Neptunium sorption

Figure 2. Sorption and Desorption Isotherms for Neptunium Sorption on Mabton Interbed Solids, (a) Oxidizing Conditions, (b) Reducing Conditions.

In general, sorption of Np(V) by aluminosilicates is expected to be low in waters at Yucca Mountain (Turner and Pabalan, 1999 Turner et ai, 1998). A4S for sorption of neptunium by zeolites and tuff particles were typically less than lOmLg in waters from that site (Tien et al, 1985 Runde, 2002). The low neptunium sorption is due to the relative dominance of the poorly sorbed hydrolyzed species Np02(OH)(aq) and the anionic Np02C03 species in solution. In contrast. 

Bertetti F., Pabalan R. T., and Almendarez M. (1998) Studies of neptunium sorption on quartz, cUnoptiloUte, montmoriUo-nite, and alumina. In Adsorption of Metals by Geomedia (ed. E. Jenne). Academic Press, San Diego, CA, pp. 131-148. 

Triay I. R., Furlano A. C., Weaver S. C., Chipera S. J., Bish D. L., Meijer A., and Canepa J. A. (1996) Comparison of Neptunium Sorption Results Using Batch and Column Techniques. Los Alamos National Laboratory. 

Bertetti, F.P., R.T. Pabalan, and M.G Almciulaiv/. I Sinclies of neptunium sorption on quartz, clinop-... 

Neptunium V sorption on hematite (a-Fe20 ) in aqueous suspension the effect of CO2. Radiochim. Acta 85 3348 Kolb, E.D. Caporaso, A.J. Laudise, R.A. 

Combes, J. M., Chisholm-Brause, C. J. et al. 1992. EXAFS spectroscopic study of neptunium(V) sorption at the a-FeOOH/water interface. Environmental Science and Technology, 26, 376-382. 

Del Nero, M., Ben Said, K., Made, B., Clement, A. Bontems, G. 1998. Effect of pH and carbonate concentration in solution on the sorption of neptunium(V) by hydrargilite Application of the non-electrostatic model. Radiochimica Acta, 81, 133-141. 

Del Nero, M., Advocat, T., Jollivet, P. Bontems, G. 1999b. Sorption of neptunium (V) on an alteration gel of alumino-borosilicate glasses and on synthetic silicate gels. Proceedings 2nd International Symposium on Nuclear Fuel Cycle Safety Engineering Research Facility NUCEF 98, Hitachinaka, Ibaraki, Japan, 584-595. 

After a few years of storage, the main radioactive heat emitters in HLW are 90Sr and 137Cs. In addition, extremely long-lived actinides—neptunium, plutonium, americium, and curium—should be collected for transmutation in the future. Therefore, different flowsheets can be proposed for waste processing. It is possible to extract each radionuclide in the special extraction (sorption) cycle, for example, uranium and plutonium in the PUREX process, and after that, minor actinides (MAs) by the TRUEX process,4 strontium by the SREX process,5,6 and cesium by sorption7 or extraction.8... 

Sorption isotherms were also measured for sorption of selenium, technetium, tin, radium, uranium, neptunium, plutonium, and americium on the reference Mabton Interbed solids. The GR-1A groundwater composition was used in these experiments. Two temperatures (23°C 2°C and 60°C 1°C) were used, and both oxidizing and reducing conditions were used for each radionuclide. 

Tin and americium were so extensively sorbed under all conditions that isotherm data could not be obtained. These elements are not significantly mobile in the Mabton Interbed aquifer. Values of Freundlich constants for technetium, radium, uranium, neptunium, and plutonium are given in Table IV. The Freundlich equation did not fit the selenium sorption data very well probably because of slow sorption kinetics or precipitation. Precipitation was also observed for technetium at 23°C for concentrations above 10 7M. This is about the same solubility observed for technetium in the sandstone isotherm measurements. Linear isotherms were observed only in the case of radium sorption. In general, sorption on the Mabton Interbed was greater than on the Rattlesnake Ridge sandstone. This is probably due to the greater clay content of the Mabton standard. 

The sorption and desorption behavior of uranium is similar to neptunium. Figure 3 shows that hysteresis is more important for uranium sorption under reducing conditions than under oxidizing conditions. Values of Ng/Nd are 10 and approximately 200 for oxidizing and reducing conditions, respectively. 

Variables found to be significant for sorption of cesium, strontium, technetium, selenium, neptunium, plutonium, americium, and radium on sandstone and tuff at 23°C are given in Table VII. They are ranked in order of significance where more than one variable was found to be significant. The ( + ) and (-) signs indicate whether sorption is increased or decreased. 

Effects of Hanford High-Level Waste Components on the Solubility and Sorption of Cobalt, Strontium, Neptunium, Plutonium, and Americium... 

Chemical components in the waste solutions potentially could affect radioelement solubility and sorption reactions, and thus enhance or reduce radionuclide transport. The effects of 12 chemical components on the solubility and sorption of cobalt, strontium, neptunium, plutonium, and americium were studied to... 

Neptunium and plutonium sorption behaviors were remarkably similar, implying that they had similar sorption reactions and solution species. Both NaOH and NaA102 decreased neptunium and plutonium sorption. Several explanations can be offered to rationalize this behavior. First, NaOH and NaAlO, may have reacted with the sediment minerals to yield solids of lower sorptive capacity. Aluminate ion, as an anionic species, also may have competed with the similar neptunate and plutonate anions for sorption sites. Finally, sodium hydroxide may have stabilized the hydrolyzed Np02(0H) and Pu02(0H)2" species in solution, as was shown in the solubility tests, and prevented sorption. Explanation of the effect of NaOH and NaA102 on neptunium and plutonium sorption will require further investigation. 

The presence of either HEDTA or EDTA resulted in significantly lower neptunium and plutonium sorption. Complexation of the neptunium and plutonium by HEDTA and EDTA may have caused the reduced sorption. However, this evidence for complex formation was not consistent with the observations made in the solubility studies (HEDTA increased and EDTA decreased neptunium solubility neither affected plutonium solubility). Thus, HEDTA and EDTA may have decreased neptunium and plutonium sorption through some undetermined effect on the sediment minerals. 

Sorption Prediction Equations. Equations predicting radioelement distribution coefficients, K s, as arithmetic functions of component concentrations were obtained for sorption of strontium, neptunium, plutonium, and americium on two Hanford sediments. These equations, presented in Table VH and derived from statistical fits of Box-Behnken experimental designs, were generated for strontium in terms of sodium ion, HEDTA, and EDTA concentrations. Prediction equations for neptunium and plutonium sorption were derived from NaOH, NaA102, HEDTA, and EDTA concentrations. Americium sorption prediction equations were based on NaOH, HEDTA, and EDTA concentrations. 

The sorption behaviors of neptunium and plutonium were similar, thus confirming their suspected similarities in solution chemistry. Both NaOH and NaA102 decreased neptunium and... 

In summary, the solubility and sorption reactions of cobalt, strontium, neptunium, plutonium, and americium were found to be dependent on HLW compositions. Evidence revealed the formation in HLW of organic complexes of cobalt, strontium, and americium, and of hydroxide complexes of neptunium(V) and plutonium(V). Sorption reactions were dependent on radioelement complex formation and suspected waste/sediment reactions. These data can aid in assessing effects of future HLW processing operations as well as in judging the feasibility of continued storage of HLW in existing tanks. 

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