Enhanced sorption and fixation of radiocaesium in soils amended with K‐bentonites, submitted to wetting–drying cycles

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Summary: The use of bentonites as soil amendment has met with little success in reducing plant uptake of radiocaesium. However, bentonites exchanged with K + have pronounced Cs + binding capacity when subjected to wetting–drying cycles. Fifty‐four different bentonites were collected and characterized for cation exchange capacity and chemical composition. The radiocaesium interception potential (RIP) increased up to 160‐fold (mean 25) when the bentonites were converted to the K‐form and subjected to wetting–drying cycles. This increase in radiocaesium sorption was ascribed to a collapse of the clay sheets into an illite‐like structure, and was most pronounced in bentonites with a high layer charge. The RIP values of K‐bentonites subjected to 25 wetting–drying cycles ranged from 0.22 to 44.3 mol kg −1 . The RIP yields, i.e. the RIP in soil–bentonite mixtures expressed per unit bentonite added, were even higher and ranged up to 99 mol kg −1 . This upper limit is about 10‐fold higher than the RIP value of illite (∼ 10 mol kg −1 ), the principal 137 Cs sorbent in soils of temperate climates. Wetting–drying also promoted fixation of radiocaesium in soils amended with K‐bentonites. About 30% of added 137 Cs could be desorbed with 1 m ammonium acetate (NH 4 Ac) from an unamended soil after 25 wetting–drying cycles, while only between 8 and 21% of 137 Cs could be desorbed from a soil amended with bentonite and a K‐salt. These findings support the proposition that addition of K‐bentonite may be effective in reducing availability of 137 Cs in soils.

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