Sorption of tetravalent plutonium and humic substances onto kaolinite

The sorption of tetravalent plutonium onto kaolinite, a clay mineral, has been studied as a function of pH. The sorption studies have been performed by batch experiments under aerobic and anaerobic conditions (glove box). A pH range of 0-11 has been investigated with plutonium concentrations of 3.5 x 10(-7)-6.9 x 10(-9) M and a solid phase concentration of 4 g/L. A sorption edge at about pH =1 and maximum sorption around pH = 8.5 has been found under aerobic and anaerobic conditions. In the presence of CO2 at pH > 8.5, the sorption of plutonium is decreased due to the formation of soluble carbonate complexes. This is supported by speciation calculations for Pu(IV)-hydroxocarbonate species in aqueous solution. Depending on the pH, 1%-10% of the sorbed plutonium is desorbed from the kaolinite and released into the fresh solution. For comparison with the behavior of Pu(IV), the sorption of the redox-stable Th(IV) onto kaolinite has also been investigated. Furthermore, the sorption of humic substances (HS) onto kaolinite has been studied as a function of pH and for varying concentrations of HS as a prerequisite to understand the more complex ternary system: plutonium, humic substances, and clay. It has been found that the sorption of Aldrich humic acid onto kaolinite is generally higher than that for Gorleben fulvic acid.