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Sorption and physicochemical state of Np on the goethite (a-FeOOH) surface were studied.The oxidation state of Np on the a-FeOOH surface was studied by the liquid extraction. The neptunium complexes formed on the surface were studied by X-ray photoelectron spectroscopy. The ionic and elemental composition of the goethite surface and NpO 2 + complexes with a-FeOOH were determined from the XPS data.No Np(IV) and Np(VI) compounds were detected. Neptunyl(V) Np(V)O 2 + forms complexes with the surface groups of a-FeOOH. The equatorial plane of these complexes is occupied by the oxygen atoms of a-FeOOH and water and/or carbonate group CO 3 2! .Migration, bioavailability, and hence toxicity of radionuclides depend on their physicochemical state in the environment [1]. The mobility of radionuclides is essentially determined by processes occurring on the mineral3natural water interface (sorption, redox reactions) and types of radionuclide complexes formed on the surface of minerals and suspended particles. Neptunium mainly occurs in nature in the form of neptunyl(V) ion NpO 2 + . This is a linear dioxo cation with the formal charge of +1. Since the electron density on the central atom is low, NpO 2 + is relatively inert chemically and hence readily migrates in the environment. Under these conditions, it can be reduced to Np(IV) or oxidized to Np(VI)O 2Goethite is an abundant iron-containing mineral having high sorption capacity for heavy metal ions and radionuclides [335]. In this work we studied sorption of Np(V)O 2 + on the goethite (a-FeOOH) surface and the surface complexes of neptunyl(V) by both liquid3liquid extraction allowing separation of neptunium species in different oxidation states and by X-ray photoelectron spectroscopy (XPS). The latter method is used to determine the physicochemical state of radionuclides in the environment. Reduction of Cr(VI) to Cr(III) on the goethite surface by Fe(II) traces [6] and interaction of uranyl with calcite and diabase [7] were studied by XPS previously. EXPERIMENTALGoethite (a-FeOOH, sample I) was prepared by the procedure in [8]. Fe(NO 3 ) 3 . 9H 2 O (50 g, chemically pure grade) was dissolved in double-distilled water (825 ml). A 2.5 M KOH solution (200 ml) was added. The resulting solution was heated at 60oC for 24 h with continuous stirring. The precipitate was washed several times with double-distilled water and acetone and dried at 40oC for 2 days. For sorption experiments, an a-FeOOH suspension was prepared.Sample II was prepared as follows. A goethite suspension (0.0374 g of sample I with 1.6 0 10 18 nm 2 free surface area of the mineral and 5.2 0 10 36 mol of the sorption centers) was placed in a centrifugation tube, and a supporting electrolyte (0.1 M NaClO 4 ) was added. Then 237 Np(V) was added in an amount corresponding to approximately 10 36 M total Np concentration. The pH was adjusted at 7.2 + 0.2, and the solution was continuously stirred for several days. The mother liquor was separated by centrifugation at 6000 rpm. After that, an aliquot of the mother liq...

Humic acids (HAs) and fulvic acids (FAs) are the main components of natural waters that can influence the migration ability of cations. They form stable complex compounds with actinide elements that can change their solubility. In addition, HAs could be responsible for migration of colloidal species. The role of HAs in formation of geochemical barriers in actinide migration in the geosphere is discussed on the basis of results of laboratory experiments and speciation of plutonium and neptunium in field samples collected from the regions contaminated by human-made radionuclides. 175 I. V. Perminova et al. (eds.),