Effect of pH and ionic strength on sorption of Eu(III) to MX-80 bentonite: batch and XAFS study

Abstract

Sorption of radionuclides on MX-80 bentonite has been studied extensively because of its high sorption capacity and low penetrability. Herein, MX-80 bentonite was characterized by acid–base titration, XRD and FTIR in detail. The sorption of Eu(III) from aqueous solution to MX-80 bentonite was investigated as a function of contact time, solid content, ionic strength and pH under N₂ conditions. The experimental data was performed with the diffuse layer model (DLM) with the aid of FITEQL 3.1 code. The site densities are 2.52×10⁻⁴ mol/g for [≡XOH] and 1.54×10⁻⁴ mol/g for [≡YOH], and acidity constants as pK_a are pK_XO⁻=6.772, pK_YOH2⁺=−1.68, and pK_YO⁻=4.145. The sorption of Eu(III) on MX-80 bentonite consists of ≡YOEu²⁺ species at low pH values and ≡XOEu(OH)⁺ species at high pH values. The sorption isotherms were simulated by Langmuir, Freundlich and Dubini–Radushkevich (D–R) models, respectively, and the results indicated that Langmuir model fitted the sorption data better than the Langmuir and D–R models. XAFS technique was applied to characterize the local structural environment of the adsorbed Eu(III), and the results indicated that Eu(III) was bond to O atoms at a distance of about 2.43 Å as ≡Y/XO-Eu²⁺ at low pH values.