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ECN publication
Measuring the specific caesium sorption capacity of soils, sediments and clay minerals
Koning, A. de; Konoplev, A.V.; Comans, R.N.J.
Published by: Publication date:
ECN Biomass, Coal and Environmental Research 14-9-2010
ECN report number: Document type:
ECN-W--07-049 Article (scientific)
Number of pages:

Published in: Applied Geochemistry (), , 2007, Vol.22, p.219-229.

Two methods to quantify the specific Cs sorption capacity of soils and sediments, which is generally believed to be associated with the Frayed Edge Sites (FES) of illitic clay minerals, are described in detail and are critically reviewed. The first method is a direct measurement of the FES capacity, while the second quantifies the combined parameter KCs D _ ½Kþ_ð¼ KCðK ! CsÞ _ ½FES_Þ, i.e. the product of the FES capacity and the affinity of these sites for Cs. Both methods use the bulky AgTU-complex to mask non-specific sorption sites for Cs and are applied to a number of different soils and pure minerals. Measurement of the FES capacity of pure illite is straightforward. It is shown that the measured capacity is independent of the saturating ion, but does depend on particle size. This method could not be successfully applied to a peat bog soil with 90% organic matter, because the necessary correction for non-specific Cs sorption by the large pool of organic exchange sites overpasses the capacity of the small FES fraction. Measurement of the combined parameter KCs D _ ½Kþ_ is shown to be more appropriate in such cases. Application of the FES capacity method to the hydrous aluminosilicate mineral allophane, an important soil constituent of Andisols, shows that the AgTU-complex is unable to block all non-specific sorption sites for Cs on this mineral. The KCs D _ ½Kþ_ measurements show evidence of a very small number of specific Cs sorption sites on allophane, much smaller than inferred from the FES capacity measurement. The FES capacity of the clay mineral vermiculite is difficult to quantify because the high Cs concentrations that are needed to measure the FES capacity probably cause a collapse of the vermiculite interlayers, thereby creating more high-affinity sites for Cs. The KCs D _ ½Kþ_ method, in which only trace concentrations of Cs are used, is shown to be more appropriate for soils containing substantial amounts of vermiculite. It is concluded that both the direct FES capacity measurement and the measurement of the combined parameter KCs D _ ½Kþ_ can be very useful methods to isolate and characterise Cs-selective sorption sites in soils and sediments, but that results should be interpreted with great care.

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