Examination of competitive lanthanide sorption onto smectites and its significance in the management of radioactive waste

Abstract

The competitive effect of La and Lu (analogues of radionuclides appearing in radioactive waste) in the sorption in four smectites was examined. Sorption and desorption distribution coefficients (K(d); K(d,des)), and desorption rates (R(des)) were determined from batch tests in two media: deionized water and, to consider the influence of cement leachates, 0.02 mol L(-1) Ca. The competitive effect was lower when high-affinity sites were available, as in the water medium at the lowest range of initial lanthanide concentration, with high K(d) for La and for Lu (5-63×10(4) L kg(-1)). Lower K(d) was measured at higher initial concentrations and in the Ca medium, where Lu showed a stronger competitive effect. This was confirmed by fitting the sorption data to a two-solute Langmuir isotherm. The desorption data indicated that sorption was virtually irreversible for the scenarios with high sorption, with an excellent correlation between K(d) and K(d,des) (R(2) around 0.9 for the two lanthanides). Assuming that radioactive waste is a mixture of radionuclides, and that Ca ions will be provided by the cement leachates, this would reduce the retention capacity of clay engineered barriers.

Effects of the presence of Fe(0) on the sorption of lanthanum and lutetium mixtures in smectites

“…given lanthanide with increasing initial concentrations in the range tested (from around 0.1 to 9 meq 260 L -1 ) at a constant concentration of the competitive lanthanide. The decrease observed was 261 sometimes greater by two orders of magnitude, as was the case with La in the FEBEX smectite after 262 incubation at 80 o C. This finding was similar to what was previously observed in these bentonites 12 without Fe pretreatment (Galunin et al, 2010(Galunin et al, , 2011. Changes in K d due to the variation of the 264 initial lanthanide concentration were higher than previously reported as a result of changes in the 265 two clays tested and because of the incubation temperature.…”

“…The decrease in K d values from the 274 water to the Ca medium were nearly two orders of magnitude, especially in the FEBEX bentonite 275 when La was the target solute. The results corroborated previous data reported for the sorption of 276 mixtures of La and Lu in the absence of Fe(Galunin et al, 2011). The effect of Ca on the 277 quantification of K d was explained in terms of the potential competitive influence of the Ca ions, 278 which were sorbed at the interlayer space and displaced the lanthanides to less specific sites because 279 the sorbed concentration of Ca was much higher than that of the two lanthanides (Galunin et al, 280 2011).…”

“…especially in the FEBEX bentonite, suggesting a higher competitive effect of Lu over La as was 270 observed in previous reported experiments(Galunin et al, 2011). 271The composition of the medium influenced K d .…”

“…Assuming that a main objective of the Langmuir fitting is to allow comparisons with similar 351 systems, the obtained b and K 1 parameters were compared to those obtained from the sorption 352 fitting of samples not subjected to Fe pretreatment (Galunin et al, 2011). Ratios could not be 353 calculated because the b and K i of the untreated samples originated from a more complete dataset, 354 but the b values obtained here were generally similar or lower than those obtained with the 355 untreated samples, thus indicating that the-Fe incubation seemed to provoke a decrease in the 356 sorption capacity of the Fe-incubated bentonites.…”

Characterization of radioactive contaminants and water treatment trials for the Taiwan Research Reactor's spent fuel pool

“…However, spent organic materials may be decomposed by high levels of radioactive energy generated by alpha decay and release of combustible gases; therefore, further stabilization is necessary prior to long-term storage or disposal [9]. Inorganic materials are much preferred for use in the treatment of LRWs, which contain alpha-emitters, due to their radioactive stability [10,11]. Inorganic filters manufactured from ceramics, metal, graphite, or any combination of these materials have been produced or are under development in an attempt to overcome the radiation stabilitylimiting features of polymeric membranes.…”