____

01

I0 20 Irrodiotion Time, hr

Fig. 2. Alpha Radiolysis of DTPA by ~44Cm in 4MHNO.~, [Sm-DTPA] = 0.0531M. [244Cm]= 0.49 g/l. Points experimental; line calculated from Equation 1.

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N E D E. BIBLER

Table 1. Hydroxide precipitation of 244Cm from a solution of Cm-DTPA*

Time after

Elution (days)

% 244Cm precipitated as curium hydroxide NH4OH (1M) NaOH (1M)

0.73 6.6 33.0

89 39 > 99

> 99 > 99 > 99

*[~44Cm] = 5.2g/1., [DTPA] = 0.038 in 4M HNOa.

is shown in Table 1. After 6.6 days, only 39 per cent of the Cm could be precipitated as the hydroxide using NH4OH. The remaining Cm precipitated as the oxalate. After 33 days, these oxalate ions were radiolytically destroyed as indicated by the complete precipitation of Cm as the hydroxide using NH~OH. As shown by Table 1, OH- ions from NaOH effectively compete with C204 = ions at all doses.

Studies of radiolysis mechanism When ionizing radiation passes through a dilute aqueous solution, the following overall reaction occurs [9] H20 --~ H, e~q, OH, H2, and H202 The unstable intermediates-H atoms, aqueous electrons, and OH radicals-then react with DTPA or other solutes present. Figure 3 presents the changes in the -0.008

I

r

I

-0.006

1- -0'004 <1 - 0'002

O0

I 2"0 Dose,

I I 4.0 6"0 I0 t9 eV/ml

8.0

Fig. 3. Dose Dependence of the Gamma Radiolysis of 0-1M Sm-DTPA. ©, 0.4M H2SO4; A, 4M HNO3. 9. For a review of the radiolysis of water and aqueous solutions see: J. K. Thomas, in: Advances in Radiation Chemistry(Edited by M. Burton and J. Magee), Vol. 1, p. 103, John Wiley, New York ( 196 9).

G a m m a and alpha radiolysis

1421

concentration of D T P A as a result of these reactions in the y-radiolysis of 4M HNO3 and also of 0.4M HzSO4 containing Sm-DTPA. From the slopes of the respective straight lines in Fig. 3, the initial G values (molecules destroyed per 100 eV of energy absorbed) for the disappearance of D T P A [G(-DTPA)] are 5-2 in 4M HNO3 and 6.5 in 0.4M H2SO4. Similar linear results were obtained for the a-radiolysis, and the values for G(-DTPA) were 1.1 and 1.3 molecules/100 eV, respectively. The smaller values in 4M HNO3 may result from some of the energy being absorbed directly by the NO3- ions rather than H20 molecules in this relatively concentrated solution[10]. Also, the NO3- ions may be protecting the D T P A by reacting with a portion of the aqueous electrons or H atoms [9]. Degassing the solutions of D T P A in 4M HNOa or 0.4M HzSO4 did not change the values for G(-DTPA) in the y-radiolysis, indicating that dissolved oxygen does not significantly affect the mechanism. In these solutions, CO2 and H2 were formed with G values of 6.5 and 4-2 molecules/100 eV, respectively. Certain conclusions regarding the mechanism for the destruction of D T P A can be drawn from the relations between the observed values for G(-DTPA), G(H2), and G(CO2) and published values[1 l] for the initial production of the reactive intermediates H, OH, and eaq (GH, GoH, and Geff,, respectively) in 0.4M H2SO4. The following equalities result for gamma radiolysis. G(-DTPA) = G ( C O 2 ) ~-- 6.5 = G~.~,,+ G H + GOH

(2)

G(H2) = 4"1 = GH + G.*

(3)

These relationships suggest the following simple mechanism in 0-4M H 2 S O 4 t

(4)

H + DTPA --~ H2 + P1 Jr- C O 9 O H + D T P A ~ H 2 0 + P2.4_ C O

2 .

