Aqueous Nitric Acid Radiation Effects on Solvent ... - ACS Publications

Chapter 16

Downloaded by UNIV OF ARIZONA on December 8, 2012 | http://pubs.acs.org Publication Date (Web): October 15, 2010 | doi: 10.1021/bk-2010-1046.ch016

Aqueous Nitric Acid Radiation Effects on Solvent Extraction Process Chemistry Stephen P. Mezyk,*,1 Thomas D. Cullen,1 Gracy Elias,2 and Bruce J. Mincher2 1Department

of Chemistry and Biochemistry, California State University, Long Beach, 1250 Bellflower Blvd., Long Beach, CA 90840 2Aqueous Separations and Radiochemistry Department, Idaho National Laboratory, P.O. Box 1625, Idaho Falls, ID 83415-6150 *[email protected]

The use of aqueous nitric acid is ubiquitous in the nuclear industry especially as the diluent for dissolved used fuel in nuclear solvent extraction processes. These acidic solutions are therefore exposed to the radioactive decay energy from the actinides and fission products contained in the fuel. The radiolysis of aqueous nitric acid produces a mixture of radical and molecular species especially the •NO2 and •NO3 radicals. We have measured the absorbance of these radicals in pulse-irradiated 6.0 M nitric acid and find that at this acidity no •NO2 species was detectable. This, coupled with slow rate constants for reaction of •NO2 with many organic solutes, suggests that •NO3 radical is the more important reactive species. However, most literature values for •NO3 reaction with organic solutes have been made in acetonitrile, rather than aqueous acidic solution, so here we have compared values available in both solvents, including our own measurements, to obtain a quantitative correlation between the two solvents. The implications for the mechanism of the reaction with some ligand solutes are discussed.

© 2010 American Chemical Society In Nuclear Energy and the Environment; Wai, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2010.


Introduction Every year some 10,000 tonnes of spent fuel is discharged from nuclear reactors across the world. For the majority of this fuel that is ultimately slated for repository disposal, the prior partitioning of the long-lived alpha emitters and high-yield fission products would reduce its long-term radiotoxicity and significantly shorten the time needed for safe confinement (1). Planned partitioning strategies for light-water reactor fuels are likely to be based on solvent-extraction technologies. These technologies would selectively remove the uranium, plutonium, and minor actinide actinides from the remainder of the fission products. All of these extraction systems will have to work under highly acidic and radioactive conditions. Therefore, the design and implementation of a practical reprocessing system requires extraction ligands that are robust. In the aqueous phase, or at the aqueous-organic interface in the solvent extraction process where the metal ion complexation occurs, both thermal and radiolytically-induced degradation of the ligands and solvents will occur. The full understanding of this chemistry, particularly establishing the quantitative impact of the radiolysis occurring in aqueous nitric acid solutions, will potentially allow the minimization of interfering ligand degradation chemistry (2). The radiolysis of nitric acid solutions has been extensively studied for many years (3). The NO3• radical is one of the most important intermediates formed by radiolysis of nitric acid solutions and many investigations have been carried out on its formation mechanisms (2, 4–11) and yields (2, 8, 9). These investigations have identified two major NO3• formation pathways in aqueous acidic media; through the reaction of hydroxyl radicals (•OH) with undissociated nitric acid molecules (2):

and the direct action of radiation on nitrate anions:

The reaction of nitrate anions with H2O+ has also been proposed (9):

The chemical reactivity of the NO3• radical has been well-studied in the gas phase (12–14) due to its important nighttime chemistry in the atmosphere. In the solution phase a number of rate constants for NO3• radical reactions in acetonitrile (15–22) have also been determined, but only a few investigations have studied this radicals’ reactions in aqueous solution (2, 10, 18, 21, 23–29). Moreover, much less study on the NO3• radical reactivity has been done in other solvents (10, 18). The purpose of this study was to establish NO3• radical reactivities for some important ligands used in nuclear waste reprocessing, and to gain insight into the mechanisms of this reactivity through the use of model ligands under anticipated acidic, real-world, reprocessing conditions. The correlation of measured rate constants in acetonitrile and acidic water has also performed, in order to establish 194 In Nuclear Energy and the Environment; Wai, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2010.

a predictive capability for larger reprocessing ligands that are only sparingly soluble in acidic aqueous solution.


Experimental All of the chemicals used in this study (Fischer, Sigma-Aldrich) were of reagent grade or higher purity, and used as received. Aqueous 6.0 M HNO3 solutions were made by diluting concentrated nitric acid with Millipore Milli-Q, charcoal-filtered (TOC 18.0 MΩ) water. At this acidity, some 20% of the nitic acid exists in its non-dissociated form (2). Care was taken to ensure that all chemical solutions cooled to room temperature before use. The radiolysis of neutral pH water gives a mixture of radicals and molecular products according to the following equation (30):

The coefficients of each species are absolute yields in μmol Gy-1. Under these highly acidic conditions, no free hydrated electrons exist, as fast reaction with protons to form hydrogen atoms, and nitrate ions to form NO32-•, occurs by:

To improve our initial yield of NO3• radicals, formed by reactions (1) and (2), the nitric acid solutions were pre-saturated with N2O gas, which converted some hydrated electrons into •OH radicals, via the reaction

All radicals were generated by irradiating the 6.0 M HNO3 solutions with an 8 MeV electron beam, generated by a linear accelerator at the University of Notre Dame Radiation Laboratory. This system has been described in detail previously (31). This accelerator system gives 2-10 ns pulses of electrons, of doses ranging from 3-15 Gy, and the resulting chemical reactions were observed on the microsecond timescale using a transient absorption detection system (32). These irradiation conditions gave NO3• radical concentrations of the order of 10-20 μM, with solute concentrations chosen to ensure pseudo-first-order kinetics were maintained. Recombinant water radiolysis products, such as hydrogen peroxide, react slowly on time scales that are much too long to interfere with these radical measurements. A continuous-flow cuvette was used to prevent build-up of interfering absorptions due to long-lived products. Solution flow rates were adjusted so that each pulse irradiation was performed on a fresh sample, and multiple traces (5–20) were averaged to produce a single kinetic trace. Absolute hydroxyl radical concentrations (dosimetry) were measured using the transient absorption of (SCN)2•- at 475 nm, using 0.010 M potassium 195 In Nuclear Energy and the Environment; Wai, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2010.

thiocyanate (KSCN) in N2O-saturated solution at natural pH (33). These measurements were performed daily. Rate constant error limits reported here are the combination of experimental precision and estimated compound purities.


