Influence of some Seawater Components on the Sorption Behaviour of Neptunium(V)

Abstract

Neptunium / Sorption / Laboratory study / Seawater / Chemical composition SummaryThe fate of neptunium in the aquatic environment is dependent upon, amongst other processes, the extent of uptake by particulate material. In oxic natural waters it exists predominantly in the V-valent state. A number of laboratory experiments have been carried out, using 239 Np tracer, particulate material prepared from Irish Sea sediment and various synthetic solutions, to determine the influence of chemical parameters on the sorption behaviour of Np(V) in marine waters. The data suggest that uptake is highly dependent upon solution pH both as a result of changes in charge on particle surfaces and, at high pH values (>9.0), from the formation of hydrolysis species. Np(V) uptake by marine particulate material was essentially independent of ionic strength within the pH range of natural seawater. The presence of Mg 2+ , Ca 2+ and HC0 3~ ions was found to inhibit Np(V) sorption ; removal of Np(V) was, however, increased from solutions which were supersaturated with respect to CaC0 3 . The data indicate that, although Np(V) sorption in seawater is controlled by interactions between Mg 2+ , Ca 2+ and HC0 3 " ions, the effect of the combined presence of all three constituents is more complex than might be expected from the action of individual components.

Effect of pH and Carbonate Concentration in Solution on the Sorption of Neptunium(V) by Hydrargilite: Application of the Non-Electrostatic Model

“…Percent sorption reaches indeed a maximum value of about 43% at pH 8.5. These sorption trends observed with changes in carbonate concentration are in good agreement with previous studies [4,7,8]. In our experiments, the percent sorption of Np(V) at pH 8.5 is decreased from «75% to «43%, when the total concentration of carbonate in Solution is increased firom 10"' M to 10"^ M, that is to say when log [COf"] is increased from « -5 to « -4.…”

“…Adsorption approaches 90% at the highest pH studied (pH 9). A strong pH-dependence was akeady reported for the sorption of several actinides on oxihydroxides [4,[18][19][20][21][22][23][24][25], or for the sorption of Np(V) on Fe-or Al-oxihydroxides, kaolinite, quartz, clinoptilolite or montmorillonite [4][5][6][7][8][9][10][11][12][26][27][28][29][30]. Adsorption can indeed be viewed as a surface complexation reaction, where metal ions or actinides interact with surface hydroxyl groups in an analogous manner than for hydroxy complexes formation in Solution.…”

“…However, although carbonate has been identified as the most relevant inorganic ligand with respect to actinide complexation in groundwaters, numerous uncertainties remains on its influence upon the rate and extent of Np(V) uptake and upon its role in Np(V) sorption processes. In a qualitative way, an increase of the concentration of carbonate in Solution has been found to reduce the magnitude of Np(V) uptake on several minerals [4][5][6] and on sediments [7,8]. Np(V) in carbonate groundwaters has been reported to be sorbed at the surface of the sediments with very low sorption ratios of about 1 to 6 ml • g \ depending on the specific surface area of the sediments and on the prevailing carbonato complex [3].…”

“…Np(V) in carbonate groundwaters has been reported to be sorbed at the surface of the sediments with very low sorption ratios of about 1 to 6 ml • g \ depending on the specific surface area of the sediments and on the prevailing carbonato complex [3]. Reduced Np(V) uptake in presence of carbonate has been interpreted to be due to the formation of dissolved Np(V) carbonato complexes with a lower surface affinity than the neptunyl cation NpOJ [3,7,8], or to the competition between carbonate ions and neptunium for surface sites [4].…”

Speciation of Radionuclides

“…Laboratory experiments (McCubbin and Leonard, 1995) suggest that sorption of Np V by particulate matter is highly dependent upon pH with hydrolysis species forming at pH Ͼ 9.0. Uptake of Np V by marine particulates is essentially independent of ionic strength within the pH range of natural seawater.…”