X-ray photoelectron spectroscopy investigation of the interaction of U(VI) and Fe(III) with natural humic acid in aqueous solutions

Abstract

773The determination of radionuclide speciation and factors influencing radionuclide migration in the environment is an important task in current radioecological research. Humic substances are present throughout the environment and are the main organic component in natural waters. Uranium can form strong complexes with humic substances in natural waters leading to increased solubility of uranium and an enhancement of its mobility. As a result of forty years of uranium mining in the Ore Mountains (Erzgebirge) in Germany and Czech Republic, the danger occurred that uranium present in flooded mine shafts, rock piles, seepage and tailing waters can reach the drinking water reservoirs of the population in this region. This problem is also important for several regions in Russia. To improve the predictive capabilities of transport models used for risk assessment of abandoned uranium mines and evaluation of remediation strategies for contaminated sites, it is important to obtain a better understanding of the interaction of uranium with humic substances.X-ray photoelectron spectroscopy (XPS) is a wellsuited method for determining radionuclide speciation in environmentally relevant samples [1]. XPS allows to determine the relative radionuclide concentration, its oxidation state, the relative ionic composition, and the structure of the near-neighbour environment of the ac- Dedicated to Prof. Dr. Egon Uhlig on the Occasion of his 70th BirthdayKeywords: Actinides, Iron, Carboxylic acids, Photoelectron spectroscopy Abstract. Complexes of U(VI) and Fe(III) with natural humic acid (NHA) were studied by X-ray photoelectron spectroscopy (XPS). It follows from the analysis of the uranium and iron concentrations at the surface and in the bulk of the humates that the reaction in solution is heterogeneous. The NHA reacts as a particle. In solutions containing either U(VI) or Fe(III), NHA reacts similar with Fe(III) and U(VI). Howtinide ion in the compound. Previously, we used XPS to determine the chemical state of uranium in samples containing reactor fuel from "lava" and compounds growing in it as a result of the accident at the fourth block of the Chernobyl nuclear power plant [2]. XPS was also used to study the interaction of the uranyl group with calcite and diabase [3], and humic acids [4] in aqueous solutions. In the present work we investigated the interaction of the uranyl group, UO 2 2+ , and Fe 3+ ions with natural humic acid using XPS. The aim was to clarify the possibility of the formation of uranyl compounds and iron complexes at the surface of humic-acids particles and the relative complex stability.

Heterometallic Uranyl (Hydroxyethyl)iminodiacetic Acid (Heidi) Complexes: Molecular Models for U(VI) Uptake in Complex Media

“…In the case of HEIDI, there is one less carboxylate group in the molecule and so chelation takes place primarily on the M(III) cation. We note that this also aligns with the related work by Teterin et al [11] . where the humic acid was found to preferentially bind to the Fe(III) cation over the U(VI) ion.…”

“…Teterin et al. observed that Fe(III) will preferentially bind to humic acid over U(VI) and the reaction is heterogeneous, [11] which likely contributes to the variability observed by Kuo et al [8] . However, Dublet et al.…”

“…[9] These humic acid species can increase solubility of the U(VI) in solution by changing the overall charge on the complex or hindering the ability of the U(VI) to further bind to reactive surface sites. [10] Teterin et al observed that Fe(III) will preferentially bind to humic acid over U(VI) and the reaction is heterogeneous, [11] which likely contributes to the variability observed by Kuo et al [8] However, Dublet et al reported that U(VI) in the presence of both Fe(III) minerals (ferrihydrite) nanoparticles and natural organic matter forms ternary phases under acidic and circumneutral pH values, but the exact nature of this complex was unclear. [12] Dodge and Francis provide additional clarity of the possible ternary complexes using Extended X-ray Absorption Fine Structure (EXAFS) analysis and identified a soluble molecular cluster that contains a 1 : 1 : 2 Fe : U:citric acid ratio.…”