Sorption of U(VI) onto an artificial humic substance-kaolinite-associate

An artificial humic substance-kaolinite-associate was synthesized as a model substance for natural clays containing organic matter in clay formations, soils, and sediments. The U(VI) sorption onto this model substance was studied in batch experiments as a function of pH and compared to the U(VI) sorption onto kaolinite in absence and presence of separately added humic acid (HA).
The humic substance-kaolinite-associate has a TOC content of 4.9 mg/g. It was found that the humic matter associated with kaolinite exhibits an immobilizing as well as an mobilizing effect on U(VI). Between pH 3 and 5, humic matter causes an increase in the U(VI) sorption onto kaolinite, whereas at pH above 5 the release of humic matter from the associate in the solution and the formation of dissolved uranyl humate complexes reduces the U(VI) sorption. The U(VI) sorption onto the synthetic humic substance-kaolinite-associate differs from that of U(VI) in the system U(VI)-HA-kaolinite with comparable amounts of separately added HA. Separately added HA causes a stronger mobilizing effect on U(VI) than humic matter present in the humic substance-kaolinite-associate. This can be attributed to structural and functional dissimilarities of the humic substances.