Effects of K and bentonite additions on Cs-transfer to ryegrass

Bentonite amendments are generally ineffective in reducing the soil-to-plant radiocaesium transfer but it was shown earlier that bentonites in the K-form and subjected to wetting-drying cycles had pronounced radiocaesium binding capacities. We have investigated the effect of wetting-drying (WD) on Radiocaesium Interception Potential (RIP) development in 3 K-bentonites and K-bentonite soil mixtures, using a variety of procedures: homogenisation of the bentonites with K through dialysis (KB), or partial transformation of the bentonite to the K-form in presence of a solution of K2CO3 (KL) or in presence of solid K2CO3 (KS). Of the three strategies tested, addition of K2CO3 (solid) at a dose of 2 meq per g clay and adding the K-bentonite mixtures to the soil resulted in the highest RIP increase after 20 WD cycles. The procedure giving the highest RIP yield is the most practical for further applications and was used in a pot experiment under greenhouse condition. When expressing the RIP increase of the soil-bentonite mixtures per unit bentonite added (RIP-yield), 28 to 110-fold RIP increases were observed up to a value of ~60000 meq kg-1 (6 times higher than the RIP for illite). The beneficial effect following K-bentonite application was shown to be dependent both on a sorption enhancement effect (direct RIP effect) and fixation effects (indirect RIP effect). Greenhouse testing proved that the RIP effects observed in greenhouse could be predicted making use of the sorption data from the laboratory tests. It is advised to perform the soil-amendment with bentonites with high initial sorption RIP and a high sorption RIP increase when subjected to WD in the presence of potassium. Hypothised Transfer Factor (TF)-reductions of at least 10-fold could result when mixing ~1 % bentonite, like Otay bentonite (RIP-yield 99000 meq kg-1 after WD in presence of K if only fine particle size (<1 mm considered) with the contaminated ploughing layer.