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.