Interaction of iron (II) (III) with humic acids in groundwater

The safety assessment of geologic disposal of high-level radioactive wastes needs complete evaluation ofthe complexation behavior of metal ions with naturally-occurring organic materials under various conditions. In groundwater, organic matters such as humic acids are expected to play an important role in regulating the migration of radionuclides by forming complexes and they may enhance the transport of radionuclides to the biosphere through groundwater flow. The metal ions present in groundwater are known to be associated with organic matter implying a strong effect on the organic material-radionuclide interaction. Since iron is one of the metal ion originally occurring in groundwater, the complexation of Fe(II)(III) with Aldrich humic acid become important to be studied. Humic acids have a complication in their compositions, structures and molecular weights causing the effect of polyelectrolyte nature and heterogeneous composition in the complexation with metal ion. The interaction of Fe(II)(III) with humic acid was investigated by examining the change of logtJ with pH and ionic strength with using ion exchange method in the experiment.

Radiotracer Fe was diluted with 0.01M HCl to give a stock solution. Fe was added to the mixture of ion exchanger (Amberlite 200CT, Na-type) and aqueous phase containing various concentration of organic matter (humic acid or polyacrylic acid), for Fe(II)-humate interaction, 0.02M hydroxylammonium chloride and buffer reagents (a mixture of 0.02M solutions of 2-(N-morpholino)ethanesulfonic acid (MES) and tris(hydroxymethyl)aminomethane (THAM))were added. While for Fe(III)-humate interaction, HCl solution was added to control the pcH. Concentration of Fe(II)(III) was varied from 10·8 to 10 4 M with ionic strength 0.1 and 1.0 M NaCI. After shaking the mixture for 150 min, the radioactivity of Fe remaining in the solution was measured and compared with the initial added radioactivity. The remaining part of aqueous phase was used for the pH measurement.