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

Abstract

Bentonite amendments are generally ineffective in reducing the soil-to-plant radiocaesium transfer but have previously been shown that bentonites in the K-form having been 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 three K–bentonites and K–bentonite soil mixtures, using a variety of procedures: homogenisation of the bentonites with K through dialysis (K B), or partial transformation of the bentonite to the K-form in the presence of a solution of K 2CO 3 (K L) or in presence of solid K 2CO 3 (K S). Of the three strategies tested, addition of K 2CO 3 (solid) at a dose of 2 meq g −1 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 ∼60 000 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 by making use of the sorption data from the laboratory tests. Optimum soil-amendment would be obtained 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 99 000 meq kg −1 after WD in presence of K if only fine particle size of <1 mm considered) with the contaminated ploughing layer.