Effect of different complexing ligands on europium uptake from aqueous phase by kaolinite: batch sorption and fluorescence studies

Abstract

Clay minerals, a ubiquitous part of the geosphere, can interact with the radioactive contaminants present in aqueous media and can alter their pathways in the geochemical cycle. The final fate of these radiotoxic metal ions in the geosphere is decided not only by nearby clay minerals but also by their organic surroundings. In the present paper, Eu(III) sorption and speciation was studied on a kaolinite–water interface in the presence of complexing ligands, such as oxalic acid, citric acid and humic acid. The % sorption was found to be dependent on the ionic strength of media in the lower pH range but independent in higher pH range. This suggested that the Eu(III) sorption follows an ion-exchange (outer-sphere) mechanism up to pH ∼ 6, beyond which surface complexation (inner-sphere) is predominantly responsible for Eu(III) sorption onto kaolinite. The addition of complexing ligands modifies the sorption profiles. Fluorescence studies showed the sorption of Eu(III) as an Eu(III)–oxalate complex onto the kaolinite surface. The effect of the addition sequence of Eu(III) and humic acid on the sorption and speciation of Eu(III) onto the kaolinite surface was investigated and found to affect the sorption behaviour of Eu(III) onto the kaolinite surface. The effect of kaolinite solubility on Eu(III) sorption and desorption was also investigated.