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

Abstract : Aluminum (hydr)oxides play an important role in the regulation of the composition of soil/water, sediment/water and other natural water systems. In this study, the interactions among Eu(III), humic acid (HA) and γ-Al2O3 were investigated using a combination of batch and extended X-ray absorption fine structure (EXAFS) techniques. Experiments were performed with varying contact times (2, 15, 60 and 180 d) at a pH of 6.5 for both the binary γ-Al2O3/Eu(III) and the ternary γ-Al2O3/HA/Eu(III) systems. In addition, two representative pH values (pH 6.5 for a near-neutral condition and pH 8.5 for an alkalescence condition) were selected to determine the sequestration mechanisms of Eu(III) in the ternary γ-Al2O3/HA/Eu(III) systems. To verify the specific binding modes and corresponding chemical species, a coordination geometry calculation and a quantitative comparison between the HA binding site concentration and the initial Eu(III) concentration were conducted along with EXAFS data analysis. The microstructure and thermodynamic stability of the formed Eu(III) species were dependent on various environmental parameters. For the binary γ-Al2O3/Eu(III) systems, quantitative analysis results of EXAFS spectra suggested the presence of two Eu(III) species within a contact time of 15 d. Using a coordination geometry calculation, the REu-Al values at ∼3.28 Å and ∼3.99 Å corresponded to the formation of edge-shared and corner-shared surface complexes, respectively. For samples reacted longer than 15 d, the appearance of an additional Eu-Eu shell at ∼3.50 Å was indicative of a structural rearrangement process, leading to the formation of thermodynamically stable surface polynuclear complexes. 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).
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Submitted on : Tuesday, October 22, 2013 - 1:49:52 PM
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S. Yang, G. Sheng, Gilles Montavon, Z. Guo, X. Tan, et al.. Investigation of Eu(III) immobilization on γ-Al2O3 surfaces by combining batch technique and EXAFS analyses: Role of contact time and humic acid. Geochimica et Cosmochimica Acta, Elsevier, 2013, 121, pp.84-104. ⟨10.1016/j.gca.2013.07.013⟩. ⟨in2p3-00875607⟩

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