The interactions of 54Mn with humic acids in freshwater systems

The interactions of 54Mn radionuclide with humic acids (HA) isolated from the sediment of a tropical estuary at Mahakam were studied in freshwater system from the Butoniga impoundment by high voltage paper electrophoresis. This way, it was possible to detect cationic, anionic and immobile 54Mn species, and to calculate their electrophoretic mobilities. In freshwater from the Butoniga impoundment in the pH range from 8.1 to 10.5 on average 69% of 54Mn is found in the cationic zone, which corresponds to the mobility of Mn(H2O)62+ ions. The immobile electrophoretic zone of 54Mn present in the freshwater at pH>10 in the absence of HA probably consists of the sparingly soluble or precipitated hydrolytic 54Mn compounds. These immobile 54Mn forms are solubilized after the addition of HA at elevated pH values. This effect is pronounced by the increased concentration of HA and the pH, especially at HA concentration of 200 mg l−1 and pH 11.0. The cationic forms of 54Mn-HA present have lower electrophoretic mobilities [u−=(−1.16±0.05)×10−4 cm2 V−1 s−1 at pH 10.5 and u−=(−1.50±0.27)×10−4 cm2 V−1 s−1 at pH 11.0] than the cationic forms of 54Mn in freshwater system without HA added [u−=(−3.09±0.16)×10−4 cm2 V−1 s−1 at pH 10.5 and u−=(−2.90±0.26)×10−4 cm2 V−1 s−1 at pH 11.0] indicating the formation of new 54Mn cationic forms after the interaction with HA.