Spectroscopic Investigation of Enhanced Adsorption of U(VI) and Eu(III) on Magnetic Attapulgite in Binary System

The coadsorption of uranium and europium on magnetic attapulgite (M-ATT) was explored through batch experiments and spectroscopic tests. The adsorption processes of U(VI) and Eu(III) onto M-ATT were highly affected by the solution pH but not by the ionic strength, indicating that the adsorption of the two metals was predominated through inner-sphere surface complexation. Adsorption isotherms demonstrated that the maximum adsorption capacities of U(VI) on M-ATT in the single-solute system (e.g., 60.48 mg g^-1 at pH 6.0) were lower than that in the binary-solute system (e.g., 63.03 mg g^-1 at pH 6.0), and the same trend was noticed for Eu(III) adsorption due to their synergistic effect. The existence of U(VI) and U(IV) species evidenced the partial reduction of adsorbed U(VI) to U(IV) by M-ATT based on XPS tests. Hence, the enhanced adsorption of U(VI) in the existence of Eu(III) is ascribed to the primary coadsorption and then the redox of adsorbed U(VI) to U(IV) by M-ATT, as well as the formation of new sorbent active sites derived from reductive coprecipitation (e.g., UO_2+x(s)), which also increased the Eu(III) adsorption on M-ATT. The findings are significant for coadsorption of radionuclides by magnetic adsorbents in environmental pollution management.