(5)

In the a-radiolysis studies in 0.4M H2SO4 the value of 1"3 for G(-DTPA) was equal to the published value for the sum of GH+GoH+GHo2[12]. This suggests that Reaction 6 may occur along with Reactions 4 and 5. HO2

+ D T P A ~ H202 + P:~

(6)

This reaction may also occur in the y-radiolysis in the presence of dissolved oxygen. The identities and ultimate fates of the reaction products P,, P2, and P:~ have not yet been determined. However, the relationships between G(-DTPA), G(CO.,), and G(H.,) with the known yields of the primary species in water radiolysis suggest that these species (P,, Pz, P.~) do not interfere with the titration of DTPA. 10. R.W. Matthews, H. A. Mahlman and T. J. Sworski, J. Phys. Chem. 74, 3835 (1970). 11. I.G. Draganic, M. T. Nenadovic, andT. D. Draganic, J. Phys. Chem. 73, 2564 (1969). 12. M. Lefort and X. Tarrago, J. Phys. Chem. 63, 833 (1959). *Gu, is the yield of H~ that is unaffected by the presence of DTPA. i l n acid solution, all the aqueous electrons are converted to H atoms by the reaction e~q + H:~O* -~ H + H~O [6].

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N E D E. BIBLER

The efficiency of decarboxylation of the D T P A radical intermediate formed in Reaction 4 or 5 can be rationalized by considering the structure of the DTPA. Recent data from the pulse radiolysis of amino acids [13] suggest that in D T P A the H atom abstraction occurs at the methylene groups. When any one of 16 of the 18 methylene H atoms in D T P A is abstracted, the intramolecular rearrangement and electron transfer reaction favored by the formation of a six-membered ring as depicted in Fig. 4 can occur. Another possible mechanism leading to decarboxylation would be a charge exchange reaction between one of the carboxylate groups and an hydroxyl radical. Evidence for this type of reaction has been obtained in the radiolysis of trimethyl acetate and also 3,3 dimethylbutyrate [ 14]. COOH

H2

2

/

X

( HOOCCH2) 2N- (CH2) 2- NCH2- CHe-N C"

(0 / H

,

c,Hz

H\ COOH Fig. 4. Possible Mechanism for the Decarboxylation of DTPA.

The effects of pH and the presence of a rare earth cation on G(-DTPA) are shown in Fig. 5. Both curves resemble typical titration curves for free and complexed DTPA[2]. For the uncomplexed D T P A , part of the decrease of G(-DTPA) in the region 0 < pH < 3.0 is probably due to known decreases in the radiolytic yields of H atoms and O H radicals as the pH increases to 3-0 [ 11, 15]. However, this cannot be the only cause for the decrease since at pH = 4, G(-DTPA) does not equal 5.8, the sum GH + Ge + GOH[11]. Another possibility 7.0

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i

5"0 DTPA

I.a I

3.o

I'0 0

2"0

4"0

6"0

8"0

I0"0

pH Fig. 5. Effect of pH and Sm a+ on G(-DTPA). O, 0. IM DTPA; A, 0. IM Sm-DTPA. 13. P. Neta, M. Simic and E. Hayon, J. Phys. Chem. 74, 1214 (1970). 14. M. Anbar, D. Meyerstein and P. Neta, J. Chem. Soc. (B), 742 (1966).

