Degryse F., Smolders E., Vandenhove H. and Cremers A.
Laboratory of Soil Fertility and Soil Biology, K.U. Leuven, Kasteelpark Arenberg 20, 3001 Heverlee,
Belgium e-mail: Fien.Degryse@agr.kuleuven.ac.be
1.
Introduction
The use of bentonites to reduce plant uptake of radiocaesium usually has little or no effect in field trials. However, Cs-selective sites are generated upon wetting-drying (W-D) of bentonites when these are converted to the K-form (Vandenhove et al.
, 2003). Therefore, addition of K-bentonite may be a potential countermeasure in soils contaminated with radiocaesium. In this study, the increase in sorption and fixation of radiocaesium was investigated in a series of bentonites treated with K and submitted to W-D cycles.
2.
Materials and Methods
Fifty-four bentonites were collected and characterized for CEC and total chemical composition using a suspension-nebulization method (Laird et al.
, 1991). Ten g of a sandy soil was mixed with 1g of bentonite and a K
2
CO
3
solution was added to the mixture (2 mmol K). The samples were subjected to
25 W-D cycles (drying at air) after 9 days incubation. The Radiocaesium Interception Potential (RIP) of the soil/bentonite mixtures and of the untreated bentonites was measured according to the method of
Wauters et al.
(1996). The effect of number of W-D cycles (up to 40) was tested for six bentonites.
To study the fixation of radiocaesium, a sandy soil was spiked with 137 Cs. Ten g of the soil was mixed with 1g of bentonite and the soil/bentonite mixtures were treated as described above. An untreated soil sample was included as control. After the W-D cycles, desorption with 1M NH
4
Ac was carried out (24 hours, S:L 1:50).
3.
Results and Discussion
The RIP of the sandy soil was 400 µeq/g. The RIP of the untreated bentonites ranged from 40 to 4070
µeq/g. The RIP of the K-bentonites and the soil/bentonite mixtures increased with increasing number of W-D cycles, but in most cases a plateau was attained around 20 W-D cycles. The RIP values of the soil/bentonite mixtures submitted to 25 W-D cycles ranged from 630 to 9850 µeq/g. The RIP values of the bentonites in these mixtures, expressed per unit amount of bentonite added, were between 1 and
350 (mean 57) fold higher than the RIP values of the corresponding original bentonites. The highest
RIP was obtained for the Otay bentonite that has a high octahedral charge. A significant positive correlation ( r = 0.73) was found between the RIP of the soil/bentonite mixtures and the layer charge of the bentonite calculated from the chemical composition (Figure 1). It is hypothesized that wettingdrying induces a collapse of the clay sheets in a more illite-like structure causing the creation of Cs-

selective sites. This process is favoured at higher layer charge of the bentonite.
Only about 10 to 20% of
137
Cs could be desorbed in the soil/bentonite mixtures subjected to 25 W-D cycles. About 30% of
137
Cs could be desorbed from an unamended soil subjected to 25 W-D cycles.
It is concluded that K-bentonites could be useful as countermeasure in soils contaminated with radiocaesium because of the increased sorption and fixation of radiocaesium upon wetting-drying.
Vandenhove et al . (2003) showed that the transfer of
137
Cs to ryegrass from a sandy soil was about tenfold lower in a soil treated with 1% K-bentonite than from an unamended soil from the 4 th plant harvest onwards.
10000
8000
6000
4000
2000
0
0.0
0.2
0.4
0.6
0.8
1.0
Figure 1. The Radiocaesium Interception Potential (RIP) of a sandy soil mixed with bentonite and
K
2
CO
3
and submitted to 25 wetting-drying cycles as a function of the layer charge of the bentonite.
References
Laird, D.A., Barak, P., Nater, E.A. and Dowdy, R. H. 1991. Chemistry of smectitic and illitic phases in interstratified soil smectite. Soil Sci. Soc. Am. J.
55, 1499-1504.
Vandenhove, H., Smolders, E. and Cremers, A. Potassium bentonites reduce radiocaesium availability to plants. Eur. J. Soil Sci.
Accepted for publication.
Wauters, J., Elsen, A., Cremers, A., Konoplev, A.V., Bulgakov, A.A. and Comans, R.N.J. 1996.
Prediction of solid/liquid distribution coefficients of radiocaesium in soils and sediments. Part one: a simplified procedure for solid phase characterisation. Applied Geochem. 11, 589-594.