The influence of humic acid on the pH-dependent sorption of americium(III) onto kaolinite

This work investigates the sorption of americium [Am(III)] onto kaolinite and the influence of humic acid (HA) as a function of pH (3-11). It has been studied by batch experiments (V/m = 250:1 mL/g, C _Am(III) = 1 × 10^-5 mol/L, C _HA = 50 mg/L). Results showed that the Am(III) sorption onto the kaolinite in the absence of HA was typical, showing increases with pH and a distinct adsorption edge at pH 3-5. However in the presence of HA, Am sorption to kaolinite was significantly affected. HA was shown to enhance Am sorption in the acidic pH range (pH 3-4) due to the formation of additional binding sites for Am coming from HA adsorbed onto kaolinite surface, but reduce Am sorption in the intermediate and high pH above 6 due to the formation of aqueous Am-humate complexes. The results on the ternary interaction of kaolinite-Am-HA are compared with those on the binary system of kaolinite-HA and kaolinite-Am and adsorption mechanism with pH are discussed. Effect of different molecular weight of HA, with three HA fractions separated by ultrafiltration techniques, on the Am sorption to kaolinite were also studied. The results showed that the enhancement of the sorption of Am onto kaolinite at the acidic pH conditions (pH 3-4) was higher with HA fractions of higher molecular weight. Also, the Am sorption over a pH range from 6 to 10 decreased with decreasing molecular weight of HA.