Temperature and Ionic Strength Effects on Neptunium Speciation in Simplified Brine Systems Donald Reed, Michael Richmann, Marian Borkowski, Danielle Cleveland, Julie Swanson and Jean-Francois Lucchini LA‐UR 13‐25475 Repository Science and Operations Program, Los Alamos National Laboratory, 1400 University Drive, Carlsbad NM USA
Overview WIPP TRU Repository Summary of Observations Environmental Chemistry of Np Neptunium primarily exists in the environment in the Np(IV) and Np(V) oxidation states, although there are some high-pH (pH >12) conditions where Np(VI) is also stabilized. Under oxic conditions in near- • New band formation for both Np(V) and Np(IV) + make spectroscopy a good tool to investigate the neutral pH, NpO2 is the predominant species and is relatively mobile in the environment due to its high solubility and low tendency toward hydrolysis and complexation. Anaerobic microbial processes and effects of ionic strength and temperature on iron(0, II) will reduce neptunium to the Np(IV) oxidation state, which has an analogous environmental speciation. chemistry to Pu(IV), e.g., very strong hydrolysis, very strong tendency towards polymer/colloid formation, • For Np(V), the effect of ionic strength is a blue shift and low solubility. For this reason, Np(IV) can be predominant under most anoxic conditions in brine or in the peak location but little/no change in the peak low ionic-strength groundwater. Experimental Approach and Results shape. Increased temperature also caused a blue Absorption spectrometry was used to establish the speciation of neptunium. A Varian CARY 5000 shift, but peak shape is also significantly impacted. spectrometer, equipped with a circulating water bath for temperature control (0 to 90oC) was used to obtain • A weak chloride complex and borate complexation temperature-specific spectra. These spectra were obtained in gas-tight cuvettes by temperature- lead to new band formation with Np(V). equilibrating the reference and sample cuvette prior to obtaining a baseline correction and then spiking in • Np(IV) hydrolysis and complexation using the neptunium (typically 5-40 µL) to obtain the spectrum as function of complexation, ionic strength and spectroscopic methods leads to isobestic point temperature. Pseudo-Voigt (Lorentzian/gaussian) fitting was done to model the absorption peaks and de- Rationale for the Research convolute the peaks and calculate λ and the half-height width of the spectra obtained. formation that can be used for formation constant max Neptunium, as NpO + or Np4+, is a key actinide in Data from the following sets of experiments are reported: 1) the effect of ionic strength on the Np(V) 2 determination. aquo peak in simplified systems, 2) the effect of temperature on the Np(V) aquo peak, 3) Np(V)-borate performance assessment at long-times in a HLW complexation, and 4) the inorganic/organic complexation and hydrolysis of Np(IV). New band formation is repository. An understanding of the effects of Future Plans: almost always seen for both the Np(IV) and Np(V) systems investigated. Small, but significant, changes are temperature and ionic strength on its speciation is • Investigate the effects of temperature on the noted with temperature and ionic strength that need to be accounted for to use the spectral changes to needed to address the possible long-term geologic carbonate, borate and chloride complexes of Np(V) calculate formation constants. Pitzer data are obtained that can be used to model ionic strength effects. disposal of HLW in a repository in salt that is • Hydrolysis and complexation study with Np(IV) as These data collectively begin to address the speciation of neptunium as a function of temperature and analogous to the WIPP TRU site. ionic strength to understand its potential mobility in a salt-based thermal repository concept. a function of temperature