Results and Discussion The electron pulse radiolysis of only a 6.0 M HNO3 aqueous solution gave a transient spectrum (see Figure 1) whose characteristic absorption bands between 550 and 700 nm identified this absorption as being due to the NO3• radical. The NO3• radical absorption spectrum in the gas phase is sharper than seen here, and slightly red-shifted (34, 35). Our spectrum is shown in comparison to that obtained by the laser flash photolysis of cerium(IV) ammonium nitrate under the same conditions, and also for that of NO2•, which is obtained by the •OH radical oxidation of the nitrite ion (36).

One marked difference for our spectrum under these highly acidic conditions is seen in the wavelength range 300 – 450 nm, where the literature peaks for the two radicals was not observed. While the flash photolysis spectrum could have some interference from the remaining Ce(IV) product, the NO2• radical absorption would have been expected to have been formed from the subsequent decay of any formed NO32-• (30):

or through reaction of the formed hydrogen atoms with nitrate (37):

followed by NO3• oxidation of nitric acid (2):

196 In Nuclear Energy and the Environment; Wai, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2010.


Figure 1. Measured spectrum of NO3• radical (squares) in 6.0 M HNO3 in comparison to the literature spectra for NO3• (circles, from Ce(NO3)63photolysis) and NO2• (triangles, from •OH radical oxidation of NO2-). The lack of NO2• radical formation suggests that under these highly acidic conditions, an alternative decay pathway of the NO32-• radical occurs. One possibility is perhaps through the formation of H2NO3:

which might be a more stable species than the intermediate monoprotonated radical formed by single proton addition to NO32-• (Equation 9). However, the NO3• radical kinetics can still be determined through monitoring the absorbance change at 640 nm (see Figure 2). The lifetime of the NO3• radical in only the acidic solution is long, > 50 μs, and the decay follows first-order kinetics. Upon addition of a solute, the decay becomes faster, and by plotting the fitted exponential decay kinetic paramaters against the solute concentration, the secondorder rate constant can be determined. This is shown for NO3• radical reaction with lactic acid in Figure 2b:



Figure 2. a) Decay of the NO3• radical at 640 nm in N2O-saturated 6.0 M HNO3 with 0.141 (squares),, 19.58 (circles) and 65.57 (triangles) mM lactic acid added. b) Transformed second order plot of fitted pseudo-first-order decay kinetics of (a) plotted against lactic acid concentration. Solid line is weighted linear fit, corresponding to reaction rate constant of k = (2.15 ± 0.17) x 105 M-1 s-1 (R2 = 0.990).

Table 1. Summary of NO3• radical reaction rate constants for some ligands used in nuclear waste reprocessing Ligand

kNO3 M-1 s-1

Lactic acid

(2.15 ± 0.17) x 105

Tributyl phosphate

(4.3 ± 0.7) x 106

Cs7SB

(2.71 ± 0.03) x 109


Table 2. Summary of measured NO3• reaction rate constants for compounds in acetonitrile and acidic water. Values of this study in bold log10

log10

Methanol

6.37

5.32

Ethanol

6.83

6.29

1-Propanol

6.71

6.33

1-Butanol

6.84

6.28

tert-Butanol

5.36

4.75

1-Pentanol

6.83

6.38

1-Hexanol

6.79

6.52

1-Heptanol

6.90

6.56

1-Octanol

6.93

6.76

Propen-3-ol

8.35

8.34

Ethylene Glycol

6.82

5.88

Acetone

5.38

3.64

Acetaldehyde

7.36

5.69

Benzene

6.0

6.0

Anisole

9.36

9.65

4-methylanisole

10.11

9.69

Toluene

8.11

9.23

3-nitrotoluene

5.78

7.45

1,2-Dimethylbenzene

9.57

9.39

1,3-Dimethylbenzene

9.23

9.11

1,4-Dimethylbenzene

9.79

9.03


Chemical

Kinetic data obtained by this approach for several extraction ligands that had sufficient solubility in water are summarized in Table 1. These rate constants varied over four orders of magnitude with the faster values observed for species containing aromatic rings. It has previously been shown that in the condensed phase, the NO3• radical can react via both electron or hydrogen atom transfer, depending on the ionization potential of the solute. The latter reaction occurs for simple aliphatic alcohols, (see Table 2 for measured literature values) (30) where H• atom abstraction occurs from the –OH group (21). Analyses of these data showed a sub-group reactivity of ~7 x 104 M-1 s-1, ~7 x 105 M-1 s-1 and 2.4 x 106 M-1 s-1 for primary, secondary, and tertiary carbon hydrogen atom abstraction,



respectively (23). These reaction rate constants are slower than the corresponding values determined in acetonitrile, utilizing laser flash photolysis (15–22). However, the relative NO3• reactivity in acetonitrile also occurs similarly; it has been shown that formation of an initial complex between NO3• and aromatic alkylbenzenes in CH3CN which decays in

Aqueous Nitric Acid Radiation Effects on Solvent ... - ACS Publications

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