Effects of zeolite and vermiculite addition on exchangeable radiocaesium in soil with accelerated ageing

Soil amendments with zeolite and vermiculite were expected to prevent radiocaesium (Cs-137) dissolution and uptake from the soil by plants. In this study, we investigated how zeolite and vermiculite added to soil influence the radiocaesium fixation with ageing. Zeolite and vermiculite were mixed with soil (1 wt%) before or after the addition of carrier-free caesium-137 (Cs-137) to soils with different radiocaesium interception potential (RIP), which is related to the capacity of the soil to fix trace radiocaesium. Then, the soils were exposed for repeated wet and dry cycles to accelerate Cs-137 immobilization, and its extractability by 1 mol L-1 ammonium acetate was determined before and after 30 dry and wet cycles. Before accelerated ageing (i.e before thy and wet cycles), when Cs-137 was adsorbed on the soil before the addition of the amendments, the addition of zeolite and vermiculite caused a decrease in the amount of exchangeable Cs-137 in low-RIP soils but an increase in the amount of exchangeable Cs-137 in high-RIP soils. The amount of exchangeable Cs-137 was significantly decreased after accelerated ageing regardless of the application of amendments. However, radiocaesium fixation with accelerated ageing was partly inhibited by the addition of zeolite, regardless of the RIP values. The Cs-137 adsorbed on highly selective sites in zeolite is exchangeable by 1 mol L-1 ammonium acetate. Thus, because a portion of the Cs-137 is selectively adsorbed on highly selective sites in zeolite, the redistribution of Cs-137 to frayed edge site followed by ageing-induced fixation was likely limited. However, when Cs-137 was adsorbed on the soil before the addition of the amendments, the addition of zeolite and vermiculite had little influence on the amount of exchangeable Cs-137. In conclusion, the use of realistic doses of zeolite and vermiculite as agricultural amendments is not effective in enhancing the immobilization of radiocaesium in soil.