Effect of Ionic Strength on the Np(V) Spectrum Effect of Temperature on the Np(V) Spectrum The Np(V) spectrum was obtained in perchloric, LiCl and NaCl media up to ionic o strengths of ~ 8-10 M. In perchloric media, increased ionic strength did not change The effect of temperature (20 – 90 C) was established for Np(V) in 0.1 M HCl the band shape, but led to a blue shift in the peak position (~ 2.7 nm). In high ionic- for the aquo species. The increase in temperature resulted in a steady blue strength systems with chloride present (NaCl and LiCl) a red shift of up to 3 nm was shift in the absorption maximum (~ 2 nm) and a corresponding decrease in observed when spectra were obtained and this was correlated with a change in the the line width. These changes, although small, are significant when de- peak shape. This was interpreted, by peak deconvolution, as due to the formation of convoluting the neptunium spectra at variable temperature given the very a weak Np(V)-Cl complex peak at 983 nm. Spectra below show the fitted peak in small spectral shifts observed in the complexes formed (e.g., chloride and perchlorate media, the peak shift as a function of ionic strength (also perchlorate borate). These data provide baseline spectra that are needed to evaluate the media), and the peak shifts due to chloride complexation in sodium and lithium effects of temperature on complexation. chloride brines (bottom two spectra). Np (V) Aquo Peak Position in Np(V) Aquo peak in perchloric acid (fitted) Sodium Perchlorate Media 981.0 0.020 Aquo - 980.4 nm Np(V) Aquo Line Width as f(T) FWHM - 7.2 nm Np (V) Aquo Peak in 0.1 M HCl as a 980.5 in 0.1 M HCl Function of Temperature 1.130 0.015 981.0 980.0 1.125 979.5 0.010 980.5 Abs 979.0 1.120 Wavelength (nm) Wavelength 978.5 0.005 1.115 980.0 978.0 1.110
0.000 977.5 960 965 970 975 980 985 990 995 1000 0246810 979.5 1.105 Line width /1E12 width Line Wavelength (nm) [perchlorate]/M centerLine (nm) Np (V) behavior in LiCl 1.100 979.0 0.008 1.095 Wavelength at 1.68 M LiCl Temperature vs Line width/1E12
Wavelength at 2.75 M LiCl Temperature vs Line center
Wavelength at 4.01 M LiCl 1.090 Wavelength at 6.51 M LiCl 978.5 0.006 0 20406080100 0 20406080100 T (deg. C) Temperature (deg. C) 0.004 Abs
0.002
0.000 950 960 970 980 990 1000 1010 Wavelength (nm)
Complexation of Np(V) by Borate Hydrolysis and Complexation of Np(IV) and Associated Spectroscopy
In the presence of borate, Np(V) forms a 1:1 complex that leads to new band Np(IV) Hydrolysis Aging Effect on Np(IV) formation (λmax = 987 nm) as shown below. This forms an isobestic point and has a log(K) of ~ 2.11. Formation Constant Avg Log (K) apparent = 2.11 ±0.08
0.012
Wavelength vs 25 mM Wavelength vs 10 mM 0.010 Wavelength vs 5 mM Wavelength vs 1 mM
0.008
Abs 0.006 Variable Borate Concentration 0.004
0.002
0.000 960 980 1000 1020 1040 Wavelength Np(V) Peak Positions for Various Complexants Phosphate Complexation Succinate Complexation 0.07 0.020 Aquo Wavelength vs aquo 0.06 Wavelength vs pH=6 Wavelength vs pH=7 Isobestic point Isobestic point Wavelength vs pH=8 NaCl satellite 0.015 0.05 Wavelength vs pH=8.5 Wavelength vs pH=9 Log(K) = 12.9 966 nm peak
0.04 971 nm peak Log K ~ 4.2 Constant Borate 0.010 Abs Abs 0.03 1st borate and Variable pH 2nd carbonate 0.02 0.005 1st carbonate
0.01
0.000 0.00 960 980 1000 1020 960 965 970 975 980 985 990 995 1000 Wavelength Wavelength (nm)
Acknowledgements This research was performed by the Actinide Chemistry and Repository Science Team (Los Alamos, Repository Science and Operations Program) at the Carlsbad Environmental Monitoring and Research Center (operated by New Mexico State University). Research was in part funded by the Waste Isolation Pilot Plant, Department of Energy, Carlsbad Field Office (Russ Patterson program manager) and as a radionuclide solubility project for DOE/NE (Prasad Nair program manager).