MOL.20060427.0175

Thermodynamics of Neptunium(V) Fluoride and Sulfate at Elevated Temperatures

Linfeng Rao,' Guoxin Tian,' Yuanxian Xia' and Judah I. Friese'

'Lawrence Berkeley National Laboratory, Berkeley, CA 94 720 'Pacijk Northwest National Laboratoty, Richland, WA 99352

Abstract - Complexation of neptunium(l/)with fluoride and sulfate at elevated,temperatures was studied by microcalorimetry. Thermodynamic parameters, including the equilibrium constants and enthalpy of protonation offluoride and sulfate, and the enthalpy of complexation between Np(V andfluoride and sulfate at 25 - 70°C were determined. Results show that the complexation of Np(V withfluoride and sulfate is endothermic and that the complexation is enhanced by the increase in temperature - a threefold increase in the stability constants of NpO2F(aq) and NpO2SOi as the temperature is increasedfiom 25 to 70°C.

1. INTRODUCTION characteristic absorption peak in the near IR region and the concentration of Np(V) was determined by the Neptunium is one of the radionuclides of concern in absorbance at 980 nm using the molar absorption the post-closure chemical environment in the proposed coefficient of 395 M-lcm-'. Gran's potentiometric method Yucca Mountain repository because of its mobility and [4] was used to determine the concentration of perchloric long half-life (2.14 x lo6 years). It is likely that 237Np,lZ9I acid in the Np(V) stock solution. Solutions of fluoride and and 99Tcwill be the major contributors to the potential sulfate were prepared by dissolving solid NaF or Na2S04 total annual dose from the repository beyond 10,000 years in water. The ionic strength of all the working solutions in [I]. this study was maintained at 1.O M (25OC) by adding appropriate amounts of sodium perchlorate as the The postclosure chemical environment is expected to background electrolyte. be at near neutral pH, slightly oxidizing, and at elevated temperatures for thousands of years [l]. Np(V) is 1I.B. Microcalorimetry expected to be the most stable oxidation state under these conditions. Its complexation with inorganic ligands that Calorimetric titrations were performed at 25,40,55 could be present, such as OH', F-, SO:' and C03'-, plays and 70°C to determine the equilibrium constants and the an important role in determining its migration in the enthalpy of protonation of fluoride and sulfate, and the repository. To predict the migration behavior of enthalpy of complexation of Np(V) with fluoride and neptunium, it is necessary to have sufficient and reliable sulfate. The titrations were conducted on an isothermal thermodynamic data on its complexation at elevated microcalorimeter (Model ITC 4200, Calorimetry Science temperatures. However, such data are scarce and scattered Corp.). The performance of the calorimeter has been for 25"C, and nonexistent for elevated temperatures [2]. tested by measuring the enthalpy of protonation of To provide reliable thermodynamic data, we have started tris(hydroxymethy1)-aminomethane The result investigations of the complexation of actinides at elevated (THAM). was 447.7 0.2) kJ.mo1-I at 25"C, in excellent agreement temperatures. Thermodynamic parameters, including f with the value in the literature [3]. Details of the formation constants, enthalpy and heat capacity of microcalorimeter are provided elsewhere [SI. complexation are experimentally determined. This paper summarizes the results of the complexation of Np(V) with In the titrations of the protonation of fluoride and fluoride and sulfate at 25 - 70°C. sulfate, the initial cell solutions (- 0.90 cm3 at 25°C) containing fluoride or sulfate were titrated with I-IC104 in 11. EXPERIMENTAL increments of 5 pl. In the titrations of the complexation of Np(V), the initial cell solutions (- 0.90 cm3 at 25°C) 1I.A. Chemicals containing Np(V) were titrated with solutions of fluoride or sulfate in increments of 5 pl. Multiple titrations were All chemicals were reagent grade or higher. Water conducted for each ligand at each temperature. For each from a Milli-Q system was used in preparations of all the titration, n additions were made (usually n = 40 - 50), solutions. The stock solution of Np(V) in perchloric acid resulting in n experimental values of the heat generated in was prepared based on procedures previously reported the reaction cell (Qexj,wherej = 1 to n). These values [3]. The oxidation state of Np(V) was confirmed by the were corrected for the heat of dilution of the titrant (Qdilj), -150

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