The association properties of Pu with aquatic humic substances in a 0.01M NaClO4 solution at pH 6–8 were studied on the basis of molecular size distribution. Seven humic substances were isolated from river
water and groundwaters using XAD extraction technique. They were used for comparing their effects on the association of Pu.
In the presence of humic acid, the dominant molecular size of Pu was 100–30 kDa. In the presence of fulvic acid, Pu exhibited
three dominant molecular sizes: 30–10 kDa, 30–5 kDa, and less than 5 kDa. The association of Pu-humus complexes might be controlled
by the molecular size distribution of humic substances and characteristics of their respective size fractions.