G a m m a and alpha radiolysis

1423

is that at this pH, aqueous electrons are not converted to H atoms and that aqueous electrons are not as effective in destroying DTPA as are H atoms. G(-DTPA) decreases at pH values > 6.5 because of a change in the ionic form of the DTPA; the yields of the primary species are not affected by pH changes in this region [ 11, 15]. Between pH 6.5 and pH 10 the following equilibrium exists since pK4 for D T P A is 8.5[2]; however the lower reactivity of H D T P A 4remains unexplained at present. H2DTPA ,~- ~ H D T P A 4- + H +

(7)

As indicated in Fig. 5, coordination around a metal cation significantly lowers G(-DTPA). These results were obtained with 0-1M Sm-DTPA; thus, it is unlikely that this decrease results only from other solutes or recombination reactions competing with Sm-DTPA for e~q H atoms, or OH radicals. Rather, we suggest that the intermediates react with Sm-DTPA, but that the resulting radicals cannot decompose in a manner that destroys the complex. Decarboxylation of the radical-complex would be hindered either by direct coordination of the carboxyl groups to the metal, or by an increased rigidity of the ligand that prevents the formation of the intermediate depicted in Fig. 4. Further, the Sm-DTPA radical may be sufficiently stable so that it can further react to become a stable molecule again. For example, the product of the addition reaction between e~q and Sm-DTPA 5- probably produces a Sm(II) intermediate as in the similar reaction of Eu(III) EDTA[16]. The Sm(II) species could efficiently react by charge exchange with either OH radicals or the product of the reaction of OH and Sm-DTPA. In either case the complex is not destroyed. However, the final stable complex may no longer be Sm-DTPA but probably contains a ligand that is now slightly different from DTPA. Because G(-DTPA) was insensitive to pH changes between 4.0 and 6.5 and also because the acidity at pH 6-0 is low enough to inhibit the conversion of aqueous electrons to H atoms, further experiments to identify the attacking species were performed at pH = 6.0. As the initial DTPA concentration was lowered, G(-DTPA) decreased to a constant value of 2.6 below concentrations of 0.005M. With [DTPA] = 0.002M, the effect of various solutes on G(-DTPA) was determined (Fig. 6). Those solutes [Fe(CN)64- and n-C4HgOH] that effectively scavenge OH radicals rather than aqueous electrons [9] significantly lowered G(-DTPA). On the other hand, H202, a solute which converts aqueous electrons to OH radicals (Reaction 8), increased G(-DTPA) to 5.2 molecules• 100 eV. e~q +H202 ~ OH + O H

(8)

These results indicate that the OH radicals rather than the aqueous electrons are destroying the DTPA. The competition for OH radicals between DTPA and other OH radical scavengers during irradiation can be described by Equation (9), 15. G. V. Buxton, Radiat. Res. Rev. 1,209 (1968). 16. M. Anbar and D. Meyerstein, Trans Faraday Soc. 65, 1812 (1969) and S. Gordon in: Pulse Radiolysis (Edited by M. Ebert, J. P. June, A. J. Swallow and J. H. Baxendale), p. 285, Academic Press, New York (1965).

1424

N E D E. B I B L E R

6o t

,

,

,

~ 4o/~ .~.oo o

,

!

.

, _ oc~H~oHoc I , . -_

.oo~

.004

.oo~ [~], M

.oo~

.oo.

Fig. 6. Effect of Solutes on the G a m m a Radiolysis of D T P A . [ D T P A ] = 2.0 x 10-3M, pH = 6.0.

after applying the steady state hypothesis to the concentration of the intermediates. In this equation, ks is the rate constant for the reaction of OH radical with the scavenger S. G(-DTPA) -1 = (GoH)-' + ks[S](GoHks)[DTPA] -~

(9)

From the slopes of plots of G(-DTPA) -1 against [S] for the various scavengers and the known values for ks, Gor~, and [DTPA], values for k5 can be calculated. The results of experiments using three different OH radical scavengers with widely different values of ks appear in Fig. 7, and the calculated values for ks, in Table 2. The agreement among the three values for k5 confirms that OH radicals rather than aqueous electrons are destroying DTPA. The effect of Br- ions on G(-DTPA) is summarized in Table 3. Values for G(-DTPA) are not reduced by 1M Br-, even though Br- ions are efficient scavengers of OH radicals (k = 1.6 × I & M -1 sec-a[17]). On the basis of the appropriate rate constants and the relative concentrations of Br- and D T P A in this '7

