Adsorption of Eu(III) onto bentonite and phyllite: A comparative study

Abstract

Disposal of radioactive wastes in underground repositories necessitates knowledge on adsorption and mobility of radionuclides in host rocks (geologic barrier) and in engineered barriers, including clay barrier. The batch adsorption of Eu(III) (a homologue for trivalent radionuclides) onto phyllite compared to bentonite was studied as a function of solution pH (4.5 and 7.0), solution to clay rock ratio (10:1, 100:1, 500:1 and 1000:1), and various Eu(III) concentrations (0.01–190 mg/L; 0.658 × 10−7–1.25 × 10−3 M). The experimental data were interpreted using the isotherm models of Langmuir, Freundlich, Dubinin-Radushkevich, Tóth, and Sips. Adsorption/desorption experiments and bonding strength calculations showed that the adsorption behavior depends on the mineral composition of sorbents, solution pH, the initial concentration of Eu(III), and liquid: solution ratio (L:S). The cation exchange within the interlayer space of montmorillonite is the main adsorption mechanism in bentonite. Cation exchange on the minerals surface, chemical reactions leading to the precipitation of new phases, the electrostatic effect at a low initial concentration of Eu (III), and pH > pHPZC are adsorption mechanisms in phyllites. Solution pH has a pronounced effect on the Eu(III) adsorption onto phyllite due to surface protonation. Fe-oxides and hydroxides play a significant role in the adsorption/desorption of Eu(III) on phyllites. The best fitting was obtained for three-parameter isotherm models of Sips and Tóth. The mechanism of Eu(III) binding is complex and does not follow the ideal monolayer adsorption. While the maximum adsorption capacity of phyllite is 2.5 to 6.6 times lower than of bentonite, depending on the solution pH, it is high enough to guarantee efficacious and durable removal of actinides from the contaminated solutions, particularly at their low concentrations. Phyllites adsorption and mechanical properties make them suitable additives to bentonite in a clay barrier.