Effects of weak organic acids on the size distribution and size-dependent metal binding of humic substances as studied by flow field-flow fractionation

Yamashita, Yuji*; Saito, Takumi

Humic substances are natural organic matters with heterogeneity in the size distribution and composition of there functional groups. In this study, the effects of organic acid, which are often used as pH buffering agents, on the size of a humic substance and size-dependent binding of metal ions on the humic substance were investigated by flow-field flow fractionation (Fl-FFF). Comparing the three organic acids, Tris, MES, and MOPS, the size of purified Aldrich humic acid (PAHA) changed in the presence of Tris and MES, suggesting that the interaction with these organic acid modulated the molecular structure of PAHA; on the other hand, MOPS hardly affect the size of PAHA. Size-dependent binding of europium (Eu) and uranium (UO) were also studied by Fl-FFF in the presence of MOPS. The binding of these metal ions were not homogeneous with respect to the size of PAHA; they exhibited high affinity to PAHA fractions with 5 nm hydrodynamic diameter.