Two kinds of samples have been tested to investigate the adsorption of plutonium in soil under natural conditions. In a soil sample containing only about 5% of organic materials, no significant leaching of plutonium was observed with three kinds of extractants, namely, 1N-ammonium acetate, 5%-EDTA and 0.1N-sodium citrate, while some plutonium was leached with 0.1N-citric acid. The results of solubility tests made with natural soil organic acids, namely, humic and fulvic acids, showed that solubilization of plutonium by these acids is unlikely to occur in this soil sample. On the other hand, in another soil sample rich in organic materials (about 50%), significant leaching of plutonium was observed with all the extractants tested. The leaching of plutonium was especially very high with 0.1N-sodium citrate (about 60%) ; sodium citrate showing an initial pH of 8.4 probably solubilized some of organic materials present. Experiments made on extraction of organic materials with alkali solution also suggested that as much as 60-70% of plutonium in this soil sample was associated with some organic acids, whose carbon content was assumed to be 30-40% of the total organic carbon in the soil sample. This finding is very important from a view point of the uptake of plutonium by plant because plutonium associated with such organic acids is presumed to exist abundantly in cultivated soils which are quite rich in organic materials.
Yamamoto et al. (Thu,) studied this question.