Investigation of Eu(III) immobilization on γ-Al2O3 surfaces by combining batch technique and EXAFS analyses: Role of contact time and humic acid

For the ternary γ-Al2O3/HA/Eu(III) systems, the EXAFS-derived structural parameters indicated the formation of 1:1 type B ternary complexes and binary corner-shared complexes at pH 6.5 after 2 d. In contrast, the Eu(III) sequestration mechanisms at pH 8.5 were mainly attributed to the formation of 1:2 type A ternary complexes and binary edge-shared complexes. Considering the high proton dissociation constant of strong HA phenolic sites (8.8) and the high metal loading in the present study, the weak HA carboxylic sites are predominantly involved in Eu(III) complexation at pH 6.5 and 8.5. The time-dependent variation tendency of the Eu(III) chemical species formed in the ternary systems may arise from Eu(III)-induced HA agglomeration, binding of Eu(III) ions on stronger HA binding sites and migration of Eu(III) ions to less sterically accessible sites in the HA macromolecule structures. The adsorbed HA could accelerate Eu(III) immobilization at the γ-Al2O3/water interfaces and could enhance the thermodynamic stability of the formed chemical species. The findings presented in this study could provide important microcosmic information for the prediction of the long-term behaviors of Eu(III) and the relevant Ln/An(III) in a geological environment rich in aluminum hydr(oxides).