Colloid Formation and Sorption of Radionuclides in Natural Systems

Abstract

Radionuclides iti the geosphere /Sorption ratios /Colloid formation/Radioactive Cs + ¡Radioactive Sr 2+ /Radioactive Ce 3+ Summary Natural systems comprising sediments and groundwaters of low and of high salinity are investigated with respect to the formation and the influence of coarsely dispersed particles and finely dispersed particles (colloids). Equations are derived for the sorption equilibria in such systems and it is shown that the efficiencies of mutual separation of coarsely dispersed particles, finely dispersed particles and sediments (efficiencies of filtering) strongly influence the sorption ratios that are observed.In groundwater without sediments the formation of colloids of radioactive Ce 3+ is demonstrated, whereas Cs + and Sr ì+ do not form colloids under these conditions, as expected.The amounts of elements and of colloids given off by the sediments into the groundwaters are measured for the samples of low and of high salinity and the influence of agitation is shown.Sorption ratios of radioactive Cs + , Sr î+ , and Ce 3+ are investigated as function of the ratio of the groundwater volume to the mass of the sediment without filtration, after filtration through 0.45 Mm filters and after ultrafiltration. The results are discussed on the basis of the equations derived. It is shown that Cs + ions are sorbed mainly on the clay particles and that the larger amount of these clay particles has grain sizes greater than 0.45 Mm under the experimental conditions of smooth shaking. Sr 2+ ions are mainly sorbed on the sediments with a relatively low sorption ratio. Ce 3+ ions form colloids ("Eigenkolloide") that are retained by ultrafiltration.

Elemental dissolution study of Pu-bearing borosilicate glasses

“…However, previous studies of Pu-bearing colloids in the environment and within laboratory experiments have shown that colloids can be smaller than the 0.45 lm size fraction filters used in this study [31]. Thus, Pu-bearing colloids can be separated from solution only through ultra-filtration (<0.4 lm to >500 MW pore size membranes) [26,31]. No attempt to subject solutions to ultra-filtration was made during the course of this investigation.…”

“…Dissolved uranium, which is present under oxidizing conditions as the uranyl cation (UO 2þ 2 ), is complexed by hydroxide and carbonate anions to yield a number of anionic uranyl-hydroxide and -carbonate species. The anionic nature of aqueous uranium species under alkaline conditions hinders sorption to any silica-rich material that may be present as a result of glass dissolution or spallation [26]. However, geochemical modeling (MINT-EQA2) [27] indicates that the concentration of calcium and strontium reached solubility limits for CaUO 4 and SrUO 4 across the entire pH range investigated.…”

Review on subsurface colloids and colloid-associated contaminant transport in saturated porous media

“…137 Cs and 239 Pu) to colloidal particles consisting of clay minerals, oxides, and humic substances [66,149,[161][162][163]. Complexes with organic matter can result in humic colloids that are transported under favorable conditions of solution composition and water content [164 -167].…”

Near-field/altered-zone models report

“…The formation and stability of colloids, and sorption of radionuclides to pseudocolloids are influenced by solution conditions including radionuclide complexation, pH, redox state, and ionic strength (Bidoglio et al, 1985;Buddemeier and Hunt, 1988;Lieser et al, 1986;Maiti et al, 1989;Olofsson et al, 1981;Ramsay, 1988;Sanchez et al, 1985). These factors will be modified significantly by introduced materials in the NFE, especially if large quantities of concrete are used in repository construction.…”