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

• Effects of weak organic acids on the size of humic acid (HA) were studied at pH 7.
• UV/vis and fluorescence fractograms of HA were taken by flow field-flow fractionation.
• Positively charged weak acid increased the HA size whereas negatively one decreased.
• Size-dependency of UO22+ and Eu3+ binding by HA was investigated.
• HA particles with or more than 5 nm had greater affinity for both UO22+ and Eu3+.

Size distribution of humic substances is a central issue when one considers stability against aggregation and transportation of HS with bound metal ions. In the present study, the effects of low molecular weight weak organic acids, which are commonly used as pH-buffering agents, on the hydrodynamic size of purified Aldrich humic acid (PAHA) at pH 7 were investigated by means of flow field-flow fractionation (Fl-FFF). The modal size of the PAHA fractogram with 3-morpholino propane sulfonic acid (MOPS) was similar to that obtained without organic acids, whereas that with tris(hydroxymethyl)aminomethane (Tris) increased and that with 2-(N-morpholino)ethanesulfonic acid (MES) decreased relative to that without organic acids. Differences in the charges of the weak organic acids were likely the cause of this difference in the fractograms. After selecting MOPS as an appropriate pH-buffering agent, the size-dependency of metal cation binding (UO22+ and Eu3+) by PAHA was studied at low loading conditions (10−6–10−8 M). The binding of the metal ions was not homogeneous with respect to the size of the PAHA particles; larger particles had greater affinity for both UO22+ and Eu3+. The modal size for the hydrodynamic diameter of PAHA was 2 nm, whereas the modal size for the bound metal ions per unit mass of PAHA was 5 nm. This contrast indicates that PAHA particles with 5 nm sizes strongly bind UO22+ and Eu3+ in terms of their affinity and the number of relevant sites. Size variation of PAHA induced by the addition of multivalent cations was insignificant in the present study.