Uranium contamination of surface environments is a problem associated with both U-ore extraction/processing and situations in which groundwater comes into contact with geological formations high in uranium. Apart from the environmental concerns about U contamination, its accumulation and isotope composition have been used in marine sediments as a paleoproxy of the Earth's oxygenation history. Understanding U isotope geochemistry is then essential either to develop sustainable remediation procedures as well as for use in paleotracer applications. We report on parameters controlling U immobilization and U isotope fractionation by adsorption onto Mn/Fe oxides, precipitation with phosphate, and biotic reduction. The light U isotope (U) is preferentially adsorbed on Mn/Fe oxides in an oxic system. When adsorbed onto Mn/Fe oxides, dissolved organic carbon and carbonate are the most efficient ligands limiting U binding resulting in slight differences in U isotope composition (δU = 0.22 ± 0.06‰) compared to the DOC/DIC-free configuration (δU = 0.39 ± 0.04‰). Uranium precipitation with phosphate does not induce isotope fractionation. In contrast, during U biotic reduction, the heavy U isotope (U) is accumulated in reduced species (δU up to -1‰). The different trends of U isotope fractionation in oxic and anoxic environments makes its isotope composition a useful tracer for both environmental and paleogeochemical applications.

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Chromium conversion coatings are used as decorative finishes and to improve the corrosion protection and strengthen the wear resistance of metallic surfaces. Chromium electroplating frequently involves the use of hexavalent chromium (Cr). To reduce environmental impacts, several EU directives restricted its use to threshold values of 0.1% Cr(VI) by weight per homogeneous material in vehicles, and 1000 mg kg À1 of Cr(VI) in electronic and electrical equipment. There are few analytical procedures reported that comply with legislative demands, therefore, in our work a selective, quantitative and sensitive analytical procedure for determination of Cr(VI) in the corrosion protection coatings was developed. The investigation was performed on metallic plates homogeneously treated by chromium conversion or hard chrome coatings on copper or zinc electroplated steel surfaces. An alkaline solution (pH 12) was used for ultrasonic extraction of Cr(VI). Speciation analysis was performed by anion-exchange HPLC-ICP-MS. Species interconversions during the analytical procedure were followed using enriched isotopic solutions of 50 Cr(VI) and 53 Cr(III). To prevent Cr(III) oxidation, Tris, EDTA or MgCl 2 was added along with a double isotopically enriched spike and extractions were performed over different time periods. Under optimal conditions that prevent any species interconversion (30 min ultrasonic extraction at 70 C using 2% NaOH + 3% Na 2 CO 3 + MgCl 2 as an extraction agent), six consecutive extractions were necessary to quantitatively extract Cr(VI) from the protective layers; its content was determined by speciated isotope dilution ICP-MS. Concentrations in the samples investigated ranged from 2 to 7 ng of Cr(VI) per mm 2 . The accuracy of the HPLC-ICP-MS determinations was checked by analysis of the certified reference material CRM 545, Cr(VI) in welding dust, using both external calibration and speciated ID-ICP-MS. Good agreement was obtained between the determined and the reported certified values (AE0.7% for external calibration and AE0.2% for speciated ID-ICP-MS). The high sensitivity of the procedure developed (LOQ 0.0107 ng Cr(VI) per mm 2 on a surface of 250 mm 2 ) and the possibility to use external calibration for quantification of separated Cr(VI) instead of ID-ICP-MS allows its application to be extended to the routine laboratory use.

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