o

2"0

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Y/

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1.0

0 0

I 0.5

I 1.0

[F~Cn)~"-], ,~M

0

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1

5-0

I0'0

[o-coH~o~], ,,,~

0

r

i

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I

50

60

[,-co~oH], ,,,M

Fig. 7. D e t e r m i n a t i o n of the Rate C o n s t a n t for the Reaction of O H Radicals and D T P A . [ D T P A ] = 2.0 × 10-aM, pH --- 6.0. 17. M. A n b a r and P. N e t a , Intern. J. Appl. Radiat. Isotopes 18, 493 (1967).

Gamma and alpha radiolysis

1425

Table 2. Values for k~OH+DrPA~determined at 25°C using various scavengers* Scavenger Fe(CN)64n-C4HaOH t-C4H9OH

k s ( M -1 s e c - l ) t

1.1 × 101° 2.2 × 109 2.8 × 10s

k~OH+DTI,A ) ( M -1 s e c 1)

1.9 × l& 3.0 x 10a 2.4 × 109 (Avg = 2.5 -4-0.4 × 109)

*[DTPA] = 0.02M, pH = 6.0. tRef.[16].

Table 3. Effect of Br- ions on G(-DTPA)* Solute None Br-, 1M Br-, 1M; n-C4HgOH, 0.007M Br-, 1M; Fe(CN)64-, 0.0005M

G(-DTPA) 2.6 2.6 2"5 (0.76t) 0.76 (0.93t)

*[DTPA] = 0.002, pH = 6'0. tValue in absence of Br- ions. s y s t e m , it is a p p a r e n t t h a t all t h e O H r a d i c a l s r e a c t w i t h B r - ( R e a c t i o n 10) r a t h e r than with DTPA. O H + B r - --> O H - + B r (10) A l s o , in t h e p r e s e n c e o f 1M B r - t h e f o l l o w i n g e q u i l i b r i u m lies p r e d o m i n a n t l y to t h e r i g h t (k = 2"2 × l @ M - ' ) [18]. B r + B r - ~ Br2-

(11)

T h e r e f o r e , in t h e B r - - D T P A s y s t e m , t h e d e s t r u c t i o n o f t h e D T P A m u s t b e d u e to Br2-. N o t e t h a t in t h e p r e s e n c e o f 1 M B r - , n - C 4 H g O H is n o l o n g e r a n e f f e c t i v e s c a v e n g e r w h i l e F e ( C N ) 6 4 - still p r o t e c t s t h e D T P A b y s c a v e n g i n g t h e Br2i n t e r m e d i a t e s . S i n c e it is r e a s o n a b l e t h a t Br2- a n d F e ( C N ) 6 4 - c a n r e a c t b y c h a r g e e x c h a n g e , Br2- m a y a l s o r e a c t w i t h D T P A b y a c h a r g e t r a n s f e r m e c h a n i s m ( R e a c t i o n 12). Br2- + H 2 D T P A 3 --~ H 2 D T P A 2- + 2 B r -

(12)

A similar electron transfer reaction has been proposed for the oxidation of various t e r t i a r y a m i n e s b y CIO2 [19]. W h e t h e r s u c h a r e a c t i o n c a n o c c u r b e t w e e n D T P A a n d O H r a d i c a l s is c u r r e n t l y b e i n g s t u d i e d . Acknowledgment-The information in this article was developed during the course of work under Contract AT(07-2)-1 with the U.S. Atomic Energy Commission.

18. B. Cercek, M. Ebert, C. W. Gilbert and A. J. Swallow, in: Pulse Radiolysis(Edited by M. Ebert, J. P. June, A. J. Swallow and J. H. Baxendale), p. 83, Academic Press, New York (1965). 19. L. A. Hull, G. T. Davis, D. H. Rosenblatt and C. K. Mann, J. Phys. Chem. 73, 2142 (1969).

JINC Vol. 34 No. 4